valorization of apple pomace by extraction of valuable ... comparison is based on an extensive...

Valorization of Apple Pomace by Extractionof Valuable CompoundsCamila A. Perussello , Zhihang Zhang, Antonio Marzocchella, and Brijesh K. Tiwari

Abstract: Apple pomace is a promising source of carbohydrates, proteins, amino acids, fatty acids, phenolic compounds,vitamins, and other compounds with a vast range of food applications. This review focuses on the valorization ofapple pomace towards the recovery of the main compounds, namely pectin and polyphenols. Applications, advantages,and drawbacks of conventional extraction (acidic medium under high temperatures) compared with novel extractiontechnologies are presented. The comparison is based on an extensive literature review of research on extraction ofvaluable compounds from plant matrixes, particularly apple pomace. Novel extraction techniques involving enzymes,electric field, ultrasound, microwave heating, pressurized liquid, and super/subcritical fluid are also discussed. Thesetechniques offer several advantages, including shorter extraction time, increased yield, reduction—or suppression—ofsolvents, and minimization of the environmental impact. This paper may help researchers and food industry professionalson the scaling-up and optimization of eco-friendly extractions of pectin and phenolic compounds.

Keywords: apple pomace, extraction, pectin, polyphenols

IntroductionApples are one of the most consumed fruits worldwide and are

among the major sources of phytochemicals and antioxidants inthe human diet. Approximately 70 million tons of apples are pro-duced worldwide per year (Massias and others 2017). Considerableamounts of apple are required to manufacture apple juice, cider,jam, and vinegar, generating large volumes of residue, known asapple pomace. In Germany, 200 to 250 ktons of wet apple pomaceare produced per year, while pomace production from Japanese,Iranian, US-American, Spanish, and New Zealand apple process-ing was reported to amount to 160, 97, 27, and 20 ktons per year,respectively. The overall global apple pomace production was ex-pected to exceed 3600 ktons per year in 2010 (Kammerer andothers 2014).

Apple pomace is mainly composed of skin and flesh (95%), seeds(2% to 4%), and stems (1%). It contains many nutrients that differdepending on the variety, origin, and processing technology priorto its generation. O’Shea and others (2015) reported a composi-tion for apple pomace of 9.0% moisture, 2.27% fat, 2.37% protein,1.6% ash, 84.7% carbohydrate, 5.6% starch, 54.2% total sugar, andhigh quantities of calcium, potassium, and magnesium. Dhillonand others (2013a) described the chemical components of applepomace in more detail: total nitrogen (6.8 g/kg DW), total carbon(6.8 g/kg DW), cellulose (127.9 g/kg DW), hemicellulose (7.2 to

CRF3-2017-0010 Submitted 1/12/2017, Accepted 6/15/2017. Authors Perus-sello, Zhang, and Tiwari are with Dept. of Food Chemistry and Technology, TeagascFood Research Centre, Ashtown, Dublin 15, Ireland. Author Marzocchella is withDept. of Chemical Engineering, Materials and Industrial Production, Univ. of NapoliFederico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy. Direct inquiries to authorPerussello (E-mail: [email protected]).

43.6 g/kg DW), lignin (15.3 to 23.5), pectin (3.5% to 14.3% db),total carbohydrate (48.0% to 83.8% db), fiber (4.7% to 51.1% db),protein (2.9% to 5.1% db), lipids (1.2% to 3.9% db), reducing sug-ars (10.8% to 15.0% db), among which glucose (22.7%), fructose(23.6%), sucrose (1.8%), arabinose (14% to 23%), galactose (6%to 15%), and xylose (1.1%). Many mineral components were alsoreported, including Ca (0.06% to 0.1%), Fe (31.8 to 38.3 mg/kg,dry weight basis), Mg (0.02% to 0.36%) and P (0.07% to 0.076%).Since it is rich in nutrients, discard of apple pomace in today’s ap-ple processing industry is a waste of resources. Typical by-productsof the apple industry are illustrated in Figure 1. Furthermore, be-ing perishable and highly biodegradable, especially where freshapple pomace is concerned, the vast amount of nutritious po-mace from the large-scale modern apple processing industry cancause a serious environmental problem. Exploitation/valorizationof apple pomace can reduce environmental impact and meet therequirement of sustainable development of the large-scale appleprocessing industry.

In order to lower costs related to waste processing, applepomace is traditionally used as crude feed, concentrate feed orforage extender for livestock despite its low protein content andless metabolizable energy content in comparison with corn silage,yielding an average quality hay with the main function of energyreplacement (Rust and Buskirk 2008; Zhong-Tao and others2009). However, apple pomace is a good source of digestiblefiber for cattle and humans. In fact, apple pomace is a rich sourceof digestible fiber, pectin, and phenolic compounds (Walterand others 1985; Kumar and Chauhan 2010; Rana and others2015). Figure 2 summarizes the possibilities of transforming appleby-products into industrial commodities. Many studies on the ex-ploitation of apple pomace have been carried out. Typically, applepomace has been used as raw material for fermentation - usually

776 Comprehensive Reviews in Food Science and Food Safety � Vol. 16, 2017C© 2017 Institute of Food Technologists®

doi: 10.1111/1541-4337.12290

http://orcid.org/0000-0003-3312-5179

Valorization of apple pomace . . .

Figure 1–Apple processing by-products (Adapted from Rabetafika andothers 2014).

solid-state fermentation—because it bears high levels of carbo-hydrate, vitamin, dietary fiber, and many other vital nutrientsnecessary for the growth of microorganisms. In most of theseapplications, the targets were metabolic by-products of specificmicroorganisms, often fungi, cultivated in the raw materials,which are valuable biotechnological commodities, such as citricacid (Hang and Woodams 1986), lactic acid (Gullon and others2008), various enzymes (for example, cellulases, hemicellulases,β-glucosidase, and pectinase) (Hours and others 1988; Dhillonand others 2011, 2012), biopolymers (for example, chitosan,xanthan, and gum) (Stredansky and Conti 1999; Vendruscolo andNinow 2014), and even aroma compounds (Dhillon and others2013a; Madrera and others 2015). Dhillon and others (2013b),for instance, made full use of the apple pomace by employing it to

produce citric acid by fermentation using fungal mycelium followedby further extraction of chitosan as a coproduct from the fermenta-tion waste. In another important fermentation application of applepomace, its carbohydrate fraction was hydrolyzed by enzymes(for example, cellulase and hemicellulase) generated by incubatedcultures such as Saccharomyces cerevisiae MTCC 173 to producebiofuels like bioethanol. Biofuels are an international trend for therequirements of the growing energy demands of large populations.As an abundant, available, and renewable natural resource, agro-industrial wastes such as apple pomace seem to have immensecommercial potential for bioethanol production (Raganati andothers 2015). Furthermore, the use of those agro-industrial wastesto manufacture biofuels can reduce not only the competitionbetween crop usage for food and nonfood applications but alsothe environmental pressure caused by disposal of wastes.

In addition to application in fermentation, apple pomace canbe valorized by extraction of its valuable components. It has beenwell reported that apple pomace is a valuable source for the recov-ery of polyphenols, dietary fibers, and pectin. There is a growinginterest in food compounds with potential health-protecting ca-pacities. Said compounds, if derived from plants, are referred to asphytochemicals. They scarcely contribute to the nutritional valueof food products but may play a major role in maintaining hu-man health. Phytochemicals of fruits and vegetables have strongantioxidant and anti-proliferative activity. One of the main groupsof these phytochemicals is plant phenols, and many studies havereported phenolics in apple pomace. McCann and others (2007)investigated the effect of phenolics–rich apple waste extract on keystages of colorectal carcinogenesis, namely DNA damage (Cometassay), colonic barrier function (TER assay), cell cycle progression(DNA content assay), and invasion (Matrigel assay). According tothese authors, the crude extract of apple phenolics can protectagainst DNA damage, improve barrier function, and inhibit in-vasion. Kalinowska and others (2014) compiled and summarizedinformation about apple phenolic compounds and their biologi-cal properties, with particular emphasis on health-related aspects.The results of this research suggest that on one hand that apple po-mace is likely to be an excellent natural resource for these healthycompounds. On the other hand, apple pomace is rich in pectin,

Figure 2–Potential applications of apple by-products.

C© 2017 Institute of Food Technologists® Vol. 16, 2017 � Comprehensive Reviews in Food Science and Food Safety 777


thus it has been widely utilized in the food, cosmetics, and phar-maceutical industries as a stabilizer, gelling agent, and thickener.Recently, pectin has been used for cardiovascular disease ther-apy, induction of prostate cancer cells apoptosis, colon-specificdrug delivery, anti-inflammation, probiotic growth promotion,and even as a new raw material for porous materials (Wang andLu 2014). Kumar and Chauhan (2010) assessed the effect of pectinextracted from apple pomace using 2 different varieties of apple.Pectin was characterized by lipase inhibition effect and the applevariety exhibited influence on the inhibition function.

Schieber and others (2003) combined recovered pectin andpolyphenols from apple pomace. The authors used a food-gradehydrophobic styrene-divinylbenzene copolymerisate to separatepolyphenols from pectin in acidic dried pomace. After elu-tion with methanol, the polyphenolics were concentrated un-der vacuum, stabilized by lyophilization, and characterized byhigh-performance liquid chromatography. The predominant com-pounds were phloridzin, chlorogenic acid, and quercetin glyco-sides. The removal of oxidized phenolic compounds resulted insignificant decolorization of the pectin extract.

The following sections will discuss the extraction of apple po-mace’s main valuable compounds, pectin and polyphenols.

PectinPectin is a structural heteropolysaccharide contained in the

middle lamella, primary, and secondary cell walls of terrestrialplants such as apple, citrus, and apricot. It is particularly abun-dant in the nonwoody parts of the plants. Pectin consists pre-dominantly of galacturonic acid (GalA) residues, with varyingproportions of the acid groups presented as methoxyl esters,and a certain amount of neutral sugars presented as side chains.Polysaccharide in pectin exists mainly in the form of homogalac-turonan, rhamnogalacturonan-I, and rhamnogalacturonan-II. Inpectins, there are “smooth” homogalacturonic regions and ram-ified “hairy” regions, in which most of the neutral sugars arepresent. There are several reviews about the structure and func-tionalities of pectin in literature, such as those published by Willatsand others (2006) and Mohnen (2008). It has been reported thatpectin will interact with and bind to polyphenols such as procyani-din, which is released from cells (mainly from vacuoles) during ap-ple processing (for example, by pressing). The interaction results inlow extractability of pectin, but also a low degree of methylation.Oxidation of polyphenols would result in further lowering of ex-tractability and degree of methylation (Le Bourvellec and others2009).

Pectin is widely used in the food industry as a gelling agent,emulsifier, and carrier polymer for the encapsulation of food in-gredients, helping protect and promote the controlled release ofbiomolecules. It is also used in edible coatings and biodegrad-able packaging of food. In the pharmaceutical and medical fields,pectin is used to enhance drug and gene delivery, wound healing,and even tissue engineering, where it promotes bone tissue re-generation (Munarin and others 2011). According to Willats andothers (2006), apple pomace and citrus peel are the main sourcesof extracted pectin. A diagram illustrating the production of pectinis shown in Figure 3.

Extraction kineticsLimited research regarding extraction kinetics of pectin from

apple pomace was carried out. However, factors such as extrac-tion temperature, pressure and other technology related operatingconditions, solvent (type, polarity, molecular weight), solid/liquid

ratio, biomass particle size and particle size distribution are knownto affect the extraction kinetics. Panchev and others (1989) studiedthe extraction kinetics of pectin from apple pomace using 0.5%nitric acid at temperatures of 60, 70, and 80 °C, obtaining higherextraction rates for higher temperatures. Cho and Hwang (2000)investigated the extraction kinetics of pectin from apple pomaceusing HCl solution at different temperatures. The extraction fol-lowed a 2-step model: the polysaccharide in insoluble form in themiddle lamella, primary and secondary cell walls of the raw ma-terials became soluble in an acidic environment followed by aciddegradation. The 2 first-order kinetic coefficients for the 2-stepreactions increased with temperature, but at different rates. Thepredicted results showed that the soluble polysaccharides reachedmaximum yield sooner at a higher temperature, and the maximumcontent was also slightly higher. According to Wang and Weller(2006), the solvent/matrix ratio and matrix particle size also in-fluence extraction kinetics of pectin by ultrasound, consideringthe attenuation of the ultrasound intensity for large-sized parti-cles. El-Nawawi and Shehata (1987) obtained higher yields withsmaller particle sizes due to increased contact area between matrixand solvent and the small diffusion path of the solvent throughthe matrix. Nonetheless, too fine particles may hinder solventflow throughout the material. In addition, particle size distribu-tion should be quite narrow to avoid small bed voidage—smallparticles could fill the interspace of large particles—and increaseof pressure drop trough the particle bed (Richardson and others2002). As discussed by Adetunji and others (2017), moisture con-tent and particle size of the plant matrix are also important inmicrowave assisted-extraction kinetics, but especially concerningpectin yield.

Conventional extraction methodsPectin is soluble in water but not in alcohol or any other organic

solvents. Based on its solubility characteristics, pectin is tradition-ally extracted from plant tissues such as apple pomace in aqueoussolvents and then recovered by precipitation using organic solvents.However, it is difficult to have pectins dissolved by water alonebecause they are bounded to cell wall in plant tissues. Therefore,traditionally acid (quite often HCl) solutions are used to extractpectin from plant tissues. Pectin from apple pomace is usually ex-tracted using acid solution (pH 1.5 to 3.0) at high temperature(Wikiera and others 2015b). Cho and Hwang (2000) extractedpectin from apple pomace using HCl solution at pH 1.8, 70 to100 °C for 15 to 240 min. Panchev and others (1989) used 0.5%nitric acid at 60, 70, and 80 °C to perform the extraction. O’Sheaand others (2015) used a stronger acid (1 M HCl) solution at asolid/liquid ratio of 1:15 w/v to carry out the extraction in anincubator (150 rpm) for 3 or 7 h at room temperature, and thepectin yields were 7.36% and 7.84%, respectively. Wikiera and oth-ers (2016) reported tests of apple pomace treated with an H2SO4

solution of pH 2.0 (20 mL per 1 g of material) at constant shaking(200 rpm) at 85 °C for 3 h. Kumar and Chauhan (2010) extractedpectin from apple pomace in a reflux condensation system at 97 °Cfor 30 min using dilute hydrochloride acid or citric acid solutionat pH 2.5 with a solid/liquid ratio of 1:50. Min and others (2011)used oxalic acid/ammonium oxalate solution (0.25%, pH 4.6) forthe extraction of pectin from apple pomace at a solid/liquid ratioof 1:40 (w/v) at 85 °C for 1 h. Pectin yield from apple pomaceranged from 3.50% to 14.32% (O’Shea and others 2015).

Dissolving pectin directly from apple pomace would introducemany water-soluble compounds into the solvent, thus increasingimpurity of the pectin product. In research conducted by Min

778 Comprehensive Reviews in Food Science and Food Safety � Vol. 16, 2017 C© 2017 Institute of Food Technologists®


Figure 3–Schematic representation of the pectin production (Source: Adetunji and others 2017).

and others (2011), the apple pomace matrix was pretreated be-fore the dissolution of pectin by a solvent, apple pomace powderwas treated 4 times with ethanol (85%) for 70 °C for 20 min torinse the samples and then filtered with miracloth (Merck KGaA,Darmstadt, Germany). The residue (10 g) was mixed with oxalicacid/ammonium oxalate (0.25%, pH 4.6, 400 mL), which wasmaintained at 85 °C for 1 h. Sharma and others (2015a) introducedpolarity index of solvent [isopropyl alcohol (polarity index 3.9),

acetone (polarity index 5.1), or dimethyl sulfoxide (polarity index6.5)] as a scalable factor in the optimization of the pectin extractionfrom Tamarindus indica L. pulp. The authors used a 3 by 3-full factorexperimental design to evaluate the effect of solvent polarity, ex-traction temperature, and pulp concentration on yield and purity.The degree of esterification (DE), methoxyl content, and galac-turonic acid were regarded as the purity descriptors. Temperature(25 to 75 °C) and pulp concentration were more relevant to yield,



while polarity was more pronounced in the purity of the pectin.The extraction was completed at pH 1 adjusted with an HCl so-lution for 3 to 4 h at different temperatures. The pectin yields at3 and 7 h extraction were 7.36% and 7.84%, respectively, withinthe range of pectin yield from apple pomace (3.50% to 14.32%)reported in other research (O’Shea and others 2015).

