ORIGINAL PAPER

Characterization of Industrial Onion Wastes(Allium cepa L.): Dietary Fibre and Bioactive Compounds

Vanesa Bentez & Esperanza Moll & Mara A. Martn-Cabrejas & Yolanda Aguilera &Francisco J. Lpez-Andru & Katherine Cools & Leon A. Terry & Rosa M. Esteban

Published online: 12 February 2011# Springer Science+Business Media, LLC 2011

Abstract The food industry produces a large amount ofonion wastes, making it necessary to search for possible waysfor their utilization. One way could be to use these onionwastes as a natural source of high-value functional ingre-dients, since onion are rich in several groups of compounds,which have perceived benefits to human health. The objectiveof this work is to gain knowledge of any differences betweenthe different onion wastes obtained from industry and non-commercial bulbs to use them as food ingredients rich inspecific compounds. The results showed that brown skin andtopbottom could be potentially used as functional ingredientrich in dietary fibre, mainly in insoluble fraction, and in totalphenolics and flavonoids, with high antioxidant activity.Moreover, brown skin showed a high concentration ofquercetin aglycone and calcium, and topbottom showedhigh concentration of minerals. Outer scales could be used assource of flavonols, with good antioxidant activity and contentof dietary fibre. However, inner scales could be an interestingsource of fructans and alk(en)yl cystein sulphoxides. Inaddition, discarded onions (cvs Recas and Figueres) couldbe used as a good source of dietary fibre, and cv Recas also asa source of phenolics compounds.

Keywords Alk(en)yl cystein sulphoxides . Antioxidantactivity . Dietary fibre . Flavonols . Fructans . Onion wastes

AbbreviationsACSOs alk(en)yl cystein sulphoxidesDF Dietary fibreDM Dry matterDP Degree of polymerizationFOS FructooligosaccharidesFRAP Ferric reducing ability assayGAE Galic acid equivalentsICP-MS Inductively coupled plasma mass spectrometryIDF Insoluble dietary fibreMCSO (+)-S-methyl-L-cysteine sulphoxideNSC non-structural carbohydratesPCSO (+)-S-propyl-L-cysteine sulphoxidePECSO trans-(+)-S-1-propenyl-L-cysteine sulphoxideQE Quercetin equivalentsSDF Soluble dietary fibreTDF Total dietary fibreTFA Trifluoroacetic acid

Introduction

Onions (Allium cepa L.) are the second most importanthorticultural crop worldwide, after tomatoes, with currentannual production around 66 million tonnes. Over the past10 years, onion production has increased by more than 25%[1]. Lately, there has been an increase in demand forprocessed onions which has led to an increase in wasteproduction. Accordingly more than 500,000 tonnes ofonion waste are produced annually in the European Union,mainly from Spain, UK and Holland [2]. The main onionwastes include onion skins, two outer fleshy scales androots generated during industrial peeling, and undersized,malformed, diseased or damaged bulbs. These wastes

V. Bentez (*) : E. Moll :M. A. Martn-Cabrejas :Y. Aguilera :F. J. Lpez-Andru : R. M. EstebanDepartamento de Qumica Agrcola, Facultad de Ciencias/Instituto de Ciencias de la Alimentacin (CIAL),Campus de la Universidad Autnoma de Madrid,28049 Madrid, Spaine-mail: vanesa.benitez@uam.es

K. Cools : L. A. TerryPlant Science Laboratory, Cranfield University,Bedfordshire MK43 0AL, UK

Plant Foods Hum Nutr (2011) 66:4857DOI 10.1007/s11130-011-0212-x

represent an environmental problem, since onion wastes arenot suitable for fodder in high concentrations, due to theonions characteristic aroma, and neither as an organicfertilizer because of the rapid development of phytopatho-genic agents [2]. Therefore, a possible solution could be touse onion wastes as a source of food ingredients, sinceonion are rich in several groups of plant compounds, suchas dietary fibre (DF), fructooligosaccharides (FOS), flavo-noids and alk(en)yl cystein sulphoxides (ACSOs), that haveperceived benefits to human health [3]. Therefore, it isnecessary to study the composition of each industrial onionwaste to know its potential use as food ingredient.

Onion composition is variable and depends on cultivar,stage of maturation, environment, agronomic conditions,storage time and bulb section [4, 5]. Water makes up themajority (8095%) of the fresh weight of onion. Up to 65%or more of the dry weight may be in the form of non-structural carbohydrates (NSC) which include glucose,fructose, sucrose and FOS [6]. The main FOS in onionbulbs are kestose, nystose and fructofuranosylnystose [7].Health benefits of these carbohydrates have been widelyreported in the past and their prebiotic effect demonstrated[8]. Onion also showed an important quantity of totaldietary fibre (TDF) and a good soluble:insoluble dietaryfibre ratio (SDF:IDF) that will be connected with differentmetabolic and physiological effects [9].

