book chapter_microwave heating technology

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8 Microwave HeatingTechnologyM. Benlloch-Tinoco, A. Salvador,D. Rodrigo, and N. Martínez-Navarrete

8.1 INTRODUCTIONThermal technologies have been at the core of food preservation and production for many years.However, despite the fact that heat treatments provide the required safety prole and extensionof shelf life (Osorio et al., 2008), some more recent thermal technologies, for example, micro-wave energy, are being explored in an attempt to nd alternatives to conventional heating methodsthat essentially rely on conductive, convective, and radiative heat transfer and lead to dramaticlosses of both desired sensory properties and nutrients and bioactive compounds (Picouet et al.,2009). Currently, given the recent increased demand for health-promoting foods with fresh-likecharacteristics (Elez-Martínez et al., 2006), the industrial sector is showing a greater interest in thedevelopment and optimization of novel food preservation processes, intending to meet consumerexpectations by marketing a variety of high-quality, minimally processed food products in whichthe required safety and shelf-life demands are achieved but the negative impact on quality attributesis minimized (Señorans et al., 2003).

Microwave energy might replace traditional heating methods, at least partially, providing foodproducts of superior quality with extended shelf life (Elez-Martínez et al., 2006; Picouet et al.,2009; O’Donnell et al., 2010). This technology can be considered as a key factor in food innovationto successfully differentiate products (Deliza et al., 2005) or to nd new uses for foods by helpingto develop novel ways to process them.

In this chapter, the fundamental mechanisms of microwave heating are presented, followed bya review of the microwave systems and equipment used at an industrial level and the applications

CONTENTS8.1 Introduction ................ ................ ................ ................ ................ ................. ................ ......... 2978.2 Principles of Microwave Heating ............... ................ ................ ................. ................ ......... 2988.3 Microwave Heating Systems and Equipment ................ ................ ................ ................ ....... 2988.4 Industrial Applications in Food Processing...................... ................ ................ ................ ....3008.5 Establishing a Novel Thermal Preservation Process: Kinetic Data Analysis ..... ..... ..... ....... 3008.6 Microwave Preservation of Fruit-Based Products: Application to Kiwifruit Puree ......... .... 303

8.6.1 Microbial Decontamination ............... ................ ................ ................ ................ ....... 3048.6.2 Enzyme Inactivation .................................................................................................3068.6.3 Impact on Sensory Properties .............. ................. ................ ................ ................ .... 3088.6.4 Impact on Nutrients and Functional Compounds ......... ..... .............. ..... ..... .............. . 3118.6.5 Shelf Life of Fruit-Based Products ............... ................ ................ ................ ............ 312

8.7 Conclusions and Future Trends ............... ................. ................ ................ ................ ............ 315Acknowledgments ................ ................ ................ ................ ................ ................. ................ ......... 315

References .............. ................ ................. ................ ................ ................ ................ ................ ....... 315

© 2016 by Taylor & Francis Group, LLC


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of this technology in unit operations in the food industry. This chapter also deals with the kineticdata analysis of microwave processes and describes the impact of microwaves on microorganisms,enzymes, nutrients, and bioactive compounds, and also on the shelf life of a fruit-based product.Finally, conclusions and future improvements for the application of microwave energy to industrialfood processing are discussed.

8.2 PRINCIPLES OF MICROWAVE HEATINGMicrowave energy is transported as an electromagnetic wave (0.3–300 GHz). When interceptedby dielectrical materials, microwaves produce an increase in product temperature associated withdipole rotation and ionic polarization (Schubert and Regier, 2010). Molecular friction of permanentdipoles within the material takes place as they try to reorient themselves with the electrical eld ofthe incident wave, generating heat that is dissipated throughout the food material (Salazar-Gonzálezet al., 2012). Additionally, microwave interaction with polar molecules results in the rotation ofmolecules in the direction of the oscillating eld and, in turn, collisions with other polar mol-

ecules occur, a fact that also contributes to heat generation (Salazar-González et a l., 2012). All thesemolecular movements occur to a greater extent in a liquid than in a solid medium. Microwaves arenonionizing, and their quantum energy is several orders of magnitude lower than other types ofelectromagnetic radiation, meaning that microwave energy is suf cient to move the atoms of a mol-ecule but insufcient to cause chemical changes by direct interaction with molecules and chemicalbonds (Vadivambal and Jayas 2007; Schubert and Regier 2010).

Typically, microwave food processing uses a f requency of 2450 MHz for home ovens, and 915and 896 MHz for industrial heating in the United States and Europe, respectively (Wang and Sun,2012). This type of technology involves volumetric heating, which means that the materials canabsorb microwave energy directly and internally. For this reason, in comparison with conventionalheating methods, microwaves lead to a faster heating rate, thus reducing process time (Huang et al.,2007; Queiroz et al., 2008; Igual et al., 2010).

8.3 MICROWAVE HEATING SYSTEMS AND EQUIPMENTMicrowave technology has been steadily gaining importance in the food processing area. Evidenceof this is the enormous sales rates of household ovens and the increase in the spread of microwaveovens throughout the industrialized world. In the last few years, approximately 10 million micro-wave ovens have been sold annually in the United States and Europe (Schubert and Regier, 2006).