Enzymatic methodsTraditional pectin extraction using acid and high temperature is

not eco-friendly and also can lead to rapid corrosion of equipment(Wikiera and others 2015b). Furthermore, the innovative valoriza-tion of the processing by-products is an important part of the de-velopment of sustainable production methods in large-scale foodprocessing. One way to accomplish sustainable valorization, safeprocessing without the use of organic solvents, and a high qualityof the recovered products is to introduce nonsolvent, biocatalysis-based techniques for upgrading the low-value food processing by-products. Enzymatic extraction is one of the most popular novelmethods for extraction of pectin from agro-industrial by-products.

Not only acid but also some specific enzymes can enhancepectin release from the cell wall or the middle lamella. Further-more, employment of enzymes other than acid can result in ad-ditional benefits. Acid hydrolyzes pectin by randomly breakingglycoside bonds into smaller oligosaccharides or even monosac-charides, but enzyme would be more specific to some chemicalbonds, meaning that enzymatic extraction produces larger pectinmolecules. Acid also hydrolyzes the carboxyl bond from galactur-onic acid reducing DE of pectin, while suitable enzymes can avoidthe drawbacks from acid extraction and achieve higher DE. Manyresearchers have employed specific enzymes to hydrolyze cellulosein the cell wall to extract pectin from apple pomace. Enzyme hasbeen utilized as early as 1991 in the extraction of pectin from ap-ple cell walls (Renard and others 1991b). These researchers usedarabinanases, galactanase, and pectin lyase in a buffer parallel, andfound that pectin lyase extracted the highest amount of uronicacid, while arabinanases and galactanase led to the extraction oflimited quantities of pectin. Thibault and others (1988) used an en-zyme preparation from Bacillus subtilis to extract pectin from apple,and the enzyme preparation contained endo-arabinanase, endo-galactanase, and residual endo-pectate lyase. Minimal amounts ofpectin were extracted from the apple samples. Renard and others(1991a) employed pure polysaccharidases and their combinationsto extract pectin. The addition of endo-β-(1,4)-glucanase to theseenzyme formulations increased the amounts of high-molecular-weight material in the extracts.

Wikiera and others (2015b) also used enzymatic extractionmethods to extract pectin from apple pomace. A multicatalyticenzymatic preparation (Celluclast 1.5L preparation) was employedat dosages between 25 and 75 μg/g pomace (solid/liquid ra-tio: 1 g/15 mL). The extraction was carried out at 59 °C, pH4.5 and stirring rate of 200 rpm for 18 h. The highest yieldwas 19%. Wikierra and others (2016) reported another extrac-tion of pectin from apple pomace using enzymes at pH 5.0 and40 °C for 10 h under constant stirring. The utilized enzymeswere endo-β-1,4-glucanase (EC 3.2.1.4, produced by filamen-tous fungus Trichoderma viride, Sigma/Aldrich Chemical Co., Cat.No. C9422) and endo-β-1,4-xylanase (EC 3.2.1.8, from T. viride,Sigma/Aldrich Chemical Co., Cat. No. X3876). The employmentof endo-xylanase resulted in the highest extraction efficiency ofpectin (19.8%). The product also contained the highest levels ofprotein (4.38%) and phenols (1.34%). Endo-β-1,4-glucanase ledto a pectin yield of about 15.2%, and phenol yield of 1%. Min and

others (2011) employed an enzymatic preparation (Viscozyme L,Novozymes, Bagsvaerd, Denmark) containing 1.2 × 10−4 fun-gal β-glucanase units to extract pectin from apple pomace. Theresearchers pre-treated apple pomace (through a 50-mesh sieve)by washing and homogenization followed by heating in an au-toclave at 121 °C for 10 min before the enzymatic extraction.The extraction was carried out at 40 °C for 1 h. The yield usingthe enzymatic method was only 4.6%, lower than the yield bythe chemical method (7.7%). However, the former DE was 69%,higher than the latter (58%).

Other methodsBesides acids and enzymes, other chemicals such as chelat-

ing agent (Renard and others 1991a) and surfactants like ethy-lene glycol, glycerol, diethylene glycol, and monohydric alcohols(Kirtchev and others 1989) are used to improve extraction of pectinfrom apple pomace. Renard and others (1991a) enhanced the en-zymatic extraction of pectin from apple pomace using a chelatingagent to partially depectinate the cell wall material, while Kirtchevand others (1989) accelerated extraction and increased pectin yieldby 55% to 90% adding low molecular alcohols in concentrationsfrom 1% to 3% to the acid extragent. Furthermore, Kirtchev andothers (1989) found that the addition of alcohols increased thepectin gel strength.

According to a review published by Adetunji and others (2017)on recent advances in pectin extraction, the extraction process in-cludes 2 main events: hydrolysis and solubilization of the pectin.Therefore, extraction depends on the degree and velocity of hy-drolysis of the protopectin from the cell wall, solubilization ofthe extracted pectin, and achievement of a saturation state by theextraction solvent. Not only enzyme-aided extraction, but alsoextractions using physical auxiliary methods, such as subcriticalwater, microwave heating, ultrasound, and pulsed electric field,may enhance hydrolysis and solubilization of pectin, resulting inhigher extraction rates and yields.

Subcritical water, for example, is the liquid water submitted tohigh pressure to achieve temperatures higher than the boiling pointwithout changing phase. The extraction using subcritical water asa solvent has proved to be effective for hydrolysis of lignocel-lulosic materials and enhancement of pectin extraction rates andyields from citrus peel (Wang and others 2014). Zakaria and Kamal(2016) investigated the supercritical water extraction process forfood and pharmaceutical purposes. Using a setup consisting of awater reservoir coupled with a high-pressure pump, an extractioncolumn located inside an oven, and a valve to regulate the pressurewithin the system, the compound of interest diffuses from the innerplant material to its surface aided by the highly pressured water, istransported into the bulk solvent, and is then eluted out of the ex-traction column (Figure 4). Afterward, the extracted componentsare subjected to filtration, precipitation, drying, standardization,and characterization (Adetunji and others 2017). Citing other au-thors, Adetunji and others (2017) attribute the high mass transferrates and the possibility of extracting either ionic and nonioniccompounds to factors such as higher diffusion coefficient, lowerviscosity, lower surface tension, increased vapor pressure, and de-creased the dielectric constant of pressurized-water. Additionally,the subcritical water extraction produces higher quality extracts,uses a lower amount of solvents, and has lower cost in comparisonwith other extraction techniques (Zakaria and Kamal 2016).

Wang and Lu (2014) extracted pectin from apple pomace usingsubcritical water (140 to 160 °C, 5 to 15 min) with a solid/water



Figure 4–Diagram of a subcritical water extractor (Source: Zakaria and Kamal 2016).

(S/W) ratio between 1:4 and 1:14. They obtained as optimal con-ditions 140 °C and 5 min with 1:14 of S/W ratio, as longer extrac-tion time and higher extraction temperature would degrade themolecules. According to this work, lower S/W ratios favor masstransfer during extraction due to the lower viscosity of pectin.The maximum pectin yield in the tests was 16.75%. Wang andothers (2014) employed this technology to extract pectin fromapple pomace and citric pomace. Pomace particles (through a100-mesh sieve) were mixed with distilled water at a solid/liquidratio of 1:30. The extraction procedure was completed in an au-toclave. The extraction temperature setting for apple pomace was130, 150, and 170 °C. The extraction duration was 5 min. Themaximum pectin yield (16.68%) was obtained at a temperature of150 °C.

Microwave-assisted extraction also provides higher extractionrates and a lesser requirement of chemicals. Microwaves areelectromagnetic waves in the frequency range of 300 MHz to300 GHz (Decareau 1985). While conventional heating of foodsinvolves conduction and/or convection - heat flux from exter-nal surface toward the core of the material—microwave heating isbased on the conversion of oscillating electromagnetic field intothermal energy: oscillation of polar molecules of the material athigh frequency generate heat throughout the material and a fastheat transfer may be established. Therefore, microwave heating isexpected to aid the extraction of biocompounds, such as pectin,from plant materials. When assisted by microwave heating, pectinextraction from orange peels provided a higher yield than the con-ventional method (using acid) and shorter time (15 min comparedwith 3 h) according to Yeoh and others (2008), and a higher ex-traction yield (15.47% compared with 18.13%) and shorter time(60 min compared with 21 min) in the study of Guo and others(2012). The latter authors also obtained different characteristicsfrom the pectin extracted by the 2 methods, such as slightly loweractivity energy (kJ/mole) and lower intrinsic viscosity (L/g) for themicrowave-aided extraction. As distinct features characterize the2 obtained products, they can be used in different technologicalapplications.

Kratchanova and others (1994) pretreated apple wastes beforepectin extraction using microwave heating. The method provided

higher values of the degree of esterification and gel strength whencompared with the control sample. The favorable effect of mi-crowave heating on the yield and quality of pectin is ascribedfirstly to the partial disintegration of the plant tissue and hydrolysisof protopectin, and secondly, to the rapid inactivation of the pec-tolytic enzymes in the raw material. Wang and others (2007) usedmicrowave heating to extract pectin from apple pomace. Theymixed 2 g of apple pomace powder (0.6 to 1.5 mm) with HClsolution in a microwave at 2450 MHz under stirring. They op-timized extraction time (10.6 to 17.4 min), pH (1.22 to 1.78),solid/liquid ratio (0.0333 to 0.0571 w/v) and microwave power(320 to 580 W) using a response surface method. The F-test andP-value indicated that both the extraction time and pH of the HClsolution had highly significant effects on the response value, andthe quadratic of microwave power also displayed significant effect,followed by the interaction effects of pH and solid/liquid ratio.The optimum conditions of pectin extraction were extractiontime 20.8 min, pH 1.01, solid/liquid ratio 0.069, and microwavepower 499.4 W. The maximum pectin yield was as high as 23%.

Ultrasound as sound waves with frequency from 20 kHz requiresa medium—solid, liquid, or gas—for propagation, involving ex-pansion and compression of the medium molecules. As far as theultrasound-assisted extraction is concerned, the expansion cycleof the liquid solvent creates cavities that grow and collapse, in theso-called cavitation. Cavitation leads to a short period (around400 μs) at temperatures as high as 500 °C and pressures up to1000 atm, enhancing the extraction of compounds from plant tis-sues (Luque-Garcı a and Luque de Castro 2003; Azmir and others2013). Ultrasound-aided extraction results in reduced energy de-mand, extraction time and solvent requirement (Azmir and others2013). Although there is no study published regarding this tech-nique applied to apple pomace for pectin extraction, this techniquewas tested on other fruit residues. Using citric acid extraction aidedby ultrasound to extract pectin from mango peel, Wang and others(2016) decreased extraction time from 60 min to 15 min at 80 °C,obtaining a similar yield. Bagherian and others (2011) obtainedyields of 13.51% to 17.92% w/w using ultrasound during extrac-tion of pectin from grapefruit compared with 6.64% to 9.51%w/w for the conventional method. Furthermore, increasing time



of sonication from 10 to 30 min, galacturonic acid increased, whileesterification degree and molecular weight decreased. Given theseoutcomes, satisfactory results are likely to be obtained with applepomace as well.

Barba and others (2015) and Vorobiev and Lebovka (2016) dis-cussed the use of pulsed electric energy to enhance extraction fromfood materials. Citing other authors, Barba and others (2015) de-scribed the pulsed electric field technology for food processing asa treatment with electric field from 100 to 80000 V/cm in short-time pulses, in general ranging from nanoseconds to milliseconds.The electroporation ascribed to the application of electric fieldsto plant materials weakens the cell walls leading to loss of itssemi-permeability and thus enhances diffusion rates and extrac-tion yields. Additionally, being a nonthermal process, this treat-ment enhances mass transfer without hindering quality, which isessential to thermal sensitive materials such as flavors, pigments,and vitamins. Although there is no recorded publication on theuse of pulsed electric field-assisted extraction for apple pomace, itsapplication was considered successful for the extraction of otherpolysaccharides from plant tissues. Loginova and others (2010), forinstance, observed higher diffusion rates for inulin extracted fromchicory tissue for PEF-treated samples using 100 to 600 V/cm,pulses of 10−3 to 50 s and 20 to 80 °C compared with nontreatedsamples. Considering this result as well as the outcomes of severalresearch projects on extraction of compounds from plant tissues(such as sucrose from sugar beetroot, betalain from red beet, beta-carotene from carrot, and phenolics from grape) as per review byBarba and others (2015), there is evidence that the pulsed electricfield technology can enhance the extraction of pectin and othervaluable compounds from apple pomace.

Combination of novel extraction techniquesAlong with the growing trend for eco-friendly processes, indus-

try is mainly interested in reducing production costs by increasingprocess rates or increasing yields. Although there is no single tech-nology considered an ideal extraction process, there are ways toachieve a balance between product quality, production costs, anduse of solvents. As discussed in the previous sections, novel tech-nologies such as microwave heating, ultrasound, subcritical water,and enzyme-aided extraction are used as powerful tools in provid-ing high production and good products. Combinations of thesetechniques are also being tested. Peng and others (2015), for ex-ample, optimized the rheological properties and extraction yieldof sugar from beet pulp using ultrasound+microwave-aided acidextraction.

Comparison of extraction methodsThere is a keen interest in eco-friendly processes within the

food industry. Although process conditions, pretreatments, andother factors can be optimized to achieve efficient extraction,high-quality extracts, and ecologically friendly impacts influenc-ing pectin quality and yield (Table 1), techniques that reduce oreliminate the use of solvents are current trends. Extraction assistedby enzymes, microwave, electric field, or ultrasound, as well as su-percritical water extraction is considered environmentally friendlyas it reduced or eliminates the use of solvents and provides higheryield and reduced extraction time. By adjusting operational condi-tions, pectin with specific rheological and structural characteristicsis obtained. Therefore, different technological applications can becovered by these methods. Nonetheless, these novel techniquesare often difficult to scale-up and have high implementation costs,which is why most food industries still use the conventional acid

heating method. As a prior step to the industrial consolidation ofany novel technology or new application of a known processingtechnique, exhaustive testing of process conditions and investiga-tion of underlying mechanisms are mandatory in order to achieveoptimal results regarding energy demand, product quality, and pro-cess time. The main advantages and drawbacks of the current ex-traction methods applied to fruit pectin are summarized in Table 2.

Although none of the novel methods detailed above should beconsidered superior to the others, they all provide higher qualitypectin in a much shorter time than the conventional extractionmethod. Additionally, they reduce or even eliminate the solventrequirement, thus qualifying them as eco-friendly. Process cost andapplication of pectin are to be taken into account when selectingthe extraction method.