Moreover, onion is known for its flavonoid content,contributing considerably to its dietary intake inmany countries[10]. Two flavonoid subgroups are present in onion;anthocyanins, which impart a red/purple colour to somevarieties and flavonols such as quercetin and its derivativeswhich may play a role in the production of yellow and browncompounds of many other varieties [11]. In recent literature,quercetin 4-glucoside and quercetin 3,4-diglucoside are inmost cases reported as the main onion flavonols of the flesh[5, 12], whereas onion skins contain higher concentrations ofquercetin aglycon [11]. Many flavonoids are shown to haveantioxidative activity, free radical scavenging capacity, coro-nary heart disease prevention, and anti-cancer activity, whilesome flavonoids exhibit potential for anti-human immunode-ficiency virus functions [13].

The ACSOs are the flavour and aroma precursors,which, when cleaved by the enzyme alliinase, generatethe characteristic odour and taste of onion. Four ACSOshave been identified in Alliums, and the flavour variationamong species is due to differences in ACSO compositionand concentration [14]. The three naturally occurringACSOs in onion are trans-(+)-S-1-propenyl-L-cysteinesulphoxide (PECSO), which is normally found in thehighest concentration and gives rise to the compoundresponsible for the lachrymatory effect, and (+)-S-methyl-L-cysteine sulphoxide (MCSO) and (+)-S-propyl-L-cysteinesulphoxide (PCSO), which are found in smaller amounts

[15]. Allium organosulphur compounds inhibit the aggrega-tion of human blood platelets and offer the potential forpositive cardiovascular health benefits. Furthermore, thesecompounds have the ability to positively modify theantioxidant, apoptotic and inflammatory systems in mammal[16, 17].

In the present study, it has been described the content ofminerals, DF, NSC, ACSOs and flavonoids together withantioxidant activity (FRAP) in different onion wastes emanat-ing from industry and also in discarded whole onions of twoSpanish cultivars. The objective of this work is to gainknowledge of any differences among the different onionwastes obtained from industry and non-commercial bulbs touse them as food ingredients rich in specific compounds. Suchinformation may be useful to food technologists for theappropriate exploitation of each industrial onion waste assource of a specific functional compound.

Material and Methods

Material

Two cultivars, cvs Recas and Figueres, that are common incommercial production in Spain, were used in this study.Onions were supplied by Spanish onion producing industry(CEBACAT, Catalonia, Spain). Cv. Recas is a Valencianlate cycle and long-day cultivar, which is yellow, firm, withhigh density, and good storage capacity. However, cvFigueres is a Catalonian advanced cycle and long-daycultivar, with light purple colour and acceptable aptitude forstorage. The samples analyzed were not marketable onions,due to sprouting, damage to the outer scales, lost peel orbelow commercially acceptable size (

vacuum oven [9, 18]. The ash content was estimatedaccording to AOAC [18]. Total nitrogen content (N) wasdetermined in freeze-dried samples using the standardKjeldahl procedure, by nitrogen colorimetric determinationafter mineralization. Crude protein was expressed as6.25N. The concentration of potassium, calcium, magne-sium, iron, zinc, manganese and selenium was determinedusing ICP-MS (Elan 6000 PE Sciex).

Dietary Fibre Determination Mes-Tris AOAC method991.43 was used for DF determination in freeze-driedsamples [18]. DF fractions were obtained as indigestibleresidues after enzymatic digestion of non DF components.TDF was determined as the sum of SDF and IDF.

Non-structural Carbohydrates Extraction and DeterminationNSC were extracted and determined according to Jaime etal. [7] in freeze-dried samples, using Beckman CoulterLC125 HPLC system (Beckman Coulter, Brea, CA, USA)coupled to Beckman 156 refractive index detector toidentify and quantify fructose, glucose, sucrose and FOS.The separation occurred on Aminex HPX-42C column(cationic ion exchanger, 7.8300 mm, Bio-Rad, Hercules,CA, USA). Appropriate dilutions of a solution containingglucose, fructose and sucrose (Sigma Aldrich, St. Louis,MO) and fructofuranosylnystose (1F-nystose), nystose andkestose (Wako Pure Chemical Industries, Ltd., Osaka,Japan) were used as calibration standards.