Basically, a microwave system consists of three parts: (1) the microwave source, (2) the wave-guide, and (3) the applicator. The magnetron tube is by far the most commonly used microwave

source for industrial and domestic applications (Schubert and Regier, 2005). A magnetron consistsof a vacuum tube with a central electron-emitting cathode with a highly negative potential. Thiscathode is surrounded by a structured anode that forms cavities, which are coupled by the fring-ing elds and have the intended microwave resonant frequency (Schubert and Regier, 2006). Thewaveguides are elements that are used to guide the electromagnetic wave, consisting principally ofhollow conductors, normally with a constant cross section, rectangular and circular forms being ofmost practical use. Within the waveguide, the wave may spread out in so-called modes which denethe electromagnetic eld distribution within the waveguide (Schubert and Regier, 2005). The appli-cator is basically the element that contains and distributes the microwave energy that surrounds thefood product to be heated. Common applicators can be classied by type of eld conguration intothree types: (1) near-eld, (2) single-mode, and (3) multi-mode applicators. Multi-mode applicatorsplay by far the most important role in industrial and domestic uses because of the typical dimen-sions of microwave ovens (Schubert and Regier, 2005).

To date, microwave heating has not been used as successfully in the food industry as in house-holds. The development of a nonhomogeneous eld distribution has been one of the main factors


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299Microwave Heating Technology

that is l imiting the exploitation of this technology to its fullest potential in the food industry. Despitethe fact that the number of working installations increases every year, it is still considered to bequite low (Hebbar and Rastogi, 2012). Inhomogeneous eld distribution may lead to an undesiredinhomogeneous heating pattern, producing hot spots that damage the item being heated and coldspots where the item may be under-heated or under-processed, thereby compromising product qual-ity, stability, and repeatability (IMS, 2014). Bearing in mind that the homogeneity of the electro-magnetic eld distribution depends strongly both on microwave equipment features and on foodproperties, improvement of industrial microwave systems design could be a key factor to promote agreater spread of this technology in the industrialized world.

In fact, it could be claimed that microwave systems design has shown a spectacular evolu-tion over the years. Early operational systems included batch processing of, for example, yogurtin cups (Anonymous, 1980), their primary drawback being their inability to heat materials ina predictable and uniform manner. Then continuous microwave applicators were developed inan attempt to solve these problems, which allowed continuous processing to improve heatinguniformity and at the same time accomplish the high throughputs desired by the food indus-

try (Hebbar and Rastogi, 2012). Since then, microwave equipment has improved remarkably.Figure 8.1 shows a model of the industrial microwave equipment currently used by American andEuropean companies to heat, cook, and pasteurize different kind of food products. Nowadays,there is a variety of continuous microwave systems with features that address the major obstaclesto the commercialization of microwave heaters for many industrial applications. For example, thefact that microwave energy can optionally be irradiated in modern industrial ovens by one high-power magnetron or by several low-power magnetrons, or be used under vacuum conditions, asin microwave-assisted air-drying and microwave-assisted freeze-drying operations, in order toimprove the efciency of the process, can be taken as proof of this substantial evolution (Schubertand Regier, 2005; Vadivambal and Jayas, 2007).

Cold product out

Applicators

Product heatingtubes

Powersupply

Controlsystem

Magnetron(microwave

energy)

Hotproduct out

Waterload

FIGURE 8.1 Major components of a model microwave heating system currently used in the food industry.


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Since safety must always be the primary concern, thermal treatments are constrained by therequirement to achieve the target lethality, but lethality is not the only aspect to be considered;quality loss must also be taken into account. Thermal processes tend to be optimized to maximizemicrobial and enzyme inactivation and minimize degradation of sensory attributes and loss of nutri-tional value (Awuah et al., 2007; Wang and Sun, 2012). To perform this optimization, knowledge ofthe processing parameters and of the inactivation kinetics of target microorganisms, enzymes, andquality attributes is of utmost importance (Valdramidis et al., 2012).

The calculations involved in the kinetic studies used to design and optimize conventional heatprocesses are well established. However, when it comes to microwave processes, the issue becomesmore complicated, and the main concern lies in the particular form of heating that takes place dur-ing microwave exposure (Banik et al., 2003). In conventional heating, a holding period is expected,but in case of microwaves, the heating that takes place is exclusively non-isothermal (Matsui et al.,2008). Furthermore, it is usually not possible to x the parameters that affect the heating process,such as (1) the heating rate, (2) the range of temperatures at which the samples are exposed, or(3) provision of appropriate sample homogenization. At present, little is known kinetically about the

general basic relationship between microbial and enzyme inactivation and quality retention in foodsand microwave exposure.More specically, focusing on microbial inactivation, Fujikawa et al. (1992), Tajchakavit et al.

(1998), Cañumir et al. (2002), Yaghmaee and Durance (2005), and Pina-Pérez et al. (2014) haveconducted some of the few studies regarding the kinetics of destruction of foodborne pathogens andspoilage microorganisms by microwave irradiation. According to their results, microbial inactiva-tion due to microwave processing can be tted using rst-order kinetics, which has been success-fully employed to describe destruction ofCronobacter sakazakii , Saccharomyces cerevisiae , and

Lactobacillus plantarum under microwave processing (Fujikawa et al., 1992; Tajchakavit et al.,1998; Pina-Pérez et al., 2014).

When rst-order kinetics models are used to describe the inactivation process, the existenceof a linear relationship between the logarithm of the microbial population and time is assumed.Two key parameters ( D and z values) are then determined from the survival and resistance curves,respectively (Awuah et al., 2007; Tajchakavit and Ramaswamy, 1997). The D -value represents theheating time required to reduce 90%of the existing microbial population under isothermal condi-tions (Equation 8.1). The z-value represents the temperature change that results in a 90%reductionof the D -value (Equation 8.2).

log

N

N

t

D0

= − (8.1)

where N is the survivor counts af ter treatment (CFU/g) N 0 is the initial microorganism population (CFU/g)t is the processing time (s)

D is the D -value at the temperature studied (s)

log

D D

T T zref

ref =

−

(8.2)

where D is the D -value at each temperature studied (s) D ref is the D -value at reference temperature (s)T is the processing temperatureT ref is the reference temperature (s)

z is the z-value or temperature sensitivity (°C)