Separation and purificationAfter extraction, the pectin dissolved in the solution must be

separated, usually by filtration or centrifugation, followed by sed-imentation of pectin using an organic solvent, often ethanol,acetone, or methanol. Cho and Hwang (2000) recovered water-soluble polysaccharide by centrifugation at 6500 × g for 10 min,followed by sedimentation using isopropanol and drying at roomtemperature. The sediments were resolved in water and subjectedto centrifugation to remove insoluble impurities, followed byfreeze-drying. In the research by Wikiera and others (2016), af-ter extraction, the samples were centrifuged (4100 rpm, 10 min,4 °C), followed by filtration of the supernatant through blottingpaper. The pomace pellets were washed with distilled water, cen-trifuged again, and the second supernatants were filtered as well.The supernatants were added to cooled (4 °C) 96% ethanol toreach a final concentration of 70%. The precipitated pectin wascollected by centrifugation (4000 rpm, 20 min), washed with 70%ethanol, centrifuged under the same conditions, were dried at 60°C for 24 h. O’Shea and others (2015) filtered the pomace andsolvent mixture through a muslin bag, then neutralized the super-natant with NaOH (10 M), followed by centrifugation at roomtemperature (10 min, 9000 × g), and freeze-drying of the su-pernatant, without precipitation of pectin from the solution. In astudy by Min and others (2011), the mixture after extraction usinga chemical method was filtered with miracloth, and the filtrate wasmixed with 3 volumes of ethanol (96%), followed by centrifugationat 14500 × g for 10 min. The precipitates were washed with 70%and then 96% ethanol, followed by oven-drying (50 °C). Kumarand Chauhan (2010) filtered the hot acid extract in a cheeseclothbag and then precipitated the apple pectin by using alcohol–juiceproportion 2:1 (v/v). The precipitate was stirred for 10 min andthen left undisturbed for 1 h to allow pectin flotation. The pectinfloating on the surface was removed and then dried at 55 °C. Thedried pectin was dissolved in water and precipitated again usingdistilled acetone for purification. Wang and others (2007) filteredthe extract and precipitated it to obtain pectin using the samevolume of 95% ethanol. However, the above researchers did notconsider the pH value during precipitation. Kalapathy and Proctor(2001) reported that the precipitation pH at about 3.5 resulted ina significantly higher yield of pectin from soy hull than pH 2.0.At the same extraction acidity (for example, 0.05 N), pectin yieldwith precipitation pH at 3.5 was 26%, while that at pH 2.0 was19%. They attributed the higher yield of pectin at pH 3.5 to theacid dissociation constant, pKa, of pectin (3.5), at which pectin isin 50% ionized state, producing a strong gel network, caused by adesirable balance between hydrophilic and hydrophobic character,possibly leading to lower solubility in the precipitation solution.



Table 1–Factors affecting the extraction of pectin.

Factor Influence

Operation conditions 1. Temperature increases the extraction efficiency but may result in poor quality products due to excessive depolymerization anddesesterification of protopectin.

2. Extraction yield increases until an optimum extraction time, decreasing afterwards as a consequence of the ever lowerconcentration gradient in the solution.

3. Material:solvent ratio extraction efficiency and yield increase with higher amounts of solvents, but care should be taken toavoid pectin hydrolysis.

4. A low pH (1 to 3) is required to break protopectin. Higher quality pectin however at lower yields can be obtained at higher pHvalues.

Matrix properties 1. Particle size: smaller particles lead to higher rates and yield due to the larger surface area allowing contact between solventand matrix and the smaller diffusion distance to the solute. Nonetheless, particles that are too small can hinder solvent flowthrough the matrix due to surface tension of the solvent employed.

2. Particle size distribution must be low to avoid that smaller particles attach to larger ones hindering solvent flow throughoutthem.

3. Composition may influence the way pectin is attached to the material, influencing extraction at some extent.Solvent properties 1. A higher viscosity reduces mass transfer rates resulting in lower extraction yields.

2. Polarity affects the interaction between matrix and solvent, thus influencing extraction yield and rate and quality of the pectinextracted.

3. Toxicity is a factor to be considered regarding quality and safety. In food applications, generally recognized as safe (GRAS)solvents are mandatory.

Pre-treatments 1. Grinding, cutting and extrusion result in smaller particles, which increases extraction rate and yield.2. Drying operations reduce moisture, altering the bonding between pectin and material, leading to different extraction rates

and yields.3. The cell disruption induced by the application of electric field or ultrasound, be it intermittent or continuous, to plant tissues

results in higher mass transfer rates and thus higher extraction efficiency and yield.

Garna and others (2007) also adjusted the filtrate to pH 3.5 usingNaOH before ethanol precipitation.

The pectin product must be enriched in uronide content byremoving other compounds (for example, free sugars and salts) bypurification. In many pectin preparations, few individual steps arecarried out for purification of pectin or purity improvement, andthey are usually incorporated in the separation procedure. Afterprecipitation, the pectin is rinsed with the same organic solvent(Wang and others 2007; Min and others 2011; Wikiera and oth-ers 2016) before drying. Wang and others (2014), however, usedan acidic solvent and anhydrous ethanol (5% HCl [v/v] in 60%isopropyl alcohol) to wash the precipitate 3 times. In some in-stances, the crude, dried, unrinsed/unwashed pectin was dissolvedagain using water and the filtration/centrifugation process with orwithout subsequent precipitation was repeated to remove impuri-ties from the pectin (Cho and Hwang 2000; Kumar and Chauhan2010). O’Shea and others (2015) mixed a freeze-dried pectin so-lution (dissolved in deionized water at 60 °C water bath) withethanol (98%) at a ratio of 1:5, followed by centrifugation (10min, 9000 × g). The precipitant was then resuspended in water,and freeze-dried again. Sharma and others (2015a) proposed an-other method in the precipitation step to improve pectin purity.They introduced the polarity index of solvent [isopropyl alcohol(polarity index 3.9), acetone (polarity index 5.1) or dimethyl sul-foxide (polarity index 6.5)] as scalable factors in the optimization ofthe pectin extraction from Tamarindus indica L. pulp. Polarity had apronounced effect on pectin purity, although not on pectin yield.

Emaga and others (2012) developed another method to purifypectin after extraction, by combining sodium caseinate with pectinat different pHs. The authors pointed out that ethanol precipitationis not specific for pectin and would precipitate other compoundsalong with this polysaccharide. The developed method confirmedthe interaction between caseinate and pectin, which is ascribed toelectrostatic interaction and coacervation mechanism. In compar-ison with ethanol precipitation, the developed method is specificfor charged pectin and cannot precipitate neutral pectin. As a re-sult, the precipitated pectin was not the total pectin extracted fromapple pomace.

All the purifications above were carried out after extraction. Inthe research by Min and others (2011), on the other hand, be-fore the dissolution of pectin by solvents, apple pomace powderwas treated 4 times with ethanol (85%) at 70 °C for 20 min torinse the samples and then filtered with miracloth (Merck KGaA,Darmstadt, Germany). The rinsing of pomace before extractionreduced, to some extent, the introduction of some soluble com-pounds into the extracted pectin. After extraction and filtration,dialysis of the filtrate overnight, rather than washing, was employedto purify the extract.

Decolorization of pectinFew references discuss pectin color. Apple pomace undergoes

brown discoloration due to retained polyphenol oxidation activi-ties and microbial spoilage after its generation, unless dried imme-diately. Furthermore, rapid depolymerization and de-esterificationof apple pectins catalyzed by genuine and microbial pectinolyticenzymes are inevitable (Kammerer and others 2014). These pig-ments would be bound to and precipitated with pectin resultingin a deep color of pectin. Therefore, fast dehydration of applepomace has been suggested to stabilize the pomace for its furthervalorization (Kammerer and others 2014). Schieber and others(2003) developed a decolorization method for pectin extractedfrom apple pomace. They adoptively absorbed phenolics from thepectin using diluted mineral acid and found that the absorptionby a resin reduced the browning degree with a striking decreasein chroma and increase of hue angle.

Quality of pectin extracted from apple pomacePectin can be characterized by its degree of esterification,

molecular mass, neutral sugar composition, galacturonic acid per-centage, protein, total phenolic content, color, and rheologicalproperties.

Galacturonic acid. Quantification of galacturonic acid as themain component of pectin allows the estimation of the purityof the alcohol precipitated pectin. The galacturonic acid contentof acid-extracted pectin varies depending on the extraction con-ditions. Garna and others (2007) investigated pH (1.5 and 2),



Table 2–Comparison between pectin extraction methods.

Extraction method Advantages Drawbacks

Conventional/hotacidic solution

(a) Wide knowledge of the underlying mechanisms and howoperation conditions affect extraction yield and time and qualityof pectin; (b) provides reasonable extraction rates and yieldsupon optimization of process conditions.

Long extraction times in comparison with novelextraction techniques; (b) pectin degradation due tothe excessive extraction time; (c) temperatures up to80 °C increase extraction yield until a certain point,beyond which it starts to decline due to pectin thermaldegradation; (d) high solvent requirement, leading toequipment corrosion and need of effluent treatment;(e) toxicity and flammability issues related to the useof chemicals; (f) poor selectivity.

Enzyme-assisted (a) Enzymes’ high level of selectivity reduces amount of solvent andextraction time and increases yield; (b) possibility of turning LMpectin into HM pectin; (c) allows the use of mild temperatures andpH, reducing equipment corrosion and wastewater treatmentcosts; (d) environment-friendly due to lower requirement ofsolvent; (e) tailored extraction leads to higher quality pectinregarding molecular weight and degree of esterification; (f)functional and structural properties of the extractedpolysaccharides is better preserved.

(a) Requires smaller particles to increase access to pectinby enzymes; (b) cost of enzymes, which is partiallycounterbalanced by the reduced costs with acid andmaintenance/control of high temperatures; (c)difficult to scale-up.

Supercritical water (a) Higher solubility of pectin in supercritical water results inimproved extraction rates and thus shorter extraction time; (b)pressure increases pectin yield; (c) Potential for continuous orsemi-continuous extraction; (d) tmperatures as high as 160 °Cenhance solubility of pectin increasing extraction rate, whereasthermal degradation is observed as soon as 80 °C using theconventional method; (e) higher quality of the pectin extracted;(f) lower solvent requirement and consequently reduced costswith solvent and wastewater treatment; (g) the most eco-friendlyextraction method as it completely eliminates the use of solventsother than water; (h) increased safety regarding toxicity andflammability; (j) completely preserves the structure of the pectinextracted.

(a) Implementation costs, which are in some extentcompensated by replacing costly solvents with water;(b) scale-up still being studied.

Microwave-assisted Faster extraction and higher yield—extraction time is generally thelowest compared to all other extraction methods, and is morethan 10 times lower than by the conventional extraction; (b)lower solvent requirement, thus more eco-friendly than theconventional extraction; (c) homogeneous heating rather thanconcentration gradients that result from conventional heating;(d) until an optimal point, higher microwave power andirradiation time enhance yield, degree of polymerization, andgalacturonic acid content; (e) allows the use of small equipment;(f) higher degree of esterification and gel strength; (g) inactivatesendogenous enzymes from the plant tissue

Being water the main conductor of microwave energywithin a food, low water matrixes result in lowerextraction rates and yield; (b) solid/solvent ratio mustbe optimized since the acidic solution can achievesaturation and become viscous, hindering extraction;(c) solvent requirement is reduced but not eliminated,still posing problems of equipment corrosion andeffluent treatment; (d) molecular weight reductionresults from long microwave-exposition times andespecially high power due to disaggregation of pectinnetworks.

Ultrasound-assisted (a) Produces extracts of better color without altering the chemicalstructure; (b) lower energy demand and extraction time ascompared to the conventional method, leading to higher processefficiency and less detrimental environmental effects; (c) tequireslower operation temperatures, preserving material composition;(d) the use of intermittent sonication rather than continuoussonication may lead to higher yield; (e) depending on the processconditions, galacturonic acid content, degree of esterification,and molecular weight change as compared to the conventionallyextracted pectin, allowing different technological applications; (f)in general, viscosity, stability, activation energy, and gellingcapacity are much higher than using other extraction techniques

(a) Enhanced yield and extraction rates arematrix-dependent and may not be observed for someplant tissues; (b) ratio solvent/matrix must beoptimized to avoid high solid particles content, whichattenuates ultrasound intensity; (c) forms weaker gelswith increasing sonication power and time; (d) after acertain sonication time, yield starts to decrease as aresult of pectin degradation

Electric field-assisted (a) The electric field increases permeability of plant materials due tocell disruption, resulting in higher yield and extraction efficiency;(b) moderate electric field (<1 kV/cm) extraction results inpectin with higher galacturonic acid content, higher solubility inwater, and preserved fiber structure; (c) yield and galacturonicacid content can be modulated using different voltages andextraction times; (d) voltages up to 300 V increase yield by morethan 300% respective to the nontreated samples; (e) the lowerthe frequency, the higher the yield (at least in the range 20 to120 kHz) due to increased permeability and molecular mobility ofthe matrix; (f) enhanced yield is observed for any temperature upto 80 °C in comparison to the conventional extraction; (g) pulsedrather than continuous electric field further reduces heatingeffects to the plant tissue; (h) eco-friendly due to the reducedrequirement of solvents

(a) Increases yield and extraction efficiency withouteliminating the use of solvents, although the amountsof chemicals required are lower; (b) difficult toscale-up; (c) the electric field strength necessary tocause membrane permeabilization and thus increasedextraction rates is material dependent, especiallyregarding cell geometry and size.

Sources: Adetunji and others (2017), Bagherian and others (2011), Guo and others (2012), Yeoh and others (2008), Oliveira and others (2015), Knorr and Angersbach (1998), and Yang and others (2016).

extraction temperature (80 and 90 °C), and time (1, 2, and 3 h)on the content of pectin from apple pomace. Pectin content, ex-pressed in percentage in relation to the dry matter of alcoholprecipitate, varied from 24.4% to 58.4%, observed at pH 2 at90 °C for 2 h. The pH had a significant effect on the content,

and the contents at pH 2 were always superior to that found atpH 1.5. The authors suggested 3 potential reasons for the effectof pH: the lower yield of pectin extracted at pH 1.5, precipitationof nonpectin compounds solubilized in acid, and more pectin de-graded at the lowest pH. The amounts of galacturonic acid were



always higher at 90 °C than at 80 °C. Wikiera and others (2015b)compared the content in apple pectin extracted using acid andenzymatic methods (utilizing Celluclast 1.5 L. The contents forenzymatic extraction (61.0% to 70.6%) were always higher than foracidic extraction (about 56.4%). However, the commercial pectinused in their experiments had as high as 80.9% galacturonic acidcontent. Wikiera and others (2015a) analyzed the effect of differ-ent enzyme preparations and reported a content between 56.56%and 62.36%, while that for acidic extraction was 56.44%. Wangand Lu (2014) revealed that the galacturonic acid content in theoptimized compressed hot water extraction of pectin from applepomace was 48.2%. However, Min and others (2011) reported alower galacturonic acid content in a physical/mechanical pretreat-ment combined with enzymatic extraction (693.2 mg/g) than inthe chemical extraction (853.5 mg/g).

Degree of esterification. The ratio of esterified to nonesteri-fied galacturonic acid determines the behavior of pectin in foodapplications. The degree of esterification (DE), which is definedas the percentage of the total number of carboxyl groups esteri-fied, is used as a quality parameter for pectin. In nature, as highas 80% carboxyl groups of galacturonic acid in pectin can be es-terified with methanol, depending on the maturity and variety ofthe fruit. This proportion decreases to a varying degree duringpectin extraction based on the extraction conditions, such as pH,temperature, and time. DE can be evaluated by either degree ofmethylation or of acetylation. DE of extracted pectin is one of themajor factors governing its price. The DE of genuine apple pectinis about 80%, resulting in 10% higher prices. However, due tothe recent trend towards low caloric jams and the replacement ofsugars by sweetening agents, low-esterified pectins have increasedin price (Kammerer and others 2014).

The gelling properties of pectin are largely dependent on pH,molecular weight, and concentration of solutes, but in particu-lar, the degree of methylation (DM). Selection and application ofpectin in commercial food products are chosen based on the fol-lowing classification: high-methoxyl (HM) pectin (60% to 75%)and low-methoxyl (LM) pectin (20% to 40%) (Sriamornsak 2003).This is why pectins are classified as high- compared with low-ester pectins. HM pectins form gels in an acidified environmentin the presence of sugar (for example, sucrose/glucose), whereasLM pectins form gels in the presence of calcium ions. DM canvary based on the maturity of the fruit and the pectin extractionmethod (Sriamornsak 2003).

In the traditional acidic extraction of pectin, DM of pectin varieswith extraction pH, temperature and time, and usually decreaseswith lower pH or longer time. O’Shea and others (2015) extractedpectin from apple pomace using (1 M) HCl for 3 and 7 h at roomtemperature, and DEs of the pectin were 56.38% and 39.64%,respectively. Extraction at 90 °C resulted in lower DM than 80 °C,and pectins extracted at pH 1.5 were generally characterized bylower DM than those extracted at pH 2 (Garna and others 2007).Besides the factors mentioned above, different acids lead to varyingDE of extracted pectin. DEs from 2 varieties of apple pomaceextracted using HCl at pH 2.5 and 97 °C were 22.15% and 45.98%while using citric acid at the same pH and temperature were50.14% and 52.51%, respectively (Kumar and Chauhan 2010).