Total Fructans Total fructan concentration in freeze-driedsamples was measured using a fructan assay kit (Mega-zyme, Co., Wicklow, Republic of Ireland) using a colori-metric method according to the manufacturers instructions(AOAC method 999.03, AACC method 32.32.) [19].

Sulphur Content and S-alk(en)yl-L-cysteine Sulphoxides Totalsulphur content was determined using an elementalanalyzer LECO CHNS-932 (LECO, S.L., St. Joseph,Michigan,USA). The microanalysis was based on Pregland Dumas classic method, in which sample suffered atotal oxidation through an instantaneous and completecombustion which converts the sample into its combus-tion products (CO2, H2O, N2 and SO2). ACSOs weredetermined according to Mallor and Thomas [15] withslight modifications. For ACSO extraction, 10 mg offreeze-dried sample was added to 1 ml of 12:5:3 (v/v/v)methanol: chloroform:water and incubated overnight at20 C. A 700 l sample of the extract was transferred toa 1.5 ml Eppendorf tube, to which 385 l of water and315 l of chloroform were added. After mixing, thephases were separated by centrifugation at 13,000 g for30 s at room temperature, and 790 l of the upper phasewas collected into an Eppendorf tube and then freeze-

dried. This extract was resuspended in 600 l of 0.03 MHCl and filtered through a 0.2 m filter.

HPLC analysis was carried out using an Agilent 1200series HPLC system (Agilent, Berks., UK) coupled toAgilent 1200 DA G1315B/G1365B photodiode arraydetector. The injection volume was 15 l and the separationoccurred on a ZORBAX Eclipse XDB- C18 column(4.6 mm250 mm, 5 m) with an Agilent ZORBAXEclipse XDB guard column, 1.0 mm17 mm (Part no.51855921) at 25 C. The mobile phase was 0.03 M HCldegassed by sonication and run at 0.6 ml min1. The datawere presented in Agilent ChemStation Rev. B.02.01software. MCSO and PCSO were calibrated againstauthentic standards and PECSO against allyl-cysteine-sulphoxide.

Phenolic Compounds Extraction Phenolic compounds wereextracted as previously described Downes et al. [11] withslight modifications. Freeze-dried samples were weighed(1500.5 mg) and dissolved in 3 ml of 70:29.5:0.5 (v/v/v)methanol (analytical grade):water (Milli Q):HCl (analyticalgrade). After mixing well, vials were placed in a shakingwater bath at 35 C for 90 min; samples were vortexed every15 min during the extraction to mix. When the samples werecooled, they were filtered using a 0.2 m filter. Extracts werestored in a freezer at 20 C until further analysis. Theseextracts were used to determine total phenolics, totalflavonoids, total antioxidant capacity and flavonols byHPLC.

Total Phenolics, Total Flavonoids and Total AntioxidantCapacity Absorbance Assays UV spectrophotometric tech-niques were used to determine total phenolics, totalflavonoids and total antioxidant capacity (FRAP). Totalphenolics and FRAP were measured according to Terry etal. [20] and total flavonoids were determined according toDownes et al. [5].

Flavonol Determination by HPLC Flavonols were deter-mined according to Downes et al. [5] with slightmodifications. Extracts were analyzed using an Agilent1200 series HPLC system (Agilent, Berks., UK). Fla-vonols were separated on a ZORBAX Eclipse XDB-C18column, 4.6 mm150 mm, 5 m particle size (Part no.993967-902), with an Agilent ZORBAX Eclipse XDBguard column, 1.0 mm17 mm (Part no. 51855921).The mobile phase consisted of HPLC grade water with0.5 g l1 trifluoroacetic acid (TFA) (A) and acetonitrilewith 0.5 g l1 TFA (B). The gradient involved a linearincrease/decrease in the amount of solvent B in A (% B):06 min, 525%; 614 min, 2585%; 1415 min,855%. The flow rate was 0.8 ml min1. Samples(10 l) were injected and the separation took place at

50 Plant Foods Hum Nutr (2011) 66:4857

30 C. The eluted flavonols were detected with anAgilent 1200 DA G1315B/G1365B photodiode array ata wavelength of 370 nm. The data was presented inAgilent ChemStation Rev. B.02.01 software and querce-tin and quercetin glucoside concentrations were calculat-ed against authentic calibration standards (quercetin 3-glucoside, quercetin 4-glucoside, quercetin 3,4-digluco-side and quercetin; PlantChem, Sandnes, Norway), whilefor isorhamnetin glucosides, the equivalent quercetinglucoside standards were used.