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As previously mentioned, one of the main aspects to take into consideration when performing kineticdata analyses in microwave processes is the fact that, as opposed to conventional heat treatments, theheating that takes place is exclusively non-isothermal. This can be seen in Figure 8.2, which showstemperature proles of a kiwifruit puree sample subjected to different conventional and microwavetreatments. Accordingly, correction of processing time values for come-up periods is essential priorto kinetic data analyses. Time–temperature proles have to be used to calculate the effective time (t e)

(Equation 8.3), which represents the isothermal holding time at the selected reference temperaturethat causes the same level of microbial destruction as the heating actually applied, as if the micro-wave treatments had been performed under isothermal conditions (Tajchakavit and Ramaswamy,1997; Awuah et al., 2007; Matsui et al., 2008; Latorre et al., 2012). Since no holding period at apreset temperature is expected in microwave processes, the maximum temperature reached duringthe treatment is considered asT ref (reference temperature) (Matsui et al., 2008; Latorre et al., 2012).

t dt e

T t T zref = −( )∞

∫100

( ) /

(8.3)

wheret e is the effective time (s)T (t ) is the processing temperature at each processing timeT ref is the reference temperature (s)


12001000800600Processing time(s)(a)

T e m p e r a t u r e

( ° C

)

40020000

10

20

30

40

50

60

70

350300250200150

Processing time(s)(b)

T e m p e r a t u r e

( ° C

)

1005000

10

20

30

40

50

60

70

FIGURE 8.2 Mean kiwif ruit puree temperature pro le for conventional thermal processing (a) at 60°C (–),55° C (– –·), and 50° C (-----) and microwave processing (b) at 1000 W (–), 900 W (– –·), and 600 W (-----).


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Matsui et al. (2008) proposed a method for calculating D and z-values under microwave heating bynonlinear regression. According to their reports, the predicted surviving microbial population foreach microwave experimental run can be calculated from Equation 8.4. Then a nonlinear estima-tion procedure can be used to minimize the sum of squared errors (SSE) between experimental andpredicted surviving microorganisms, de ned in Equation 8.5.

log

(( ( )) )

N N

t D

dt

D predicted

e

T

T T t z

T ref

ref

re0

010

= − =

⋅−

∞

∫ / f f

(8.4)

where N is the survivor counts af ter treatment (CFU/g) N 0 is the initial microorganism population (CFU/g)t e is the effective time (s)

D T ref is the D -value at reference temperature (s)T ref is the reference temperature (s)T (t ) is the processing temperature at each processing time


SSE

N N

N N experimental predicted

=

−

log log

0 0ii

n

=∑

1

(8.5)

where N is the survivor counts af ter treatment (CFU/g)

N 0 is the initial microorganism population (CFU/g) N is the number of experimental runs

A further aspect to be taken into consideration is that the ability to properly understand and carryout kinetic data analysis in microwave heating is important not only for the accurate design of pres-ervation processes but also for the establishment of appropriate comparisons between microwaveand conventional heat treatments (Latorre et al., 2012). A comparison of microwave and conven-tional heating has been the basis of many studies dealing with microwave process applications, suchas those performed by Gentry and Roberts (2005) or Igual et al. (2010). Nevertheless, poor correc-tion of processing time values for come-up periods prior to kinetic data analysis owing to the non-isothermal nature of microwave processes may lead to mistaken interpretations, hinder comparisonof different research works, and cause conicting opinions regarding the superiority, of microwavetechnology over conventional heat treatments.

8.6 MICROWAVE PRESERVATION OF FRUIT-BASEDPRODUCTS: APPLICATION TO KIWIFRUIT PUREE

As mentioned previously, microwave energy could potentially replace conventional heating forsome specic purposes, and commercially proven applications of this technology in several foodprocessing operations are a matter of fact (Awuah et al., 2007; Vadivambal and Jayas, 2010).Nevertheless, microwaves have not yet been exploited to their fullest potential in the food industry(Picouet et al., 2009).

Nowadays, there is still a gap in knowledge concerning fundamental understanding of theinteractions of microwaves when applied to food, and published information on the impact of thistechnology on food safety, stability, and quality aspects is currently both scarce and inconsistent.


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Bearing in mind that the application of microwave heating would be justied only from the stand-point of obtaining high-quality products (Vadivambal and Jayas, 2007), in-depth research work onthe impact of microwaves on microorganisms, enzymes, bioactive compounds, and sensory prop-erties of a variety of foods might make an important contribution to expanding the use of thistechnology on an industrial level.

In the present chapter, the particular case of a kiwifruit puree has been selected as a modelfruit-based product to evaluate the impact of a microwave preservation process on some safety andquality issues.

8.6.1 M ICROBIAL D ECONTAMINATION

Thermal preservation treatments are particularly designed to minimize public health hazards andto extend the useful shelf life of food products, information regarding thermal resistance of micro-organisms, pathogenic or otherwise, being crucial to a correct understanding of their lethal effect.

In the present study, the safety of a ready-to-eat kiwifruit puree subjected to microwave heating

was investigated by checking how effective microwaves are at inactivating Listeria monocytogenes ,taken as the pathogen of greatest concern in the product (Figure 8.3). Although fruit products ofan acidic nature, such as kiwif ruit (pH = 3.4), have not been recognized as being potentially themain vehicles for foodborne illnesses, there has been increasing concern because some outbreakshave been caused by consumption of unpasteurized juices contaminated with Escherichia coli orSalmonella spp. (Bufer, 1993; Picouet et al., 2009) or of salad vegetables or mixed salads with

L. monocytogenes (EFSA, 2013). L. monocytogenes is currently recommended by the NationalAdvisory Committee on Microbiological Criteria for Foods as an appropriate target organism tobe used for fruit juices. Despite the fact that the minimum pH allowing growth of this pathogen infood products has been reported to be pH 4.6 (Carpentier and Cerf, 2011), ready-to-eat fruit-basedacidic products may still represent a potential hazard to health, given the well-known ability of

L. monocytogenes to proliferate in products stored under refrigeration for long periods.Microwave inactivation of L. monocytogenes in the kiwifruit product ( Actinidia deliciosa var.