Many innovative extraction methods improve the DM in com-parison with acidic extraction. Wikiera and others (2015b) re-ported the DM of acidly extracted apple pectin was 56.1%, andthat of commercial pectin was 66.9%. When enzyme preparationCelluclast 1.5L was used for the extraction, the DM improvedslightly, ranging between 57.4% and 58.7%. Wikiera and others

(2016) reported the DM of apple pomace extracted using endo-xylanase was 73.4%, by endo-cellulase 66.3%, by the combinationof the 2 enzymes 67.5%, by acid 56.1%, and commercial 56.9%.Wang and Lu (2014) carried out an extraction using compressedhot water. However, the DM was only as low as 30.23% whilethe commercial variety was 85.54%. DM of pectin from po-mace treated by the physical method (121 °C for 10 min) +enzyme preparation (Viscozyme L with fungal β-glucanase) was69%, while that for the chemically extracted was 58%. Wang andothers (2014) used subcritical water to extract pectin from applepomace, and pectin’s DE ranged between 83.41% and 89.69%.Kratchanova and others (1994) pre-treated apple waste using mi-crowave heating before pectin extraction. Higher values for DEand gel strength were observed compared with the control sample.The favorable effect of microwave heating on the yield and qualityof pectin is ascribed firstly to the partial disintegration of the planttissue and hydrolysis of protopectin, and secondly, to the rapidinactivation of the pectolytic enzymes in the raw material.

Antioxidant properties. Several studies report the antioxidantcapacity of pectic polysaccharides and other polysaccharides withpolyhydroxyl groups (Khasina and others 2003; Gan and Latiff2011). However, only a few papers have investigated the antiox-idant capacity of pectin from apple pomace. Wang and others(2014) found scavenging capacities of >60% DPPH and 80%ABTS for apple pomace, and Wang and Lu (2014) reported thattheir extracted pectic polysaccharides showed in vitro antioxidantcapability and inhibitory effect.

PhenolicsApple pomace’s strong antioxidant properties are ascribed to the

presence of phenolics such as epicatechin, quercetin, phloretin,chlorogenic acid, protocatechuic acid, caffeic acid, p-coumaricacid, ferulic acid, salicylic acid, phloridzin, and 3-hydroxy-phloridzin, among others (Lu and Foo 2000; Rana and others2015; Lohani and Muthukumarappan 2016). Polyphenols havean influence on sensory perception (bitterness, color, flavor, odor,and astringency) and also on oxidative stability of foods since theyact as reducing agents and thus protect living cells from gettingdamaged by the unstable molecules known as free radicals andreactive oxygen species (ROS). Free radicals are neutralized byantioxidants in the form of by-products of normal cell processes.Studies on the effects of polyphenols on human health have shownthat they protect the organism against aging, infections, hyper-tension, cancer, diabetes, asthma, brain dysfunction, osteoporosis,and cardiovascular diseases. Being an anti-diabetic compound,phloridzin, for instance, is a major target when it comes to phe-nolics from apple pomace (Pandey and Rizvi 2009; Sharma andothers 2015b). According to Masumoto and others (2009), Lavelliand Corti (2011), and Makarova and others (2015), diets richin phloridzin reduce glucose serum levels in mice and humans,improving the quality of life for diabetics. McCann and others(2007) attributed anticancer properties to phenolic compounds ofapple waste. In vitro tests applied to colon cells treated with 0.01%to 0.1% apple extract confirmed protection against DNA damageand increased barrier protection, leading to decreased rates ofaggressive mutations associated with tumors. Their study indicatesthat apple consumption decreases the risk of colon cancer.

Apples contain over 60 different phenolic compounds. The 4major phenolic groups are hydroxycinnamic acids (with chloro-genic acid as the major representative), dihydrochalcone deriva-tives (specially phloridzin), flavan-3-ols (catechin as monomers orprocyanidins as oligomers), and flavonols (quercetin and quercetin



glycosides) (Franquin-Trinquier and others 2014). Investigatingbioactive compounds in apple pomace, Grigoras and others (2013)reported that besides the compounds listed above, the majorphenols also cover benzoic acids (gallic acid) and flavonols (rutin).To date, the major class of compounds identified in apple pomaceis flavonoids, where flavonols are the largest subclass, followedby flavanols, flavanones, flavones, dihydrochalcones, and antho-cyanins. The phenolic acids identified are primarily hydroxycin-namic acid derivatives and lesser of hydroxybenzoic acids (Reisand others 2012). Further details of phenolic compounds in ap-ple pomace can be befound in the review by Dhillon and others(2013a, 2013b).

Depending upon their solubility, antioxidants can be classifiedbroadly as hydrophilic and hydrophobic. In general, water-solubleantioxidants (hydrophilic) react with oxidants in the cell, cytosol,and blood plasma, while lipid-soluble (hydrophobic) antioxidantsprotect cell membranes from lipid peroxidation (Sharma and others2015b). Reis and others (2012) reported that water-soluble andorganic solvent (methanol and acetone) soluble polyphenols canbe extracted from apple pomace, suggesting that apple pomace isa good source of both types of antioxidants.

Phenolic compounds from different plant sources, includingapple pomace, are powerful antioxidants, and antioxidant activityis related to the kind and concentration of phenolic compoundsin the food matrix. Several studies have reported high antioxi-dant activities (DPPH) for apple pomace (that is, 2.09 to 3.74 mgTEAC/g dry weight sample powder. However, these values arestrongly influenced by the extraction method and conditions, of-ten not representing the real antioxidant activity of this by-product.Rana and others (2015) found different contents of total phenolics,total flavonoids, antioxidant activity, and phenolic composition at60 °C for 30 min depending on the extraction solvent used (50%methanol, 50% acetone, and 50% ethanol, water solutions). Thehighest antioxidant activity, total phenolic content (TPC), andtotal flavonoids content (TFC) were obtained for acetone, respec-tively 3.74 ± 0.34 mg TEAC/g, 3.31 ± 0.31 mg GAE/g, and0.99 ± 0.02 mg QE/g for the dried apple pomace powder. Phlo-ridzin (1.23 ± 0.08 μg/mg extract) and quercetin (5.72 ± 0.08μg/mg extract) were extracted in higher amounts for 50% acetone,but phloretin was extracted in lesser quantities in comparison with50% ethanol (2.01 ± 0.01 μg/mg extract compared with 3.10 ±0.03 μg/mg extract). Massias and others (2015) recovered pheno-lic compounds from apple peels using subcritical fluid extraction(CO2 + ethanol 96% + water, 75:22:3 molar fraction), macerationwith ethanol, and conventional solvent extraction (acetone/water,70:30 v/v). Maceration resulted in phenolics yield of only 177 ±32 mg/100 g dry peel compared with 792 ± 26 mg/100 g dry peelfor the solvent extraction, while the supercritical fluid extraction(of 30 g peels at 25 MPa and 50 °C) yielded from 550 ± 115mg/100 g to 800 ± 25 mg/100 g depending on solvent/peel massratio (37 to 73 wet basis), mass of ground apple peels (15 or 30 g),and process regime (dynamic or static). The phenolic compositionalso changed for each method. Using water at room temperature toextract phenolic compounds from apple pomace, Reis and others(2012) obtained different TPC, TFC, antioxidant activity (FRAP),and extract composition as compared with methanol 40% and ace-tone 40% extractions. Extracts equivalent to 2566 g of gallic acid,6.696 g of quercetin, and 837 mg of catechin were obtained uponfractionation of 1 kg of dried apple pomace. Although water ex-tracted the highest amount of TPC (1.7 g gallic acid/ kg dry applepomace), TFC (4.9 g quercetin/kg dry apple pomace), and PAC(proanthocyanidins content) (440 mg catechin/ kg dry apple po-

mace), subsequent extractions with aqueous solutions of methanoland acetone extracted further amounts of TPC (around 0.5 g gallicacid/kg dry apple pomace), TFC (0.8 and 1.2 g quercetin/kg dryapple pomace), and PAC (180 and 240 mg catechin/kg dry applepomace). The water extract showed highest antioxidant activityeither by FRAP or DPPH due to the higher content of phenoliccompounds. The IC50 values (DPPH) in mg/mL were 82.0 ±8.0, 94.1 ± 10.0, and 115.4 ± 18.0 for the water, methanol, andacetone extracts, respectively, while the FRAP results were 1.20,0.30, and 0.25 g ascorbic acid/ kg dried extract.

The sample extraction is a time- and solvent-consuming proce-dure. The sample handling can decrease the quality of analyticalresults and can account for at least 1/3 of the analytical error. Inmany widely used extraction steps, solvents like methanol, ethanol,acetone and their mixture with water were used for recovery ofa wide range of polyphenols of various phenolic structures. An-tioxidants such as ascorbic acid or sulfite are added to the solventto protect the analytes from oxidation. The extraction is oftenfollowed by clean-up (liquid-liquid extraction or solid-liquid ex-traction) and pre-concentration.

The conventional recovery method regarding phenolic com-pounds from plant matrices consists of maceration or Soxh-let extraction with solvents, usually using aqueous solutions ofmethanol, ethanol, ethyl acetate, dichloromethane, and acetone(Rabetafika and others 2014; Rana and others 2015). The replace-ment of chemicals with pure water has been successfully investi-gated by Reis and others (2012), who obtained high-antioxidantapple pomace extracts through water extraction at room temper-ature. Despite not being efficient to extract quercetin glycosides,water successfully extracted flavones, flavonols, flavanols, dihy-drochalcones, and hydroxycinnamic acids. The phenolic com-pounds identified by these authors were kaempferol, quercetin,isorhamnetin, quercetin 3-O-arabinoside, quercetin 3-O-glucoside, quercetin 3-O-rhamnoside, quercetin 3-O-galactosideand rutin, epicatechin, procyanidin dimer A2, procyanidin dimerB1 or B2, procyanidin trimer C, procyanidin tetramer D,phloretin, phloridzin, phloretin 2’-O-xylosyl-glucoside, hydrox-ycinnamic acids, chlorogenic acid, and feruloyquinic acid. Al-though the authors suggest subsequent extractions using chemicalssuch as methanol and acetone to maximize extraction of phenolicsfrom apple pomace, these methods still reduce the environmentalproblems associated with the high amounts of solvents (up to 80%concentration) required in the conventional method. Methods toenhance extraction rate and yield such as ultrasonication (Goula2013), supercritical fluid extraction (Massias and others 2015), mi-crowave (Kala and others 2016), hydrodynamic cavitation (Lohaniand others 2016), pulse electric field (Lohani and Muthukumarap-pan 2016), and pressurized liquid-solid extraction (Franquin-Trinquier and others 2014) have also been tested by researchers,be it in plant tissue or apple pomace specifically. In the followingsections, a comparison between extraction methods is presented.

Apples have different concentrations and profile of phenoliccompounds depending on cultivar, maturation stage, growingconditions, storage, and part of the fruit under consideration.Studies show that organic apples have greater amounts of phenoliccompounds and thus higher antioxidant capacities than con-ventionally grown apples (Fratianni and others 2007; Petkovsekand others 2010). In a review of apple phenolic compositionperformed by Kalinowska and others (2014), different parts ofthe fruit have variable contents of specific phenolics, and thedata indicate that in general apple skins are richer in phenoliccompounds than the flesh. Accordingly, the yield and phenolic



profile of apple pomace differ based on the by-product origin,since different industries produce different residues. Le Bourvellecand others (2009) investigated the interaction between applepolyphenols (procyanidin) and the cell wall. After apple wasprocessed (for example, by pressing), procyanidin in cells (mainlyin vacuolar) was released and bound primarily to pectin comparedwith other cell wall compounds. Interaction resulted in lowextractability of pectin but low degree of methylation. Oxidationof the polyphenols resulted in further lower extractability anddegree of methylation. Because many polyphenols combine withthe cell wall, especially with pectin, after the cells are destructed,and the combination reduces pectin extractability, polyphenolsshould be extracted before pectin. Such an extraction sequencewould result in higher pectin yield and purity.

In manufacturing and preparation of antioxidants, extraction isregarded as the bottleneck. It is a time- and solvent-consumingprocedure and sample handling can decrease the quality of ana-lytical results. Solvents like methanol, ethanol, acetone and theirmixtures with water were often necessary within many widely usedextraction steps, and they recover a wide range of polyphenols ofdiverse phenolic structures, rather than specific compounds. Theaddition of antioxidants to the solvent such as ascorbic acid andsulfite is sometimes required to protect analytes from oxidation.Extraction is often followed by clean-up (liquid-liquid extrac-tion or solid–liquid extraction) and pre-concentration. Therefore,some novel extraction technologies such as supercritical fluid ex-traction and microwave-assisted extraction are widely investigatedsince they offer better control and thus shorter extraction timeand/or higher selectivity.

Conventional methodsTypical extraction procedures of phenolic compounds from

apple pomace are mostly carried out using organic solvents, suchas 70% acetone or 80% to 100% methanol (Reis and others2012). Wijngaard and Brunton (2010) used 100% methanol forthe extraction, while Alberti and others (2014) used 50% to 80%acetone. Grigoras and others (2013) carried out the extractionfrom 1 g of dried apple pomace in 20 mL of ethanol understirring for 1 h at room temperature. Peschel and others (2006)employed ethanol/water (50:50) and acetone/water (80:20)for the extraction. Different solvents resulted in various yieldsor compounds extracted. Suarez and others (2010) comparedextractions (at room temperature under stirring) of apple pomaceparticles using 70% acetone and 80% methanol. The acetonesolution yielded more polyphenols than the methanol solution.The extraction solvent resulted in different phenolic composition,but the antioxidant activity was weakly related to the phenolicconcentration. Rana and others (2015) also reported that aqueous50% acetone was more efficient for extraction (30 min at 60 °C) ofphenolic constituents from apple pomace than 50% methanol and50% ethanol. Reis and others (2012) compared the extraction ofphenolic compounds from apple pomace using water, methanol,and acetone. Water extraction was repeated 3 times at roomtemperature before methanol extraction. The residual fromwater extraction was then subjected to 20% ttto 100% methanolextraction. Subsequently, the methanol extraction residual wasextracted with acetone 20% to 100%. The water extract containedhigh amounts of phenolic compounds with high antioxidantcapacity, such as hydroxycinnamic acids, flavonols, flavanols, dihy-drochalcones, and flavone. The extracts from the organic solventextractions also contained a considerable amount of remainingphenolic compounds with significant antioxidant capacity. The

authors concluded that water is an excellent solvent for manyantioxidant components, but not for quercetin glycosides.

Other than the traditional solvents, surfactants could be used inthe extraction of phenols. Sharma and others (2015b) employedsurfactants to extract polyphenols from apple juice. The aque-ous surfactant formulations (SDS, Brij-35, Brij-58, Triton X-100,and Span-40) were compared with water and traditional solventsin terms of TPC and antioxidant capacity. Brij-58 resulted in thehighest TPC and antioxidant capacity, followed by Brij-35, whichwas even more efficient than organic solvents such as ethanol,methanol, and acetone. Adjustment of a solvent’s acidity can en-hance extraction from apple pomace, as the acid can hydrolyze theplant matrix (mainly cellulose and pectin) to some extent, result-ing in increased release of polyphenols from the matrix. Sultanaand Anwar (2008) used acidified methanol containing 1% (v/v)HCl and 0.5 mg/mL Tert-Butyl Hydroxyquinone (TBHQ) toextract aglycons of flavonol from apple. Extraction was performedat temperatures as high as 90 °C under reflux for 2 h, resulting in459.9 mg/kg flavonol content.