Statistical Analysis

The statistical analysis was performed by SPSS (version15.0). The data were analysed by one-way analysis ofvariance (ANOVA) using Duncan test. Differences wereconsidered to be significant at P0.05. Correlations werecarried out using StatGraphics v. 3.1.

Results and Discussion

Chemical Parameters

Table 1 summarizes the chemical parameters and mineralcomposition of cvs Recas and Figueres and their industrialwastes. DM content is an important quality parameter forthe onion industry and it is related to other qualityattributes, such as pungency, storage life, fructans andfirmness [21]. No cultivar differences regarding DMcontent were observed between studied cultivars. In agree-ment with other authors [9, 21], both cultivars could belabelled as fresh market with regard to their DM content.Industrial wastes reflected significant differences in theirDM contents; thus, an increase was observed from innerbulb sections towards outer bulb sections. However, skin ofcvs Recas and Figueres showed less DM than the skin ofother cultivars as red and brown onion cvs which has beenfound to be as high as 80% [11]. Recas cultivar showedmore content of crude protein than Figueres, although bothcultivars showed the same distribution of nitrogen com-pounds within industrial wastes, thus the high content wasfound in topbottom, probably because the growing apex ofthe onion bulb is located in the bottom. Moreover, differ-ences between inner and outer scales were found, suggest-ing that crude protein increased towards young scales (innerscales) and growing apex; this is in agreement with Jaime etal. [9] who indicated that nitrogen compounds in bulbripening were transported from senescenced leaves to thebulb. No significant differences were found in total ashbetween Recas and Figueres whole onion and it was founda decrease of total ash from the outer to the inner part of the Ta

ble1

Chemical

parametersandmineral

compositio

nof

whole

onions

andtheirindustrial

onionwastes

Whole

onion

Innerscales

Outer

scales

Top

bottom

Brownskin

Recas

Figueres

Recas

Figueres

Recas

Figueres

Recas

Figueres

Recas

Figueres

DM

(%)

8.70.7 a

c8.80.7 a

c6.90.2 a

c8.30.3 b

c6.30.3 a

c7.50.1 b

c13.23.5 a

c18.02.3 b

c51.91.8 a

c50.83.6 a

c

Crude

proteinA

11.80.7 b

c10.10.3 a

c15.30.3 b

d11.60.1 a

d9.30.7 a

b8.30.4 a

b15.60.2 a

d15.00.5 a

e2.30.1 a

a2.40.0 a

a

TotalashA

4.70.1 a

a4.90.1 a

c4.90.7 a

b4.40.3 a

a5.60.1 b

c4.60.0 a

b8.60.1 a

d8.20.3 a

d10.60.6 b

e9.30.1 a

e

Minerals

KB

14.10.5 a

c15.80.7 b

c15.90.2 a

d15.70.4 a

c13.90.5 b

c12.80.3 a

b11.10.1 a

b13.00.1 b

b4.20.1 a

a7.30.1 b

a

CaB

6.60.3 a

b6.60.2 a

c3.50.0 b

a1.80.0 a

a7.80.2 b

c4.20.1 a

b16.50.1 b

d10.20.2 a

d30.70.7 b

e22.10.2 a

e

MgB

1.00.1 a

b1.30.1 b

b0.70.0 a

a0.60.0 a

a0.80.0 a

a0.70.0 a

a1.40.0 a

c1.50.0 a

c1.10.0 a

b1.50.0 b

c

FeC

53.21.3 a

c100.30.2 b

b43.61.6 b

b19.60.2 a

a19.60.4 a

a19.60.4 a

a426.79.5 a

e888.98.0 b

d119.81.1 a

d419.47.2 b

c

ZnC

19.60.3 a

b23.90.1 b

d28.40.6 b

d16.20.2 a

a26.80.2 b

c18.60.1 a

b50.40.5 a

e53.80.7 b

e14.90.1 a

a21.60.4 b

c

MnC

10.20.1 a

b13.70.1 b

b11.50.2 b

c6.50.0 a

a8.10.0 b

a6.60.0 a

a20.20.4 a

d28.81.2 b

d8.30.1 a

a25.30.7 b

c

SeC

0.34

0.04

bb

0.15

0.04

ab0.93

0.02

bd

0.13

0.03

ab0.18

0.05

ba

0.06

0.03

aa0.43

0.01

bc

0.03

0.00

aa0.90

0.06

bd

0.05

0.00

aa

ValuesaremeansS

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Means

with

inrow

andonionwaste

with

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