Hayward) was determined by using the following experimental procedure. The puree was inocu-lated by adding 1 mL of a concentrated suspension of the microorganism so as to give an initial

L. monocytogenes concentration of 107 CFU/g. The product was then processed in a microwaveoven (model: 3038GC, Norm, China) provided with a turntable plate and a ber-optic probe

175150

FDAcriteria

12510075Effective time (s)

l o g N / N

0

50250−8

−7

−6

−5

−4

−3

−2

−1

0

FIGURE 8.3 Survival of L. monocytogenes under microwave processing at 1000 W (experimental (•), model(–)), 900 W (experimenta l (□ ), model (----)) and 600 W (experimenta l (▴ ), model (–·–)). The plotted values anderror bars represent the average of three replicates and the corresponding standard devia tion.


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(CR/JP/11/11671, OPTOCOM, G ermany) which was connected to a temperature datalogger (model:FOTEMP1-OEM, OPTOCOM, Germany) to continuously record the time–temperature history atdifferent points in the sample. The safety of the process was evaluated on a sample placed in thecoldest spot, previously identied (data not shown), since contaminating pathogenic microorgan-isms may survive in cold spots (Nicolaï, 1998). For each treatment, a 500 g sample was temperedto an initial temperature of 25° C and then heated in the microwave oven in a standard glass beaker(9 cm inner diameter and 12 cm height) (BKL3-1K0-006O, Labbox, Spain). Treated samples takenfrom the coldest spot were immediately cooled in ice-water until the puree reached 35° C (for 10–15 s).Then, immediately after they had been inoculated or subjected to different MW power–time pro-cesses, respectively, serial decimal dilutions of the treated and untreated samples were performedin 0.1% (w/v) sterile peptone water (Scharlab Chemie S.A., Barcelona, Spain). The medium usedfor enumerating viable cells was tryptic soy agar (Scharlab Chemie S.A., Barcelona, Spain). Theselected dilutions were incubated at 37° C for 48 h, after which the counting step was carried out.The reduction of viable cells was expressed as the decimal logarithm of the quotient of the treatedand untreated cells.

Survival curves were obtained at three power levels (600, 900, and 1000 W) with process-ing times varying between 50 and 340 s(Figure 8.3). Since L. monocytogenes inactivation undermicrowave heating was close to linearity, as previously reported by other authors forS. cerevisiae and L. plantarum (Fujikawa et al., 1992; Tajchakavit et al., 1998), the data obtained were tted to rst-order kinetics (see Section 8.5). As mentioned previously, in order to make it possible to com-pare the kinetic parameters ( D -values) obtained under microwave processing at the preset powerlevels, kinetic data transformation was performed. Treatment times were corrected, and effectivetime values were obtained. Calculated effective times represented the equivalent holding time ateach processing temperature as if the treatments had been performed under isothermal conditions(Awuah et al., 2007; Matsui et al., 2008; Latorre et al., 2012).

From the inactivation data presented in Figure 8.3, it can be claimed that in the kiwifruit pureesamples subjected to effective times higher than 75 and 82 s at 900 and 1000 W, respectively, thepasteurization objective of 5D established by the FDA (2004) was accomplished. To the best of ourknowledge, the only published study on microwave Listeria spp. inactivation in fruit-based prod-ucts is the one conducted by Picouet et al. (2009). They found a 7-log10 cycle reduction of Listeriainnocua in an apple puree subjected to 900 W for 35 s. However, conventional heat inactivation of

L. monocytogenes in different fruit substrates has been evaluated by several authors. For example,Hassani et al. (2005) reported that 5-log10 cycles of L. monocytogenes were inactivated in a refer-ence medium (pH = 4) when it was subjected to 58° C for 84 s, and Fernández et al. (2007) found a4-log10 cycle reduction when a sucrose solution (pH = 7, aw = 0.99) was maintained at 60° C for 60 s.

The effect of the processing parameters, power (W) and time (s), on inactivation of L. monocyto-

genes was evaluated statistically. Both factors were shown to affect the L. monocytogenes reductionlevel achieved signi cantly ( p < 0.05), although no signicant differences were found between 1000and 900 W. Both higher power level and higher effective time led to signicantly higher L. mono-cytogenes inactivation ( p < 0.05) (Figure 8.3). In this respect, the higher the microwave power, thelower the effective time necessary to reach the same level of inactivation. For example, in order toachieve the FDA recommendations for pasteurized products (5-log10 cycle inactivation), a consider-ably longer effective time was required at 600 W (t e = 116 s) than at 1000 W (t e = 82 s).

Kinetic parameters describing L. monocytogenes inactivation under microwave processing werecalculated (see Section 8.5), providing the following D -values: D 60° C = 42.85± 0.13 ( R2-adjusted =0.992) at 600 W, D 60° C = 17.35± 0.34 ( R2-adjusted = 0.993) at 900 W, and D 60° C = 17.04± 0.34 at1000 W ( R2-adjusted = 0.996). Although the kinetics of L. monocytogenes inactivation by thermaltreatment has been studied extensively in various foodstuffs such as beef, milk, chicken, carrot,cantaloupe, and watermelon juice (Chhabra et al., 1989; Bolton et al., 2000; Sharma et al., 2005),in reference medium (Hassani et al., 2005, 2007) and in sucrose solutions (Fernández et al., 2007),there is no information available about the survival behavior of this pathogen in fruit-based products


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under microwave heating. Cañumir et al. (2002) reported higher D -values for microwave apple juicepasteurization when the inactivation kinetics of E. coli was evaluated, ranging between D 70.3° C =25.2 s to D 38.3° C = 238.8 s for 900 and 270 W, respectively. Yaghmaee and Durance (2005) foundsimilar D -values for microwave inactivation of E. coli in peptone water at 510 W, with D 55.6° C = 30 sand D 60.5°C = 18 s. Once more, the power level effect can be evaluated by comparing the D 60° C-values. Microwave processing performed at 900 and 1000 W led to considerably faster bacteriumreduction than processing at 600 W.