Temperature is another key extraction parameter. On one hand,high extraction temperatures enhance diffusion of the compoundsfrom the plant matrix into the surrounding solvent, increasing theyield of the compounds of interest in a certain extraction time.On the other hand, high temperatures can degrade some heatsensitive compounds. In particular, high temperatures associatedwith long extraction times lead to further degradation. As a result,temperature and extraction time should be balanced to achieve anoptimal extraction. A large number of extractions of polyphenolsfrom apple pomace were carried out at about room temperature(20 to 30 °C) (Peschel and others 2006; Suarez and others 2010;Rana and others 2015). However, extraction at such temperaturesusually requires hours to achieve a desirable yield. Stirring isoften employed to increase the extraction rate. Alberti and others(2014) optimized extraction conditions with a temperature rangebetween 10 and 40 °C. They found optimal temperatures of 28°C for methanol (84.5%) and 10 °C for acetone (65%). Wijngaardand Brunton (2010) conducted a similar extraction optimizationwithin a temperature range of 26.36 to 93.64 °C. The optimaltemperature was 80 °C for ethanol (56%) and 25 °C for acetone(65%).

Some researchers prefer high temperature-short time extractionsto promote hydrolyzation of cellulose and pectin, which enhancesrelease of bound polyphenols from cell walls. Rana and others(2015) carried out the extraction at 60 °C for 30 min. The extrac-tion temperatures employed by Sultana and Anwar (2008) wereas high as 90 °C. However, at such high temperature, a refluxingsystem was necessary to avoid excessive loss of solvents, especiallyfor volatile solvents such as acetone. The refluxing system must bepowerful enough to condense as much of the evaporated solventas possible.

The solid/liquid ratio is another important factor in extraction.Sultana and Anwar (2008) mixed 5 g of sample with 25 mL ofsolvent. Peschel and others (2006), Suarez and others (2010), andGrigoras and others (2013) employed a solid/liquid ratio of 1:10(w/v), while Rana and others (2015) carried out the extractionat 1:20 w/v ratio. Wijngaard and Brunton (2010) submitted applepomace to homogenization before extraction, in which 0.25 gsample was mixed with 15 mL solvent. In the subsequent extrac-tion, the mixture was added with a solvent to 25 mL. Alberti andothers (2014) used a much higher ratio of 1:60. Once the ratio isselected, absorption of the solvent by the raw material, especiallyfor dried raw materials, must be considered. High solid/liquid



ratios may result in poor convection of the solvent during extrac-tion, affecting diffusion of phenols into the solvent. When thebulk density of the extraction raw material is low, the solvent maybe isolated and detained by the surrounding raw materials, whichwould cause locally excessive dissolved phenols, reducing the driv-ing force for the diffusion of phenols from the solid matrix intothe solvent. The ratio also affects viscosity development duringextraction of apple pomace. As apple pomace is rich in pectin,especially using pure water as a solvent at medium temperature,not only polyphenols but also pectin can be extracted into thesolvent, increasing the mixture’s viscosity. At high ratio, viscositymay increase rapidly, resulting in high resistance against phenolicdiffusion from the surface of the plant matrix into the solvent.

Pomace particle size also affects extraction. Smaller-sized rawmaterials may enhance extraction due to the high specific contactsurface and small diffusion path of solvent through the solid ma-trix. Furthermore, apple pomace consists of stem, seeds and skin,and these parts are not completely separated. After drying, applepomace has variable size and shape, and large size particles prolongthe extraction process. As a result, apple pomace is usually groundor milled into smaller particles prior to extraction. Wijngaard andBrunton (2010) milled pomace into a fine powder as raw materialfor extraction. Peschel and others (2006) also powdered pomace tobe used as extraction raw material. Suarez and colleagues (2010)ground apple pomace into 0.5 mm-particles before extraction.Pinelo and others (2008) compared the performance of 3 differentsizes of apple pomace (300, 900, and 1500 μm) during extraction.The pomace size in the experiment by Reis and others (2012) wassmall enough to pass through a 250 μm sieve.

Preparation or pretreatment of apple pomace, such as blending,milling, and grinding, affects extraction. Wijngaard and Brunton(2010) pretreated pomace samples by a 2-stage homogenization(10000 rpm for 30 s and subsequent 20000 rpm for 60 s) with ahigh solid/liquid ratio before extraction. According to Rana andothers (2015), the drying method used to obtain dry extracts alsoaffects extraction. The highest content of total phenolics (5.78 ±0.08 mg GAE/g dry weight) was recorded in freeze-dried samples,followed by oven drying and sun drying.

Although solvent extraction offers excellent recovery ofpolyphenols, it has several drawbacks, such as the use of largeamounts of organic solvents, long extraction time, limited solventchoice for health assurance, and possible degradation of targetcompounds. Many alternative methods eliminate or reduce thesedrawbacks. These are microwave-assisted extraction, pressurizedliquid extraction, ultrasonic extraction, and supercritical fluid ex-traction (Adil and others 2007). In the following sections, someinnovative methods studied to extract phenolic compounds fromapple pomace are introduced.

Enzymatic methodsIn apple pomace, many polyphenols are bound with cell walls,

particularly with the pectin in middle lamella, primary, andsecondary cell walls. The release of the polyphenols from cellwall can enhance extraction. Enzymatic hydrolysis of the cellwall, especially pectin, is expected to be a promising methodto accelerate extraction and increase yield. However, a suitableenzyme or enzyme combination should be selected to assist therelease of polyphenols from cell walls, or help reduce resistanceto mass transfer in the plant matrix. Pinelo and others (2008)investigated the kinetics of mass transfer during extraction ofphenols from apple skins under different conditions. The authorstested 3 different enzyme preparations: Pectinex Smash, Celluclast

1.5 L, and Sumizyme AP. The application of cell wall degradationenzyme before extraction improved mass transfer of phenolics.The highest difference from control (without any enzyme) wasdetected when pectinases were applied. Proteases also promoted aslight increase, but cellulases did not have a significant influence.The highest value of Fick’s module for total phenols was foundwhen all the 3 enzyme preparations were added simultaneously.Pectinase could degrade middle lamella between cells to reducethe resistance to mass transfer and to release phenols bound withpectin. Additionally, pectinase reduced the viscosity caused bythe dissolved pectin in the solvent. Proteases were added in theattempt to catalyze the breaking down of structural proteins, stiff-ening the polysaccharide domain in the primary and secondarycell wall. If weakened, this structure may become more porousand allow a higher penetration of solvent and enzymes, easing thesolvent contact and mass transfer of the phenols from the interiorof the plant matrix. Oszmianski and others (2011) maceratedfresh apple pomace with different enzymes (Pectinex Yield Mash,Pectinex Smash XXL, Pectinex XXL, Pectinex Ultra-SPL, andPectinex AFP L-4) to get purees, which were mixed with freshjuice. The mixed juice and purees showed a significantly highercontent of polyphenols than the control sample.

The enzyme dosage is also a key factor in extraction. Dosageshould be based on the sample dry weight. Pinelo and others(2008) revealed that higher dosages result in higher diffusion co-efficients, as expected. However, when enzymes are employed,the optimal working environment for the enzyme should be con-sidered. Many enzymes require certain pH and temperature toachieve maximal enzymatic ability.

The above are factors specifically related to enzymatic reac-tions. Factors affecting conventional extraction (that is, agitation,solid/liquid ratio, and particle size) also play an important role inenzymatic extraction. When it comes to particle size of raw ma-terials, for instance, smaller particles not only facilitate diffusionof phenols from the solid matrix into a solvent, but also provide ahigh specific contact surface and a small diffusion path of enzymesthrough the plant matrix. Based on the requirement of minimumconcentration of extracted components detectable by HPLC andabsorption of the solvent by dried apple skins, a solvent/solid ra-tio of 40:1 was chosen. When the traditional solvent method wasused, 1 g of dried sample was mixed with solvent (pure water,mixture of 1:20 or 1:10 (v/v) ethanol/water) in a heater with150 rpm-stirring. The extraction temperature varied between 25and 50 °C. Extraction samples were taken at 0.5, 1, 1.5, 2, 3, 5, 7,and 10 min. When the enzymatic method was used, liquid enzymepreparations Pectinex Smash, Celluclast 1.5 L and Sumizyme APwere added to the apple skin in proportion between 0% and 0.1%(v/w) enzyme/substrate (freeze-dried apple skins). Other condi-tions were the same for traditional solvent extraction except thatthe solvent was pure water. After the set time, the samples wereimmediately pasteurized at 90 °C for 60 s. It is interesting thatthe viscosity of the mixture of apple mash and water during itsenzymatic maceration (20 min at 50 °C) decreased, as measuredusing a rapid viscosity analyzer. Temperature increase, reduction ofparticle size, and the presence of ethanol in the solvent increasedthe mass transfer rate of phenolics (Pinelo and others 2008).

Ultrasound extractionThe ultrasonic extraction is a straightforward and rapid method

for the extraction and fractionation of plant materials. The ul-trasonication is principally ascribed to mechanical and chemicaleffects of the acoustic wave, which breaks the biological cell and



facilitates the release of cell contents into the extraction medium.Further, cell fragmentation dramatically increases contact areas andaccelerates mass transfer rates of target extracts into the extractionsolvent. At the same time, power ultrasound generates convec-tion of the extraction solvent, as well as micro-jet and micro-stream around the solid materials being extracted, which improvemass transfer from the solid material into the solvent. Ajila andothers (2011) reported that the extracted TPC in water, 80%acetone, and 80% ethanol was 1.24, 5.6, and 4.2 mg GAE/gdry weight sample, respectively, much lower than the values ob-tained using ultrasound and microwave extraction. It is worthmentioning that microwave-assisted extraction is a fast extractionprocess as microwave energy is delivered efficiently to materialsthrough molecular interaction with the electromagnetic field, of-fering a rapid transfer of energy to the extraction solvent and rawplant materials. Wiktor and others (2016) found a TPC content145.3% higher than the control (without ultrasound treatment)and antioxidant activity 64.5% higher. Virot and others (2010)reported the optimized ultrasound extraction increased yield byover 20% in comparison with the conventional extraction. Allthese studies revealed that ultrasound-assisted polyphenols extrac-tion from apple pomace is a suitable, rapid, sustainable alterna-tive to the conventional technique and that a process scale-up isfeasible.

Some studies have been conducted using power ultrasound toassist extraction of polyphenols from apple pomace, and most ofthem have used aqueous solutions of organic solvent (Virot andothers 2010; Grigoras and others 2013; Wiktor and others 2016).Gassara and others (2012) compared extraction yields obtainedusing 2 different solvents—80% acetone or 80% ethanol—andfound that acetone extract exhibited higher TPC (383 to 720 mgGAE/L) than ethanol extract. The same trend was observed in theantioxidant ability. Pingret and others (2012), however, proposedto use water to perform the ultrasound aided-extraction. In all ex-tractions, a 50 mM malate buffer in a pH 3.8 was used to mimetizethe fruit’s conditions. Ajila and others (2011) tested different sol-vents, pure water, 60%, 70%, and 80% ethanol, methanol, and ace-tone in ultrasound extraction, and obtained different extractabilityof polyphenol: acetone 80% > acetone 70% > acetone 60% >

ethanol 80% > ethanol 70% > ethanol 60% > methanol 80% >

methanol 70% > methanol 60% > water. The average total phe-nol content of acetone 80% extract (16.12 mg GAE/g DW) was atleast 3 times higher than that of aqueous extracts (5.78 mg GAE/gDW). However, the free radical scavenging capacity of the extractsdid not follow the same order. These results suggest that extractionby different solvents results in different polyphenolic composition,and the contribution to the scavenging capacity by differentcomponents vary based on the individual phenolic compoundsextracted. In addition to solvents alone, some additives were testedto improve extraction. Ajila and others (2011) tested the effectof a surfactant, Tween-20, on TPC and antioxidant capacity. Thesurfactant enhanced extraction, and the addition of the chemical(1% v/v) in aqueous extract provided the highest antioxidantcapacity.

Some researchers investigated the solid/liquid ratio effect on ul-trasound extraction. Pingret and others (2012) used water as a sol-vent and investigated the evolutions of extractable TPC and waterabsorbed by the material with an increase in the solid/liquid ratio.Even though the extractable TPC increased with the solid/liquidratio, the free water in the mixture was severely reduced, and theultrasound treatment required a minimum amount of free solvent.The optimal ratio was 150 mg/mL. Gassara and others (2012) used

ratios as high as 1:2 for acetone or ethanol solutions. However,it should be noted that their samples were liquid-state, fermentedapple pomace rather than dried apple pomace. Grigoras and others(2013) mixed 3 g dried apple pomace in 60 mL solvent. Virot andothers (2010) reported an optimal solid/liquid ratio of < 15% forethanol/water mixture (50%:50%, v/v).

Extraction temperature also influences ultrasound extraction.Appropriate temperature setting is necessary to avoid destructionof organic compounds as well as provide an efficient application ofultrasound. Ultrasound effects are known to decrease with tem-peratures higher than 40 to 50 °C. Extraction temperature wasoften included in the parameters of much of the research regard-ing ultrasound extraction optimization (Virot and others 2010;Pingret and others 2012). Investigating temperatures between 10and 40 °C, higher temperatures resulted in high TPC in the ex-tract, and the highest temperature was regarded as the optimalextraction temperature. Nonetheless, there are some studies inwhich ultrasound extraction was carried out at room tempera-ture (Grigoras and others 2013). However, temperature controlis a problem during the application of power ultrasound, as themechanical energy is converted into heat during the propagationof sound in the solvent, resulting in the heating of the solvent.Thus, temperature rises more rapidly when the ultrasound powerdensity is higher. In ultrasound extraction, the enhancement ofextraction is attributed not only to the ultrasound mechanical andchemical effects but also to temperature, being the simultaneous ef-fect of ultrasound and temperature important as well. Fluctuatingor inadequate temperature control during ultrasound extractionmay result in significant deviation of extraction amongst differentstudies or equipment, leading to unsatisfactory repeatability, es-pecially in the case of high ultrasound power density. Consistenttemperature control ensures a more precise scale-up. Therefore, anexternal temperature control system becomes necessary, for exam-ple, connection to an external, refrigerated circulator to keep thepomace-solvent mixture at the desired temperature. Heat transferbetween the mixture and medium surrounding the vessel wall of-ten maintains the mixture temperature. Hence, the heat resistanceof the vessel wall has to be considered to ensure effective tem-perature control of the mixture. Few researchers have describedthe temperature control applied in their studies. Virot and others(2010) explained in detail how their extraction system was set up.The ultrasound extraction was carried out in a jacketed ultrasoundcontainer (25 kHz, 150 W), where ultrasound transducers wereinstalled below the container. A stirrer was also employed duringextraction. An external thermostat was connected to the jacket tomaintain the desired solvent temperature during extraction. How-ever, the authors did not report the extract’s temperature history.

The most important operating condition for ultrasound ex-traction is the ultrasound power, even though many researchersdid not mention the ultrasound power set in their studies. Themost popular parameter associated with ultrasound power is thepower intensity. Pingret and others (2012) investigated an ultra-sound intensity range between 0.764 and 0.335 W/cm2. Virot andothers (2010) employed an ultrasound instrument in which theultrasound power was fixed (150 W). They changed the amountof sample and introduced another ultrasound power parameter,power per gram of sample, which varied from 0.058 to 0.142W/g. The yield increased with power level, but the effect of thepower level was not as strong as that of temperature and sonicationtime, most likely due to the low power levels used.

Sonication time is an important factor in the recovery of phenolsfrom apple pomace by ultrasound extraction. Ajila and others



(2011) and Gassara and others (2012, 2013) carried out completeextractions in 30 min. Pingret and others (2012) investigated theeffect of sonication time, ranging between 5 and 45 min. Yieldincreased with longer sonication time. A slight influence of thequadratic effect of sonication time was also noticed, suggesting thatthe effect of time was ever less important throughout extractiontime. The kinetic study under optimal conditions revealed that theextraction followed a 2-stage process: rapid during the first 10 minand slower after that, indicating that the effect of time decreasedafter a certain time. Virot and others (2010) investigated the effectof sonication time (5 to 55 min) on the TPC and drew conclusionssimilar to that of Pingret and colleagues (2012).