Like the results of other authors (Fujikava et al., 1992), the results obtained in this study provedthe effectiveness of microwave heating against foodborne pathogens of concern, such as L. mono-cytogenes , showing that safety can be properly ensured in fruit-based products by means of thistechnology.

8.6.2 E NZYME I NACTIVATION

Enzymes are naturally present in fruit and vegetables and can cause product deterioration in

many w ays (Whitaker et al., 2003). Enzymes such as peroxidase (POD) and polyphenol oxidase(PPO) are principally responsible for the degradation of color and nutritive value of most foodproducts of vegetable origin (Queiroz et al., 2008), while pectin methylesterase (PME) causeschanges in the rheological properties of foods by means of pectin de-esterication (Jolie et al.,2010). In view of the very negative impact that enzymes of this kind could have on kiwifruit-based products, POD, PPO, and PME were selected to check how effective microwave heatingis at inactivating enzymes in the product. To study the effect of microwave power and processtime on the inactivation of POD, PPO, and PME in the product using the minimum number ofexperimental trials (Beirão-da-Costa et al., 2006), an experimental design based on a centralcomposite design was applied (Cochran and Cox, 1957). Power and time were designed to varybetween 300 and 900 W and between 100 and 300 s, respectively. Each microwave treatmentwas carried out as described in Section 8.6.1. The temperature of the sample was recorded con-tinuously, in this case in the hottest spot, previously identi ed (data not shown). Enzyme activitywas measured in all the treated samples and also in the untreated sample, which was used as thecontrol, following the methods described by De Ancos et al. (1999) for POD and PPO and byRodrigo et al. (2006) for PME. The percentage of enzyme inactivation ( I ) was then calculatedby using Equation 8.6.

I

A A

A

F T

F

=−

× 100 (8.6)

where AF is the enzyme activity of fresh kiwifruit puree AT is the enzyme activity of treated kiwifruit puree

The results obtained showed that the inactivation of POD, PPO, and PME in the kiwifruit pureeproduced by processing in the desired range of microwave power (300–900 W) and time (100–300 s)varied from 43%± 6%to 88.0% ± 0.7%, from 11.4% ± 0.5%to 81% ± 2%, and from −19.0%± 1.3%to 57% ± 6%, respectively. These results indicate that, in kiwifruit, PME and POD were theenzymes that were most resistant and most sensitive to microwaves, respectively, while PP O showedan intermediate behavior. Similar results have been reported by other authors for this fruit as wellas for strawberry when subjected to conventional heat processes (McFeeters et al., 1985; De Ancoset al., 1999; Beirão-da-Costa et al., 2008; Terefe et al., 2010). Despite the fact that POD was themost sensitive enzyme in this case, it could still be considered as a suitable indicator of treatmentefciency since it has been reported to be very important in kiwif ruit because of its high activity andextensive contribution to the quality of this fruit (Fang et al., 2008).


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One of the microwave treatments applied (300 W–100 s) led to a promotion of PME activity. Thismight be related with the low temperature reached by the sample in this case, around 43° C, and theshort exposure time. A similar phenomenon was observed by Beirão-da-Costa et al. (2008), whofound a signicant ( p < 0.05) increase in PME activity in kiwifruit slices subjected to mild heattreatment prior to inactivation. Another sample subjected to 300 W reached 45° C, but the treatmenttime was 300 s. Under these conditions, inactivation of PME was only 4.3%(standard deviation 0.7).The temperature reached by the other samples was in the range of 60° C–100° C.

The results obtained from the enzyme inactivation study were also analyzed by means of theResponse Surface Methodology, y ielding 3D plots for POD, PPO, and PME inactivation (Figures 8.4through 8.6). As can be observed in Figure 8.4, there was a signicant ( p < 0.05) increase in PODinactivation up to a power of 800 W, decreasing slightly when a higher microwave power wasapplied. De Ancos et al. (1999) observed that inactivation of papaya POD behaved similarly undermicrowave heating. They reported an increase in peroxidase inactivation when the microwavepower increased from 285 to 570 W for 30 s of processing time. Thereafter, a higher power level(800 W) did not increase POD inactivation. In accordance with other authors, as the process time

increased, there was a linear increase in POD inactivation (Matsui et al., 2008).Figure 8.5 shows the PPO inactivation behavior as related to microwave power and processtime. As can be observed, the level of PPO inactivation rose signi cantly ( p < 0.05) as the micro-wave power increased. However, the increase in the PPO inactivation observed was smaller atgreater powers. Process time also had a signi cant ( p < 0.05) effect, leading to a greater inactiva-tion of this enzyme. Latorre et al. (2012) and Matsui et al. (2008) found that there was a greater

900800700600500400

300100

140180

220260

300

Power (W)

Time (s)

P O D

0

20

60

40

80

100

FIGURE 8.4 Response surface plot for the percentage of peroxidase (POD) inactivation in kiwifruit pureeas a function of microwave power and process time.

900800700600

500400300100 140

180 220

260300

Power (W)

Time (s)

P P O

0

20

60

40

80

100

FIGURE 8.5 Response surface plot for the percentage of polyphenoloxidase (PPO) inactivation in kiwifruitpuree as a function of microwave power and process time.


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level of PPO inactivation in red beet and green coconut water, respectively, after longer micro-wave exposure. De Ancos et al. (1999) observed that PPO inactivation in kiwifruit and straw-berry was controlled better by pre xing the power rather than the exposure time. In addition, aninteractive effect on PPO inactivation was observed between microwave power and process time.As expected, as greater microwave power was applied, the level of PPO inactivation rose fasterin samples subjected to longer treatment times than in kiwifruit puree subjected to shorter treat-ment times.

From Figure 8.6, it can be said that PME inactivation increased signicantly ( p < 0.05) asthe microwave power level rose and the processing time lengthened. Similarly, Tajchakavit andRamaswamy (1997) reported a linear relationship between time and PME inactivation duringmicrowave heating of orange juice, and Kratchanova et al. (2004) found, when microwaving orangepeel, that as microwave power increased, PME inactivation also increased.

Summarizing, in accordance with what has been reported by other authors, the results of thepresent study highlight the suitability of microwave heating for enzyme inactivation (De Ancoset al., 1999; Matsui et al., 2008; Latorre et al., 2012), which means that stability and quality can beproperly ensured during the shelf life of fruit-based products by means of this technology (Igualet al., 2010; Zheng and Lu, 2011).

8.6.3 I MPACT ON S ENSORY P ROPERTIES

Despite the fact that sensory assessment must be considered as an essential tool to guide any modi -cation of the food processing step (Di Monaco et al., 2005), there still seems to be a need for sensoryanalyses that focus on the impact that alternative technologies, such as microwaves, have on foodproduct characteristics (Da Costa et al., 2000).

In the present work, the following experimental procedure was performed to evaluate the impactof microwave processing on the most important sensory characteristics of kiwifruit puree. Theeffect of the two processing variables (microwave power and process time) was investigated simul-taneously by means of a rotatable central composite design(Section 8.6.2). Each microwave treat-ment was carried out as described in Section 8.6.1. Cooked purees were then cold stored (4°C) for24 h before sensory assessment.

A sensory panel with 11 assessors (four men and seven women), recruited from studentsand employees of the Food Technology Department (Universitat Politècnica de València) agedbetween 25 and 50, was trained over a period of 2 months (12 training sessions). Sampleswere tempered at 25° C and served in disposable standard-size plastic containers identied withthree-digit codes. In all cases, training and formal assessment were performed in a normaliz ed

900800700600500400

300100 140

180 220260 300

Power (W)

Time (s)

P M E

−26

−6

34

14

54

74

FIGURE 8.6 Response surface plot for the percentage of pectin methylesterase (PME) inactivation in kiwi-fruit puree as a function of microwave power and process time.


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tasting room. The selection of descriptors was made over two 1 h sessions using the checklistmethod (Table 8.1) (Law less and Heymann, 1998). During the training period, all the treatedand untreated samples were tasted. Tests of three different samples in each session were usedby the panelists for each descriptor until the panel was homogeneous in the ranking of thesamples. Panel members were then trained in the use of scales by using reference samples(10 cm unstructured scales for all the attributes). Panel performance was checked by an analysisof variance (ANOVA) for the discrimination ability of the panelists and the reproducibility oftheir assessments. Once the training period was over, the formal assessment was performed.To this end, a balanced complete block experimental design was carried out in duplicate (twodifferent sessions), using the Compusense® program release ve 4.6 software (Compusense Inc.,Guelph, Ontario, Canada) to evaluate the samples. The intensity of the sensory attributes wasscored on a 10 cm unstructured line scale. Samples were selected randomly and served with arandom three-digit code. All the treated samples were subjected to formal analysis, as well asthe untreated sample.

The results obtained from the sensory assessment indicated that signicant differences( p < 0.05) among samples were only found in the sensory descriptors “ typical kiwifruit color,”

“ tone,” “ visual consistency,” “ lightness,” and “ atypical taste.” As a general rule, for these vedescriptors (Figure 8.7), noticeable differences increased in treated samples compared withuntreated samples when heating intensity increased. In fact, signicant differences were notfound ( p > 0.05) between fresh kiwifruit puree and samples processed at 200 W–200 s, 300W–100 s, and 600 W–60 s as the lines in the spider plot nearly overlapped (Figure 8.7a). Figure8.7b shows greater differences in each signicant attribute between treated samples and freshkiwif ruit puree, except in “visual consistency.” Panelists considered that samples 600 W–200 sand 900 W–100 s had less lightness and a lower “ typical kiwifruit color intensity” than f resh puree( p < 0.05). These samples and also the 300 W–300 s one seemed to be signicantly ( p < 0.05)browner, or rather less green, than the fresh kiwifruit puree. Figure 8.7c shows greater differ-ences in the assessments given to samples 600 W–340 s, 900 W–300 s, and 1000 W–200 s ascompared with fresh kiwifruit puree. In general, the panelists considered that the three processedsamples had signicantly ( p < 0.05) less lightness and greenness, with a lower typical kiwifruitcolor intensity and higher atypical taste intensity; however, they had the same visual consistencyas f resh kiwifruit puree.

TABLE 8.1Attributes, Scale extremes, and Evaluation Technique Used in Descriptive SensoryAssessment of Kiwifruit Puree Treated with MicrowavesAttribute and Scale Extremes TechniqueKiwi odor intensity (low/high) ObserveAtypical odor (low /high) ObserveTypical kiwi color (low/high) ObserveTone (green/brown) ObserveLightness (light/dark) ObserveGranularity (low/high) Evenness of the sample’s surface. Take a spoonful of the sample and observe its surface.Visual consistency (low/high) Take enough quantity of kiwi puree with a spoon and drop it to evaluate its visual consistency.Sweetness (low/high) Taste the necessary quantity of kiwi puree to notice the intensity of sweetness.Acidity (low/high) Taste the necessary quantity of kiwi puree to notice the intensity of acidity.Astringency (low/high) Taste the necessary quantity of kiwi puree to notice the intensity of astringency.