In most of the studies, ultrasound was applied during the ex-traction. However, Wiktor and others (2016) used ultrasound topretreat samples before extraction. It is interesting that sampleswere treated with direct and indirect sound wave irradiation. Indirect irradiation, samples were put on a stainless-steel platformattached to a ring sonotrode. The sonotrode’s work was con-trolled by the ultrasonic processor. The ultrasound was directlytransmitted into the sample from the platform. The indirect ir-radiation was performed in a beaker immersed in an ultrasoundbath, as done in many studies. Ultrasound was irradiated to thesample through a water medium in the bath as well as in thebeaker. The ultrasound pre-treatment, regardless of the irradiationmethod, improved TPC in the extract. In comparing the 2 irradi-ation modes, direct irradiation showed a stronger effect on TPC.Another sample pre-treatment, effective for improving extractionof TPC from apple pomace, is fermentation. Gassara and others(2012) fermented apple pomace as liquid slurry by liquid-state cul-ture of Phanerochaete chrysosporium ATCC 24275, achieving higheramounts of acetone and ethanol extractable polyphenols than be-fore the fermentation. The ethanol extract exhibited a 1.5-foldTPC increase and decreased onwards. The authors suggested thatthe increase was caused by the production of carbohydrate me-tabolizing enzymes, such as β-glucosidase, and the decrease wasascribed to the polymerization and lignification of the releasedpolyphenolics by the lignifying and tannin forming peroxidasesand enzymes produced by the P. chrysosporium. However, this pre-treatment required as long as 67 h before the sample could besubjected to extraction. Ajila and others (2011) fermented applepomace before extraction but in solid-state-ferment. Spores of P.chrysosporium were inoculated in sterilized apple pomace, and thenfermentation was conducted at 37±1 °C for 14 d. The fermentedapple pomace showed higher TPC and antioxidant capacity thanthe fresh apple pomace.

Microwave extractionMicrowave-assisted extraction (MAE) is a faster extraction pro-

cess, in which microwave energy is delivered efficiently to materialsthrough molecular interaction with the electromagnetic field. Itoffers a rapid transfer of energy to the extraction solvent and rawplant materials (Ajila and others 2011).

Grigoras and others (2013) applied microwave to enhance ex-traction of phenolics from apple pomace of 4 different varieties,Gala, Golden, Granny Smith, and Pink Lady. One gram of driedapple pomace and 20 mL of solvent (ethanol, ethyl acetate, orwater/methanol mixture) were introduced in a 50 mL closed re-actor and submitted to 3 extraction cycles of 30 s at 1000 W.The reactor was cooled down to room temperature with ice be-tween each extraction cycle. The major phenols identified werebenzoic acids (gallic acid), hydroxycinnamic acids (chlorogenicacid), flavanols (catechin), flavonols (rutin), and chalcones (phlo-

ridzin). Among the terpenes, triterpenic acids as ursolic acid werethe most abundant, but triterpenic acid derivatives with coumarylgroup were also detected. Different varieties exhibited differentphenolic profiles, thus different antioxidant activity. The FRAPantioxidant potential was higher for Pink Lady, which showedhigher contents of phenolics and flavonoids, followed by Golden,Granny Smith, and Gala. Ajila and others (2011) also carriedout microwave-assisted extraction. They sealed a 1 g-sample with20 mL solvent in a Teflon container in a microwave extractor. Theextraction was conducted in the vessel for 10 min at 60 °C witha pressure of 692 kPa and microwave power of 400 W. After the10-min extraction, the container was cooled before removal fromthe extractor. The authors optimized the solvent and found 70%or 80% acetone was the best solvent for the extraction. Regardingthe effect of extraction time, 10 min resulted in the highest TPCunder the microwave operation conditions, irrespective of the sol-vent used (water, ethanol, or acetone). The extraction temperatureinvestigation showed that 60 °C led to the highest yield.

Chandrasekar and others (2015) used microwave-assisted ex-traction to recover phenolic antioxidants from the pomace of 2apple varieties—Red Delicious and Jonathan. The extraction wasoptimized regarding solvent type (70% acetone and 60% ethanol),microwave power (100 to 900 W), solvent/sample volume ratio (4to 12 mL/g dry pomace), and extraction time (30 to 180 s) usingresponse surface methodology. Higher microwave power (265 to635 W) and extraction time (61 to 149 s) increased the total phe-nolic yield in the extract. However, further values of microwavepower and extraction time exceeded the microwave unit’s pressurelimits due to solvent superheating. Solvent volume did not in-fluence microwave extraction when evaluated alone. Nonetheless,the phenolic yield increased using low microwave power combinedwith high solvent volume and high microwave power associatedwith the lower solvent volume (10.3 to 5.6 mL/g) for both applevarieties. The optimal extraction conditions were 735 W for 149s at a solvent volume of 5.65 mL/g DW for acetone and 10.35for ethanol. Both solvents were efficient in extracting phenoliccompounds from apple pomace, but TPC was higher when us-ing ethanol as the extraction solvent. The resulting TPC for RedDelicious was 6.66 mg GAE/g DW for acetone and 15.81 mgGAE/g DW for ethanol. The same trend was observed for theJonathan variety, 5.79 mg GAE/g DW for acetone and 7.65 mgGAE/g DW for ethanol. Additionally, TPC and DPPH antioxi-dant activities were higher for Red Delicious than for the Jonathanvariety. Different phenolic profile and yield were verified for eachsolvent, given their selectivity for certain compounds. Both applepomaces were found to contain phloridzin, quercetrin, quercetin,gallic acid, caffeic acid, and chlorogenic acid as the major pheno-lic compounds. Considering ethanol has a higher polarity, it ex-tracted more phenolic acids than acetone in both pomaces. MAEdecreased extraction time and solvent requirement compared withthe conventional extraction.

Bai and others (2010) also obtained good results usingmicrowave-assisted extraction of polyphenols from apple pomace.The optimal conditions regarding polyphenols yield were:microwave power 650.4 W, extraction time 53.7 s, ethanolconcentration 62.1% and solvent/raw material ratio 22.9:1. Theseconditions produced an extract with 62.68 ± 0.35 mg gallic acidequivalents per 100 g of dry apple pomace. The major polyphe-nols, as identified and quantified by HPLC, were chlorogenic acid,caffeic acid, syrigin, procyanidin B2, (-)-epicatechin, cinnamicacid, coumaric acid, phlorizin, and quercetin. Among these com-pounds, procyanidin B2 had the highest content (219.4 mg/kg).



As one of the main operational factors for microwave extrac-tion, microwave power has not yet been studied in the extractionof phenols from apple pomace. In research applied to other rawmaterials and target compounds, microwave power is regardedas a key factor for extraction. Arasi and others (2016) reportedthat the yield of the target compound, a polysaccharide fromPsidium guajava L. fruits, increased linearly with microwave powerfrom 140 to 200 W, but decreased when power exceeded 200 W,presumably due to denaturation of the polysaccharide. As a result,microwave power should be chosen carefully to achieve optimaleffects and avoid negative impact, especially when it comes toheat sensitive target compounds.

Other methodsPressurized liquid extraction is a promising technology that can

reduce extraction time, increase extraction efficiency, and preservecompounds better by combining high pressure and acceptabletemperatures. Grigoras and others (2013) employed this techniqueto extract polyphenols from apple pomace. Three grams of driedapple pomace were mixed with 3 g of Na2SO4 and extracted withethanol using 3 static cycles for 5 min each, a flush volume of65%, and a purge with nitrogen gas of 100 s at the end of eachextraction cycle. Extractions were carried out at 40 °C and undera pressure of 100 bar.

Franquin-Trinquier and others (2014) investigated phenolics(hydroxycinnamic acids and flavonoids, with the exception of pro-cyanidins) from freeze-dried apple using different solvents (puremethanol, acetone–water mixture (70%:30%, v/v)), extractiontime (1 to 15 min), sample weight (50 to 550 mg, with 7 mLsolvent), and extraction cycles (1 to 3 cycles). When pressurizedliquid extraction was employed, the pre-set conditions were as fol-lows: preheating period—5 min; solvent flush—100% of the ex-traction cell volume; purge—30 s using pressurized nitrogen at 10bars; and collection in glass vials. Each experiment was performedas 2 successive extractions at room temperature with 7 mL of oneof the extraction solvents in each extraction. Accordingly, 3 com-binations of solvent were tested: methanol + methanol, acetone-water + acetone–water, and methanol + acetone–water. Twosupernatants were collected and evaporated to dryness. The 3 sol-vent combinations were compared, revealing that the methanol +methanol was the best regarding TPC (per unit of sample) and thatthe least successful combination was acetone–water + acetone–water. On the other hand, in the extraction under atmosphericpressure, the best solvent regarding TPC was acetone–water. Theefficiency of different solvents is related to the physicochemicalproperties of the phenolic compounds studied (polarity, interac-tions with other compounds in the freeze-dried fruit matrix, andstability). However, the polynomial model developed in this studydid not exhibit satisfactory fitness for TPC experimental data. Al-though it is difficult to set a precise relationship between TPC andthe studied parameters, the predicted TPC increased with extrac-tion time and the number of extraction cycles but decreased withsample mass. Unfortunately, no further information was suppliedon the extraction temperature, a critical factor for extraction.

Another study on the extraction of polyphenols from applepeels and pulp using pressurized liquid extraction technology wasconducted by Alonso-Salces and others (2001). The extractionwas carried out by mixing freeze-dried peel (1 g) or pulp (3 g)with the dispersion agent diatomaceous earth, which was usedto reduce solvent volume, in a proportion of 1:1. The presetextraction conditions were: preheating for 5 min, 60% cell volumeof solvent to flush, 1 extraction cycle, and purge for 90 s using

pressurized nitrogen (150 psi). The effects of solvent (ratio ofmethanol to water, 40:60 to 100:0), temperature (40 and 60 °C),static extraction time (5, 10, 15 min) and pressure (1000, 1250,and 1500 psi) were investigated. The solvent ratio of 75:25 wasreported as the optimal percentage of methanol, better than puremethanol, as hydroxyl groups such as glycoside in polyphenols arehydrophilic, and they exhibit higher solubility in hydro-alcoholicmixture and in pure alcohol solution. On the other hand, the 75%-methanol solution resulted in higher RSD, reducing the method’sprecision. As for temperature, there was a slight increase in yieldfrom 40o to 60 °C, but further higher temperatures reduced theyield. This decrease can be ascribed to possible degradation due tohydrolysis, internal redox reaction, and polymerization. A cloudyextract was observed at �60 °C, and solely pure extract wasobtained only at 40 °C. With respect to the effect of time (40 °C,1000 psi, pure methanol), no difference in yield was found.Regarding the number of extraction cycles, 2 cycles were able torecover more than 80% of the total extractable polyphenols.

Supercritical fluid extraction is an alternative for the food indus-try, especially using CO2 because it has a low critical temperature(Tc = 31.1 °C), it is safe and nontoxic. Moreover, the absenceof light and air during extraction reduces the risk of degradation.However, since CO2 is nonpolar, it is not a good solvent for polarpolyphenols. The addition of organic cosolvents such as ethanol,methanol, and acetone increases the solvating power of CO2 andthe yield of extraction of polyphenols. Ethanol is a permitted co-solvent in the food industry. Consumption of ethanol is in lesseramounts than in conventional solvent extraction, and it is easilyeliminated from the extract by evaporation at room temperature.It must be noted though that when a cosolvent is added to CO2,the critical temperature of the resulting mixture is elevated (Adiland others 2007).

Massias and others (2015) adopted a supercritical fluid extractiontechnology to extract phenols from apple pomace, using CO2 +25% mol cosolvent (ethanol at 96%) at 25 MPa and 50 °C for 3 h.The high proportion of ethanol was justified by the fact that phlo-ridzin and quercetin glycosides, abundant in apple peels, containa sugar moiety that is too polar to be soluble in neat CO2. Asupercritical vessel was filled with alternating sample bed and glassbeads (2 mm diameter) bed, in order to avoid caking. The totalsample bed was set between 15 and 55 g. The vessel was pres-surized with neat CO2 up to 25 MPa, and heated to the desiredtemperature (50 °C). A static period of 20–30 min was set, fol-lowed by extraction by circulating pressurized CO2 + cosolventthrough the beds in the vessel. The overall flow rate of CO2 +cosolvent was 10 g/min. Dissolved compounds from the pomacewith the cosolvent were collected in a cyclonic collector whosebottom was plunged in ice at atmospheric pressure. The com-pounds and cosolvent in the collector’s bottom were completelyremoved from the collector every 20 min as a fraction for fur-ther analysis. After 200 min of extraction, the cosolvent pumpstopped and only net CO2 fluxed within the matrix until no co-solvent was detected in the collector, typically after 1 h. Besidesthe above dynamic procedure, the authors also proposed a stepwiseextraction procedure, in which the extraction vessel was isolatedand the flux was stopped for 30 min (so-called static period) beforeeach collection. The static/dynamic alternation was carried out for4 × 20 min, before stopping the cosolvent flow and flushing thematrix with pure CO2 as in the dynamic procedure. The concen-tration evolution of 9 phenolic compounds and global yield withextraction time was assessed, resulting in a constant extractionrate until 1.1 kg fluid was consumed (20 to 70 times the processed



weight sample), followed by a decreasing extraction rate (however,the matrix was not exhausted even after 3 h of extraction). Thestepwise procedure did not significantly improve the overall yieldand the individual yield of most phenolic compounds. The in-crease of matrix loading (from 15 to 55 g) did not enhance recov-ery, most likely the result of poor load packing and channeling inthe matrix. Accordingly, extractions from 15 g-loadings providedthe highest phenolic yield per gram of sample. The antioxidantcapacity was positively correlated with the total phenolic content,but not all phenolic compounds contributed to the capacity.

Adil and others (2007) used subcritical (carbon dioxide +ethanol) extraction technology to extract polyphenols fromapple pomace as well. The fresh pomace was freeze-dried andthen ground to small particles, sieved in the interval 0.425 to0.850 mm, featuring an average particle size of 0.638 mm.The subcritical extraction was completed in a supercritical fluidextraction system, which consisted of an extractor and 2 syringepumps that enabled cosolvent addition. One gram of dried samplewas added to a 10 mL sample cartridge. The solvent flow (CO2

+ ethanol) was conducted downward with a rate of 2 g/min.The restrictor temperature was 80 °C. The extract was collectedin ethanol and diluted to a constant volume of 3 mL. Effectsof pressure (20 to 60 MPa), temperature (40 to 60 °C), ethanolconcentration (14 to 20 wt %), and extraction time (10 to 40min) were investigated. The working parameters were optimizedusing a 3 level Box-Behnken design. The optimum pressure andtemperature were 54.6 to 57 MPa and 55.7 to 58.4 °C, respec-tively. The optimum ethanol concentration and extraction timewere 20% ethanol and 40 min, respectively. TPC and anti-radicalefficiency of the extract were 0.47 mg GAE/g sample and 3.30mg DPPH/mg sample, respectively. TPC significantly increasedduring the first period, and then the extraction rate slowed down,reaching a plateau after about 40 min, without significant TPCincrease in comparison with the first period. When it came to thesupercritical extraction with a combination of CO2 and ethanol,there was a problem related to saturation, so an addition ofmethanol over 20 wt% created 2 phases. The increase of ethanolconcentration within the studied range increased TPC andantioxidant capacity significantly by increasing the solving abilityof CO2. As ethanol concentration increased, the solubility ofquercetin in the mixture of CO2 + ethanol rose due to improvedphenol-ethanol interaction. As for pressure, TPC and antioxidantcapacity increased with operation pressure until about 50 MPa,above which TPC and antioxidant capacity did not increasesignificantly. A likely cause is the enhanced solving ability of CO2

with an increment of CO2 density at pressures below 50 MPa:density does not increase significantly at pressures above 50 MPa.Temperature exhibited a positive effect on TPC and antioxidantcapacity up to about 50°C. However, other researchers (Murgaand others 2002 and 2003) reported that when extraction pressurewas low, that is, between 10 and 15 MPa, temperature exerted anegative effect, while a positive effect was observed above thesepressures. The cross-over pressure was explained by the fact thatsolubility is controlled by the balance between solvent densityand the change in the solute vapor pressure with temperature.

As discussed in the previous chapter, subcritical water extrac-tion (SWE) replaces chemicals with pressurized water to achieveenhanced extraction rates and higher quality extracts from plantmaterials. SWE has been proved efficient in obtaining improvedphenolic contents and profiles from fruits, vegetables, spices, andflowers, according to the review by Zakaria and Kamal (2016).However, according to the same authors, the extraction of phe-

nolic compounds assisted by SWE requires optimization depend-ing on the plant material and target compounds as the pheno-lic concentration is not uniform throughout plant matrices. Theefficiency of SWE broadly observed for other plant tissues is ev-idence of the potential of this method to extract phenolics fromapple pomace. This is the most environment-friendly between thenovel technologies for the extraction of biocompounds given thecomplete replacement of chemicals with pure water.

Sharma and others (2015b) used surfactants to extract polyphe-nols from apple juice. The aqueous surfactant formulations wereSDS, Brij-35, Brij-58, Triton X-100, and Span-40. Brij-58 re-sulted in the highest TPC and antioxidant capacity, followed byBrij-35, which produced even better results than organic solventssuch as ethanol, methanol, and acetone.

Lohani and Muthukumarappan (2016) applied pulsed electricfield (PEF) to release bound phenolics from apple pomace at differ-ent flour to water ratios (FWR)—5, 8.75 and 12.5% (w/v). Afternatural fermentation, apple pomace was treated with 3 differentelectric field intensities (1, 2 and 3 kV/cm) for 500, 875, and 1250μs. At the optimal conditions of 12.5% (w/v) FWR, 2 kV/cm, and500 μs, total phenolic content (TPC) and antioxidant activity (AA)were 37.4% and 86% higher than the control sample. TPC andAA of the apple pomace were determined as 385.4mg GAE/100g DW and 780.6 μmol TE/100 g DW, respectively, as average atthe 2 kV/cm intensity. Although this work regarded a techniqueto increase antioxidant levels in apple pomace rather than enhanc-ing the extraction of bioactive compounds, it suggested that PEFcan be used as a pre-treatment prior to extraction to enhance therelease of bound phenolic compounds, thus increasing extractionrate and yield. According to Lohani and Muthukumarappan, PEFcauses minimal damage to the other nutrients in foods. The betterphenolic composition and antioxidant activity found in this workare attributed to the electroporation leading to cell disruption andconsequently the release of phenolics from the bound tissue. Indescending order, the factors affecting the release of phenolicswere flour/water ratio, electric field intensity, and treatment time.

Comparison between extraction methodsTable 3 discusses the advantages and drawbacks of the extraction

methods used to recover phenolics from apple pomace.

Other componentsBesides pectin, the other major components of apple pomace are

the cell wall polysaccharides cellulose and xyloglucan. Xyloglucancan be derivatized into a range of compounds that may be usedas thickening agents or texture modifiers as an alternative to thoseprepared from cellulose (for example, methyl-cellulose, hydrox-ypropylcellulose, carboxymethylcellulose). Another potential usefor xyloglucan is as a source of biologically active oligosaccha-rides. Xyloglucan oligosaccharides are characterized by antitumoractivity (Fu and others 2006). Ultrasound technology was em-ployed during extraction of xyloglucan from apple pomace: thesample (0.5 mm) and extractant were indirectly sonicated in anultrasound bath. The working parameters solvent (KOH) con-centration, solid/liquid ratio, and sonication extraction time wereoptimized using central-composite rotary design (CCRD) with 3variables at 5 levels.

Apples are also a good source of aromatic compounds witha wide range of food and cosmetics applications: terpenes andderivatives, norisoprenoids, and coumaran are example of suchcompounds. Kus and others (2013) extracted volatile com-pounds of apple honey using ultrasonic solvent extraction. They



Table 3–Comparison between extraction methods for phenolic compounds.

Extraction method Characteristics

Conventional Mechanism/Operation conditions: Maceration or Soxhlet extraction are carried out at high temperatures using organicsolvents, such as aqueous solutions of acetone, methanol, and ethanol at concentrations of 60% to 100%.

Advantages: Good knowledge of the underlying mechanisms and how operation conditions affect extraction yield, extractiontime, and profile of phenolic compounds; Provides reasonable extraction rates and yields upon the optimization of processconditions.

Drawbacks: Long extraction times as compared to other extraction techniques; Excessive extraction time results in degradationof phenolic compounds; High solvent requirement, leading to equipment corrosion and need of effluent treatment; Toxicity andflammability associated with some solvents; Poor selectivity.

Enzyme-assisted Mechanism/Operation conditions: A pre-treatment using enzymes such as pectinases and cellulases lead to cell walldegradation and thus higher extraction rates of phenolics, which were bound to the plant tissue.

Advantages: Lower extraction time; Produces extracts with higher content of polyphenols than the conventional method; Highselectivity of enzymes leading to extraction of target compounds.

Drawbacks: Cost of enzymes, which is somewhat compensated by the lower costs with chemicals and maintenance of hightemperatures; Difficult to scale-up.

Pressurized fluid Mechanism/Operation conditions Solvents are subjected to high pressures, enhancing penetration within the matrix tissue andthen increasing extraction of phenolic compounds.

Advantages: Higher solubility of water/solvents results in improved extraction rates and thus shorter extraction times;Increased yield of phenolics into the extracts; Lower solvent requirement, and consequently reduced costs with chemicals andeffluent treatment, and smaller ecological impacts.

Drawbacks: Implementations costs, which are in some extent compensated by the lower solvent requirement.

Microwave-assisted Mechanism/Operation conditions: Apple pomace is exposed to microwaves to weaken the plant tissues and consequentlyincrease extraction of phenolics.

Advantages: It is the fastest extraction process due to the molecular interaction with the electromagnetic field, which results infast energy transfer to the apple pomace and the extraction solvent.

Drawbacks: High microwave power or exposition time may cause degradation of phenolic compounds.

Ultrasound-assisted Mechanism/Operation conditions: Acoustic waves are applied to the apple pomace prior to extraction, damaging the plantcells and increasing release of their contents into the extraction medium. In addition, cell fragmentation increases the contactareas, accelerating mass transfer rates.

Advantages: Lower extraction time compared to the conventional method, leading to higher process efficiency and lessernegative environmental effects; Requires lower operation temperatures, preserving the material composition;

Drawbacks: Difficult to scale-up; Long sonication times leads to degradation of phenolic compounds, resulting in decreased yield.

Supercritical fluid Mechanism/Operation conditions: CO2 is used alone or together with organic cosolvents due to its solvating power.Advantages: Lower extraction time compared to the conventional method, leading to higher process efficiency and lesser

detrimental ecological effects; CO2 is safe, nontoxic and has a low critical temperature (around 31 °C); The lower operationtemperatures required help preserving the material composition; The absence of light and air during extraction reducesdegradation reactions, yielding higher quality extracts; Addition of organic cosolvents such as ethanol, methanol and acetoneincreases the solvating power of CO2 and thus the polyphenols yield.

Drawbacks: Difficult to scale-up; Not efficient in the extraction of polar polyphenols since CO2 is nonpolar.

Electric field-assisted Mechanism/Operation conditions: Electric field is applied to plant materials, increasing permeability of plant materials due tocell disruption, resulting in higher yield and extraction efficiency;

Advantages: Eco-friendly due to the reduced requirement of solvents; Increases yield and extraction efficiency.Drawbacks: Do not eliminate the use of solvents, although the requirement of chemicals is lower; Difficult to scale-up.

identified benzaldehyde, benzoic acid, terpendiol I, coumaran, 2-phenylacetic acid, methyl syringate, vomifoliol, methyl 1H-indole-3-acetate, benzyl alcohol, 2-phenylethanol, (E)-cinnamaldehyde,(E)-cinnamyl alcohol, eugenol, vanillin, and linalool.

Simultaneous Extraction of Pectin and Phenols fromApple Pomace

Pectin and polyphenols coexist in apple pomace, and theyboth are valuable compounds with bioactive functions. Water wasproved to be a suitable extraction solvent for polyphenols from ap-ple pomace. Many novel extraction technologies have been stud-ied to achieve environment-friendly extraction, and water as asolvent would be the first option in these technologies. At thesame time, during extraction of phenols from apple pomace, es-pecially using water as a solvent, and if extraction temperatureis not low, pectin from apple pomace will dissolve into the sol-vent along with the phenols. Therefore, dissolving both pectinand polyphenols from apple pomace into the solvent water isunavoidable. On the other hand, it is possible to extract themsimultaneously. However, a separation technology is needed fortheir purification. Schieber and others (2003) combined recov-ered pectin and polyphenols from apple pomace. They used a

food-grade hydrophobic styrene-divinylbenzene copolymerisateto separate polyphenols from pectin in acidic dried pomace.After elution with methanol, the polyphenolics were concentratedunder vacuum and stabilized by lyophilization. The predominantcompounds were phloridzin, chlorogenic acid, and quercetin gly-cosides. The removal of oxidized phenolic compounds resulted inconsiderable decolorization of the pectin extract.

ConclusionsThe modern apple processing industry generates vast volumes of

apple pomace annually. Therefore, it is necessary to process the po-mace to reduce environmental impacts ascribed to disposal. Manystudies were carried out to valorize the pomace to meet a sustain-able development of the apple processing industry. Extraction ofbioactive compounds is an interesting alternative for the valoriza-tion of apple pomace, as it is a good source of pectin and polyphe-nols. As conventional extraction methods for pectin usually involvean acidic and high-temperature environment, and conventionalextraction of polyphenols also requires organic solvents, the tradi-tional methods are not eco-friendly. Therefore, many innovativeapproaches, such as those assisted by enzymes, ultrasound, mi-crowave heating, pulsed electric field, and pressure, as well as their



combinations, have been studied. Some of them, that is ultrasoundand microwave extraction, are efficient enough to substitute theconventional method. However, organic solvents or acidic sol-vents may still be required. Efficient and eco-friendly extractionof bioactive compounds from apple pomace, based on innova-tive technologies deserves further investigation, especially as far asscaling-up is concerned. Furthermore, as pectin and polyphenolscan be extracted simultaneously, efficient combined extraction forboth types of compounds should be considered.

AcknowledgmentsThe authors thank the European Union’s Horizon 2020 Re-

search and Innovation Program for the financial support to theproject “Waste2Fuels - Sustainable production of next generationbiofuels from waste streams,” grant agreement N°654623. Theauthors declare that there are no conflicts of interest regarding thismanuscript.

Author ContributionsZhang and Perussello wrote the review paper, and Marzocchella

and Tiwari worked on planning and proofreading.

References

Adetunji LR, Adekunle A, Orsat V, Raghavan V. 2017. Advances in thepectin production process using novel extraction techniques: a review. FoodHydrocolloids 62: 239–50.

Adil IH, Cetin HI, Yener ME, Bayındırlı A. 2007. Subcritical (carbondioxide + ethanol) extraction of polyphenols from apple and peachpomaces, and determination of the antioxidant activities of the extracts.J Supercrit Fluids 43(1):55–63.

Ajila CM, Brar SK, Verma M, Tyagi RD, Valero JR. 2011. Solid-statefermentation of apple pomace using Phanerocheate chrysosporium—liberationand extraction of phenolic antioxidants. Food Chem 126(3):1071–80.

Alberti A, Zielinski AAF, Zardo DM, Demiate IM, Nogueira A, Mafra LI.2014. Optimisation of the extraction of phenolic compounds from applesusing response surface methodology. Food Chem 149:151–8.

Alonso-Salces RM, Korta E, Barranco A, Berrueta LA, Gallo B, Vicente F.2001. Pressurized liquid extraction for the determination of polyphenols inapple. J Chromatogr A 933(1-2):37–43.

Arasi AGMA, Rao GM, Bagyalakshmi J. 2016. Optimization ofmicrowave-assisted extraction of polysaccharide from Psidium guajava L.fruits. Intl J Biol Macromolec 91:227–32.

Azmir J, Zaidul ISM, Rahman MM, Sharif KM, Mohamed A, Sahena F,Jahurul MHA, Ghafoor K, Norulaini NAN, Omar AKM. 2013. Techniquesfor extraction of bioactive compounds from plant materials: a review. J FoodEng 117(4):426–36.

Bagherian H, Ashtiani FZ, Fouladitajar A, Mohtashamy M. 2011.Comparisons between conventional, microwave- and ultrasound-assistedmethods for extraction of pectin from grapefruit. Chem Eng Proces50:1237–43.

Bai XL, Yue TL, Yuan YH, Zhang HW. 2010. Optimization ofmicrowave-assisted extraction of polyphenols from apple pomace usingresponse surface methodology and HPLC analysis. J Sep Sci33(23-24):3751–8.

Barba FJ, Parniakov O, Pereira SA, Wiktor A, Grimi N, Boussetta N, SaraivaJA, Raso J, Martin-Belloso O, Witrowa-Rajchert D, Lebovka N, VorobievE. 2015. Current applications and new opportunities for the use of pulsedelectric fields in food science and industry. Food Res Intl 77(4):773–98.

Chandrasekar V, San Martın-Gonzalez MF, Hirst P, Ballard TS. 2015.Optimizing microwave-assisted extraction of phenolic antioxidants fromRed Delicious and Jonathan apple pomace. J Food Process Eng38(6):571–82.

Cho YJ, Hwang JK. 2000. Modeling the yield and intrinsic viscosity ofpectin in acidic solubilization of apple pomace. J Food Eng 44(2):85–9.

Decareau RV. 1985. Microwaves in the food processing industry. Orlando,CA: Academic Press.

Dhillon GS, Brar SK, Valero JR, Verma M. 2011. Bioproduction ofhydrolytic enzymes using apple pomace waste by A. niger: applications in

biocontrol formulations and hydrolysis of chitin/chitosan. BioprocessBiosyst Eng 34(8):1017–26.

Dhillon GS, Kaur S, Brar SK, Verma M. 2012. Potential of apple pomace as asolid substrate for fungal cellulase and hemicellulase bioproduction throughsolid-state fermentation. Ind Crops Prod 38:6–13.

Dhillon GS, Kaur S, Brar SK. 2013a. Perspective of apple processing wastes aslow-cost substrates for bioproduction of high value products: a review. RenSust Energy Rev 27:789–805.

Dhillon GS, Kaur S, Sarma SJ, Brar SK. 2013b. Integrated process for fungalcitric acid fermentation using apple processing wastes and sequentialextraction of chitosan from waste stream. Ind Crop Prod 50:346–51.

El-Nawawi SA, Shehata FR. 1987. Extraction of pectin from Egyptianorange peel. Factors affecting the extraction. Biol Waste 20(4):281–90.

Emaga TH, Garna H, Paquot M, Deleu M. 2012. Purification of pectin fromapple pomace juice by using sodium caseinate and characterisation of theirbinding by isothermal titration calorimetry. Food Hydrocolloids29(1):211–8.

Franquin-Trinquier S, Maury C, Baron A, Le Meurlay D, Mehinagic E.2014. Optimization of the extraction of apple monomeric phenolics basedon response surface methodology: comparison of pressurized liquid–solidextraction and manual-liquid extraction. J Food Comp Anal 34(1):56–67.

Fratianni F, Sada A, Cipriano L, Masucci A, Nazzaro F. 2007. Biochemicalcharacteristics, antimicrobial and mutagenic activity in organically andconventionally produced Malus domestica, Annurca. Open Food Sci J 1:10–6.

Fu C, Tian H, Li Q, Cai T, Du W. 2006. Ultrasound-assisted extraction ofxyloglucan from apple pomace. Ultrason Sonochem 13(6):511–6.

Gan C, Latiff Aishah A. 2011. Extraction of antioxidant pectic-polysaccharidefrom mangosteen (Garcinia mangostana) rind: optimization using responsesurface methodology. Carbohydr Polym 83:600–7.

Garna H, Mabon N, Robert C, Cornet C, Nott K, Legros H, Wathelet B,Paquot M. 2007. Effect of extraction conditions on the yield and purity ofapple pomace pectin precipitated but not washed by alcohol. J Food Sci72(1):C001–9.

Gassara F, Ajila CM, Brar SK, Verma M, Tyagi RD, Valero JR. 2012. Liquidstate fermentation of apple pomace sludge for the production of ligninolyticenzymes and liberation of polyphenolic compounds. Process Biochem47(6):999–1004.