Kiwi taste intensity (low/high) Taste the necessary quantity of kiwi puree to notice the intensity of typical kiwi taste.Atypical taste (low/high) Taste the necessary quantity of kiwi puree to notice the intensity of typical kiwi taste.Aftertaste (low/high) Assess the persistence of taste after ingesting kiwi puree.Mouth consistency (low/high) Taste the sample and evaluate its consistency during ingestion.


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Additionally, the XLSTAT 2009 program was employed to make a principal component anal-ysis (using a correlation matrix), with the aim of studying the correlation between the variousmicrowave treatments applied in the present study and the sensory attributes of kiwifruit puree.Figure 8.8 shows the rst two component maps of the principal component analysis constructed

using the sensory data . Two components were extracted that explain 80.59%of the data variability.The rst component explained most of this variance (63.83%); for this reason, it has been used todescribe all the kiwifruit puree characteristics. This component showed a positive correlation withthe sensory attributes “ typical kiwifruit color intensity,” “ kiwifruit odor intensity,” “ lightness,”“acidity,” “astringency,” and “kiwi taste intensity” and a negative correlation with the sensoryattributes “atypical odor,” “ tone,” “atypical taste,” “ visual consistency,” and “ mouth consistency.”Samples 200 W–200 s, 300 W–100 s, and 600 W–60 s were characterized by a similar acidity,astringency, color, odor, and taste to the fresh kiwifruit, owing to the less intensive treatments thatwere applied to these samples. On the other hand, when the most severe treatments were applied(600 W–340 s, 900 W–300 s, and 1000 W–200 s), the samples were characterized by a higheratypical odor and taste, higher visual and mouth consistency, and more browning. Finally, thegranularity and consistency of samples 300 W–300 s, 600 W–200 s, and 900 W–100 s were higherthan those of the other samples.

On the whole, it can be said that the application of intense treatments of high microwave powermainly affected the color and taste of the kiwifruit puree. Signicant perceivable differences

Kiwifruitcolor

Atypicaltaste

Tone

LightnessVisualconsistency

Fresh kiwifruit200–200300–100600–60

10

8

6

2

0

(a)

Kiwifruitcolor

Atypicaltaste

Tone


Fresh kiwifruit

600–340900–3001000–200

1086

0

(c)

Kiwifruitcolor

Atypicaltaste

Tone


Fresh kiwifruit300–300600–200900–100

10

8

6

2

0

(b)

4

42

4

FIGURE 8.7 Average values (on a 0–10 scale) of panel member assessments of kiwifruit color, tone, light-ness, visual consistency, and atypical taste of treated samples: 200 W–200 s, 300 W–100 s, and 600 W–60 s(a); 300 W–300 s, 600 W–200 s, and 900 W–100 s (b); and 600 W–340 s, 900 W–300 s, and 1000 W–200 s(c), compared with f resh sample.


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between kiwifruit puree samples were only found in the ve descriptors mentioned earlier, andthey increased when microwave power increased. As expected, the most severely treated samplesshowed the highest variation in these parameters.

8.6.4 I MPACT ON N UTRIENTS AND F UNCTIONAL C OMPOUNDS

Epidemiological studies suggest that the consumption of f ruit and vegetables may play an importantrole in the protection against many chronic diseases. In addition to the well-established benetsof the essential vitamins and minerals found in these products, they also provide the diet with agood source of ber and a diverse array of phytochemicals (B arret and Lloyd, 2012). More speci-cally, kiwifruit has high vitamin C and E contents and marked antioxidant activity; its vitamin Ccontent being even higher than that found in grapefruit and orange (Igual et al., 2010), citric fruitsthat are widely recognized as good sources of this bioactive compound. In fact, given its excellent

nutritional and functional characteristics, Fiorentino et al. (2009) dened kiwif ruit as a unique andprecious cocktail of protective phytochemicals.The main goal of fruit processing is to create microbiologically safe products and to extend their

shelf life so that they can be consumed all year round and transported safely to consumers all overthe world. However, processors also strive to produce the highest-quality food, attempting to mini-mize losses of nutritional and functional value (Barret and Lloyd, 2012).

In the present work, the impact of microwaves on the nutritive and functional va lue of kiwif ruitwas investigated by evaluating changes produced in the main bioactive compounds of this fruit bymicrowave pasteurization treatment. Processing conditions for puree pasteurization were chosen(1000 W–340 s) on the basis of preliminary experiments, considered in terms of the enzyme (90%of POD) and microbial (5D of L. monocytogenes ) inactivation to be achieved. Then the microwavetreatment was carried out as described in Section 8.6.1. Vitamin C, A, and E contents, total phenols,total tannins, total avonoids, and antioxidant activity were measured in the treated sample and alsoin the untreated sample, which was used as the control, following the methodology described byTaira (1995), Djeridane et al. (2006), Igual et al. (2010), and García-Martínez et al. (2012).

210PC1 (63.83%)

900–300

200–200300–100

600–60Lightness

Acidity

Kiwi taste intensity Kiwi odor intensity Typical kiwi color

Astringency Fresh

300–300

600–200

900–100

1000–200

600–340

SweetnessAftertaste

Mouth consistency Visual consistency

Atypical tasteAtypical odourTone

Granularity

P C 2

( 1 6

. 7 6 %

)

−1−2−2.0

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

2.0

FIGURE 8.8 Plot of the rst two components of the Principal Component Analysis carried out on fresh andtreated samples and sensory attributes.