Goula AM. 2013. Ultrasound-assisted extraction of pomegranate seedoil—kinetic modeling. J Food Eng 117:492–8.

Grigoras CG, Destandau E, Fougere L, Elfakir C. 2013. Evaluation of applepomace extracts as a source of bioactive compounds. Ind Crop Prod49:794–804.

Gullon B, Yanez R, Alonso JL, Parajo JC. 2008. L-lactic acid productionfrom apple pomace by sequential hydrolysis and fermentation. BioresourTechnol 99(2):308–19.

Guo X, Hana D, Xia H, Rao L, Liao X, Hua X, Wua J. 2012. Extraction ofpectin from navel orange peel assisted by ultra-high pressure, microwave ortraditional heating: a comparison. Carbohydr Polym 88: 441–8.

Hang YD, Woodams EE. 1986. Solid state fermentation of apple pomace forcitric acid production. Mircen J Appl Microb 2:283–7.

Hours RA, Voget CE, Ertola RJ. 1988. Apple pomace as raw material forpectinases production in solid state culture. Biol Waste 23(3):221–8.

Kala HK, Mehta R, Sen KS, Tandey R, Mandal V. 2016. Critical analysis ofresearch trends and issues in microwave assisted extraction of phenolics: havewe really done enough. Trend Anal Chem 85:140–52.

Kalapathy U, Proctor A. 2001. Effect of acid extraction and alcoholprecipitation conditions on the yield and purity of soy hull pectin. FoodChem 73(4):393–6.

Kalinowska M, Bielawska A, Lewandowska-Siwkiewicz H, Priebe W,Lewandowski W. 2014. Apples: content of phenolic compounds vs. variety,part of apple and cultivation model, extraction of phenolic compounds,biological properties. Plant Physiol Biochem 84:169–88.

Kammerer DR, Kammerer J, Valet R, Carle R. 2014. Recovery ofpolyphenols from the by-products of plant food processing and applicationas valuable food ingredients. Food Res Intl 65(Part A):2–12.

Khasina EI, Kolenchenko EA, Sgrebneva MN, Kovalev VV, KhotimchenkoYS. 2003. Antioxidant activities of a low etherified pectin from the SeagrassZostera marina. Russ J Mar Biol 29:259–61.

Kirtchev N, Panchev I, Kratchanov C. 1989. Pectin extraction in thepresence of alcohols. Carbohydr Polym 11(4):257–63.

Knorr D, Angersbach A. 1998. Impact of high-intensity electric field pulseson plant membrane permeabilization. Trends Food Sci Technol9(5):185–91.



Kratchanova M, Panchev I, Pavlova E, Shtereva L. 1994. Extraction of pectinfrom fruit materials pretreated in an electromagnetic field of super-highfrequency. Carbohydr Polym 25(3):141–4.

Kumar A, Chauhan GS. 2010. Extraction and characterization of pectin fromapple pomace and its evaluation as lipase (steapsin) inhibitor. CarbohydrPolym 82(2):454–9.

Kus PM, Jerkovic I, Tuberoso CI, Sarolic M. 2013. The volatile profiles of arare apple (Malus domestica Borkh.) honey: shikimic acid-pathway derivatives,terpenes, and others. Chem Biodivers 10(9):1638–52.

Lavelli V, Corti S. 2011. Phloridzin and other phytochemicals in applepomace: stability evaluation upon dehydration and storage of dried product.Food Chem 129:1578–83.

Loginova K, Shynkaryk MV, Lebovka NI, Vorobiev E. 2010. Acceleration ofsoluble matter extraction from chicory with pulsed electric fields. J FoodEng 96(3):374–9.

Lohani UC, Muthukumarappan K. 2016. Application of the pulsed electricfield to release bound phenolics in sorghum flour and apple pomace. InnovFood Sci Emerg Technol 35:29–35.

Lohani UC, Mutukumarappan, K, Meletharayl GH 2016. Application ofhydrodynamic cavitation to improve antioxidant activity in sorghum flourand apple pomace. Food Bioprod Process 100(PartA):335–43.

Lu Y, Foo LY. 2000. Antioxidant and radical scavenging activities ofpolyphenols from apple pomace. Food Chem 68:81–5.

Luque-Garcıa JL, Luque de Castro MD. 2003. Ultrasound: a powerful toolfor leaching. Trends Analyt Chem 22(1):41–7.

Le Bourvellec C, Guyot S, Renard CMGC. 2009. Interactions betweenapple (Malus x domestica Borkh.) polyphenols and cell walls modulate theextractability of polysaccharides. Carbohydr Polym 75(2):251–61.

Madrera RR, Bedrinana RP, Vallesm BS. 2015. Production andcharacterization of aroma compounds from apple pomace by solid-statefermentation with selected yeasts. LWT – Food Sci Technol 64(2):1342–53.

Makarova E, Gornas P, Konrade I, Tirzite D, Cirule H, Gulbe A, Pugajeva I,Seglina D, Dambrova M. 2015. Acute anti-hyperglycaemic effects of anunripe apple preparation containing phlorizin in healthy volunteers: apreliminary study. J Sci Food Agric 95(3):560–8.

Massias A, Boisard S, Baccaunaud M, Leal Calderon F, Subra-Paternault P.2015. Recovery of phenolics from apple peels using CO2 + ethanolextraction: kinetics and antioxidant activity of extracts. J Supercrit Fluids98:172–82.

Masumoto S, Akimoto Y, Oike H, Kobori M. 2009. Dietary phloridzinreduces blood glucose levels and reverses Sglt1 expression in the smallintestine in streptozotocin-induced diabetic mice. J Agric Food Chem57(11):4651–6.

McCann MJ, Gill CIR, O’ Brien G, Rao JR, McRoberts WC, Hughes P,McEntee R, Rowland IR. 2007. Anti-cancer properties of phenolics fromapple waste on colon carcinogenesis in vitro. Food Chem Toxicol45(7):1224–30.

Min B, Lim J, Ko S, Lee KG, Lee SH, Lee S. 2011. Environmentally friendlypreparation of pectins from agricultural byproducts and theirstructural/rheological characterization. Bioresour Technol 102(4):3855–60.

Mohnen D. 2008. Pectin structure and biosynthesis. Curr Opin Plant Biol11(3):266–77.

Munarin F, Guerreiro SG, Grellier MA, Tanzi MC, Marbosa MA, Petrini P,Granja PL. 2011. Pectin-based injectable biomaterials for bone tissueengineering. Biomacromolecules 12(3):568–77.

Murga R, Sanz MT, Beltran S, Cabezas JL. 2002. Solubility of somephenolic compounds contained in grape seeds, in supercritical carbondioxide. J Supercrit Fluids 23(2):113–21.

Murga R, Sanz MT, Beltran S, Cabezas JL. 2003. Solubility of threehydroxycinnamic acids in supercritical carbon dioxide. J Supercrit Fluids27(3):239–45.

Oliveira CF, Giordani D, Gurak PD, Cladera-Olivera F, Marczak LDF.2015. Extraction of pectin from passion fruit peel using moderate electricfield and conventional heating extraction methods. Innov Food Sci EmergTechnol 29:201–8.

O’Shea N, Ktenioudaki A, Smyth TP, McLoughlin P, Doran L, Auty MAE,Arendt E, Gallagher E. 2015. Physicochemical assessment of two fruitby-products as functional ingredients: apple and orange pomace. J Food Eng153:89–95.

Oszmianski J, Wojdyło A, Kolniak J. 2011. Effect of pectinase treatment onextraction of antioxidant phenols from pomace, for the production ofpuree-enriched cloudy apple juices. Food Chem 127(2):623–31.

Panchev IN, Kirtchev NA, Kratchanov C. 1989. Kinetic model of pectinextraction. Carbohydr Polym 11(3):193–204.

Pandey KB, Rizvszi SI. 2009. Plant polyphenols as dietary antioxidants inhuman health and disease. Oxid Med Cel Longev 2(5):270–8.

Peng XY, Mu TH, Zhang M, Sun HN, Chen JW, Yu M. 2015.Optimisation of production yield by ultrasound-/microwave-assisted acidmethod and functional property of pectin from sugar beet pulp. Intl J FoodSci Technol 50(3):758–65.

Peschel W, Sanchez-Rabaneda F, Diekmann W, Plescher A, Gartzıa I,Jimenez D, Lamuela-Raventos R, Buxaderas S, Codina C. 2006. Anindustrial approach in the search of natural antioxidants from vegetable andfruit wastes. Food Chem 97(1):137–50.

Petkovsek MM, Slatnar A, Stampar F, Veberic R. 2010. The influence oforganic/integrated production on the content of phenolic compounds inapple leaves and fruits in four different varieties over a 2-year period. J SciFood Agric 90(14):2366–78.

Pinelo M, Zornoza B, Meyer AS. 2008. Selective release of phenols fromapple skin: mass transfer kinetics during solvent and enzyme-assistedextraction. Sep Purif Technol 63(3):620–627.

Pingret D, Fabiano-Tixier AS, Bourvellec CL, Renard CMGC, Chemat F.2012. Lab and pilot-scale ultrasound-assisted water extraction ofpolyphenols from apple pomace. J Food Eng 111(1):73–81.

Rabetafika HN, Bchir B, Blecker C, Richel A. 2014. Fractionation of appleby-products as source of new ingredients: Current situation andperspectives. Trends Food Sci Tech 40(1):99–114.

Raganati F, Procentese A, Olivieri G, Russo ME, Marzocchella A. 2015.Butanol production by fermentation of fruit residues. Chem Eng Trans 49:229–34.

Rana S, Gupta S, Rana A, Bhushan S. 2015. Functional properties, phenolicconstituents and antioxidant potential of industrial apple pomacefor utilization as active food ingredient. Food Sci Hum Wellness4:180–7.

Reis SF, Rai DK, Abu-Ghannam N. 2012. Water at room temperature as asolvent for the extraction of apple pomace phenolic compounds. FoodChem 135(3):1991–8.

Renard CMGC, Schols HA, Voragen AGJ, Thibault JF, Pilnik W. 1991a.Studies on apple protopectin. III: characterization of the material extractedby pure polysaccharidases from apple cell walls. Carbohydr Polym15(1):13–32.

Renard CMGC, Voragen AGJ, Thibault JF, Pilnik W. 1991b. Cell walls:structure - biodegradation - utilisation - comparison between enzymaticallyand chemically extracted pectins from apple cell walls. Anim Feed SciTechnol 32(1):69–75.

Richardson JF, Harker JH, Backhurst JR. 2002. Coulson and Richardson’schemical engineering, vol. 2 – Particle technology and separation processes.5th ed. Woburn, MA: Elsevier.

Rust S, Buskirk D. 2008. Feeding apples or apple pomace in cattle diets.Cattle Call 13(4):2–3.

Schieber A, Hilt P, Streker P, Endreß HU, Rentschler C, Carle R. 2003. Anew process for the combined recovery of pectin and phenolic compoundsfrom apple pomace. Innov Food Sci Emerg Technol 4(1):99–107.

Sharma R, Kamboj S, Khurana R, Singh G, Rana V. 2015a.Physicochemical and functional performance of pectin extracted by QbDapproach from Tamarindus indica L. pulp. Carbohydr Polym 134:364–74.

Sharma S, Kori S, Parmar A. 2015b. Surfactant mediated extraction of totalphenolic contents (TPC) and antioxidants from fruits juices. Food Chem185:284–8.

Sriamornsak P. 2003. Chemistry of pectin and its pharmaceutical uses: areview. Silpakorn Univ J Soc Sci, Hum Arts 3:206–28.

Stredansky M, Conti E. 1999. Xanthan production by solid statefermentation. Process Biochem 34:581–7.

Suarez B, Alvarez AL, Garcıa YD, Barrio GD, Lobo AP, Parra F. 2010.Phenolic profiles, antioxidant activity and in vitro antiviral properties ofapple pomace. Food Chem 120(1):339–42.

Sultana B, Anwar F. 2008. Flavonols (kaempferol, quercetin, myricetin)contents of selected fruits, vegetables and medicinal plants. Food Chem108(3):879–84.

Thibault JF, De Dreu R, Geraeds CCJM, Rombouts FM. 1988. Studies onextraction of pectins from citrus peels, apple marks and sugar-beet pulpswith arabinanase and galactanase. Carbohydr Polym 9(2):119–31.



Vendruscolo F, Ninow JL. 2014. Apple pomace as a substrate for fungalchitosan production in an airlift bioreactor. Biocatal Agric Biotechnol3(4):338–42.

Virot M, Tomao V, Le Bourvellec C, Renard CMCG, Chemat F. 2010.Towards the industrial production of antioxidants from food processingby-products with ultrasound-assisted extraction. Ultrason Sonochem17(6):1066–74.

Vorobiev E, Lebovka N. 2016. Extraction from foods and biomaterialsenhanced by pulsed electric energy. In: Knoerzer K, Juliano P, SmithersGW, editors. Innovative food processing technologies: extraction,separation, component modification and process intensification. Sawston,UK: Woodhead Publishing, p 31–56.

Walter RH, Rao MA, Sherman RM, Cooley HJ. 1985. Edible fibers fromapple pomace. J Food Sci 50(3):747–9.

Wang X, Chen Q, Lu X. 2014. Pectin extracted from apple pomace andcitrus peel by subcritical water. Food Hydrocolloids 38:129–37.

Wang S, Chen F, Wu J, Wang Z, Liao X, Hu X. 2007. Optimization ofpectin extraction assisted by microwave from apple pomace using responsesurface methodology. J Food Eng 78(2):693–700.

Wang M, Huang B, Fan C, Zhao K, Hu H, Xu X, Pan S, Liu F. 2016.Characterization and functional properties of mango peel pectin extractedby ultrasound assisted citric acid. Intl J Biol Macromolec 91:794–803.

Wang X, Lu X. 2014. Characterization of pectic polysaccharides extractedfrom apple pomace by hot-compressed water. Carbohydr Polym102:174–84.

Wang L, Weller CL. 2006. Recent advances in extraction of nutraceuticalsfrom plants. Trends Food Sci Tech 17:300–12.

Wijngaard HH, Brunton N. 2010. The optimisation of solid–liquidextraction of antioxidants from apple pomace by response surfacemethodology. J Food Eng 96(1):134–40.

Wikiera A, Mika M, Grabacka M. 2015a. Multicatalytic enzyme preparationsas effective alternative to acid in pectin extraction. Food Hydrocolloids44:156–61.

Wikiera A, Mika M, Starzynska-Janiszewska A, Stodolak B. 2015b.Application of Celluclast 1.5L in apple pectin extraction. Carbohydr Polym134:251–7.

Wikiera A, Mika M, Starzynska-Janiszewska A, Stodolak B. 2016.Endo-xylanase and endo-cellulase-assisted extraction of pectin from applepomace. Carbohydr Polym 142:199–205.

Wiktor A, Sledz M, Nowacka M, Rybak K, Witrowa-Rajchert D. 2016.The influence of immersion and contact ultrasound treatment on selectedproperties of the apple tissue. Appl Acoust 103(Part B):136–42.

Willats WGT, Knox JP, Mikkelsen JD. 2006. Pectin: new insights into anold polymer are starting to gel. Trends Food Sci Technol 17(3):97–104.

Yang N, Jin Y, Tian Y, Jin Z, Xu X. 2016. An experimental system forextraction of pectin from orange peel waste based on the o-core transformerstructure. Biosyst Eng 148:48–54.

Yeoh S, Shi J, Langrish TAG. 2008. Comparisons between differenttechniques for water-based extraction of pectin from orange peels.Desalination 218:229–37.

Zakaria SM, Kamal SMM. 2016. Subcritical water extraction of bioactivecompounds from plants and algae: applications in pharmaceutical and foodingredients. Food Eng Rev 8(1):23–4.

Zhong-Tao S, Lin-Mao T, Cheng L, Jin-Hua D. 2009. Bioconversion ofapple pomace into a multienzyme bio-feed by two mixed strains ofAspergillus niger in solid state fermentation. Electr J Biotechnol 12(1):1–13.


valorization of apple pomace by extraction of valuable ... comparison is based on an extensive...

Documents