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The bioactive compound contents of the microwaved and control samples are summarized inTable 8.2. Unexpectedly, vitamin C was not shown to be the compound that was most sensitive tomicrowaves in kiwifruit, given that all the components analyzed in the puree decreased signi cantly( p < 0.05) as a result of the microwave pasteurization treatment except vitamin C and tota l tannins,which remained signicantly ( p > 0.05) unchanged after processing. Variations in the other compo-nent contents due to processing were calculated as the difference in each compound in the treatedpuree in comparison with the fresh puree, with reference to 100 g of fresh puree. The calculatedvalues are −100%of vitamin A, which was shown to be the compound that was most sensitive tomicrowaves, −36.2%of total avonoids, − 21.8%of total phenols, − 9.4%of vitamin E, and − 65.7%of antioxidant activity. The losses observed in the pasteurized kiwifruit are in the range typicallyexpected for pasteurization processes, which are considered as treatments severe enough to reducethe levels of most bioactive compounds present in fruit, with vitamins found to be among the mostheat-sensitive food components (Awuah et al., 2007; Rawson et al., 2011). In both microwave andconventional heat processes, simple thermal decomposition would appear to be the most likelycause for these losses, but this degradation may be a complex phenomenon that is also dependenton oxygen, light, pH, water solubility, and the presence of chemical, metal, or other compounds thatcould catalyze deteriorative reactions (Awuah et al., 2007). On the other hand, it is worth highlight-ing that the concentration of vitamin C, one of the most important bioactive compounds in kiwifruitbecause of its particularly high amount and its attributable antioxidant activity, was signicantly( p < 0.05) unaffected by microwaves. Barrett and Lloyd (2012) reviewed the effect of microwave

processing on bioactive compounds in products of vegetable origin and reported that the use ofmicrowaves leads to a higher retention of vitamin C in most fruits and vegetables than the applica-tion of conventional heating.

8.6.5 S HELF L IFE OF F RUIT -B ASED P RODUCTS

To date, many comparative studies have been conducted on the effect of microwaves and conven-tional heating on various quality aspects of f ruits (Barrett and Lloyd, 2012), pointing out advantagesof microwave heating (Huang et al., 2007). However, it should be taken into consideration that,although published data on the effect of microwaves on safety and quality are available for variousfood systems, little still seems to be known about the impact of microwaves on the shelf life andpost-processing quality loss of fruit products. Marketing of these products frequently involves astorage step, which might also contribute considerably to their nal quality, the evolution of theirproperties and the growth of microorganisms (pathogens or otherwise) during shelf life being animportant issue to study (Rodrigo et al., 2003).

TABLE 8.2Mean Values and Standard Deviation of the Vitamin C, Vitamin A, and Vitamin E Contents,Total Phenols, Total Flavonoids, and Total Tannins of Fresh and Microwaved Kiwifruit Puree

Fresh MicrowavedVitamin C (mg/100 g) 75.9 ± 1.3a 75.5 ± 1.1a

Vitamin A (mg/100 g) 0.057 ± 0.007b NDa

Vitamin E (mg/100 g) 2.45 ± 0.06b 2.22 ± 0.07a

Total phenols (mg GAE/100 g) 22 ± 2b 17.2 ± 0.5a

Total avonoids (mg RE/100 g) 1.16 ± 0.05b 0.74 ± 0.06a

Total tannins (mg GAE/100 g) 14.40 ± 0.10a 10.6 ± 0.8a

Antioxidant activity (mM Trolox/g) 5.81 ± 0.05b 1.99 ± 0.06a

a ND, not detected.b In rows, different letters denote signicant differences ( p < 0.05) according to the Tukey test.


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Having shown the suitability of microwave energy for effective inactivation of microorganismsand enzymes without strongly affecting the nutritive and functional value of the kiwifruit puree,it was proposed to investigate whether microwaves could potentially replace conventional heatingfor pasteurization purposes. To this end, the shelf l ife of a microwave-pasteurized kiwifruit pureewas compared with that of a conventionally heat-pasteurized kiwifruit puree, on the basis of theirmicrobial stability and the impact on the bioactive compounds and antioxidant activity of the prod-uct when stored at 4°C.

Accordingly, a microwave pasteurization treatment designed on the basis of the enzyme (90%ofPOD) and microbial (5D of L. monocytogenes ) inactivation to be achieved was applied (1000W–340 s) in the same way as described in Section 8.6.1. Additionally, a conventional thermalpasteurization treatment, which was equivalent to the microwave process in terms of POD and

L. monocytogenes inactivation, was carried out to establish a comparison between the two tech-nologies. The conventional thermal treatment consisted in heating the sample at 97° C for 30 s in athermostatic circulating water bath (Precisterm, Selecta, Barcelona, Spain). After the kiwifruit hadbeen triturated, 20 g of puree was placed in TDT stainless steel tubes (1.3 cm inner diameter and

15 cm length) and closed with a screw stopper. A thermocouple that was connected to a data loggerwas inserted through the sealed screw top in order to record the time–temperature history of thesample during treatment. P rior to this heating step, the samples were preheated to 25° C to shortenand standardize the come-up time (150 s). Treated samples were immediately cooled in ice-wateruntil the puree reached 35° C. The treated and untreated kiwifruit purees were then packaged inclean, sterile plastic tubes (1.7 cm inner diameter and 11.8 cm length) and then stored in darkness at4° C for 188 days. The microbial population and the concentrations of the main bioactive compoundsand antioxidant activity were measured in the samples during storage. Survival of L. monocyto-genes was evaluated as previously explained in Section 8.6.1. The total mesophilic bacteria (TMB)and yeast and mold (Y&M) counts were examined by diluting the uninoculated samples in 0.1%(w/v) sterile peptone water (Scharlab Chemie S.A., Barcelona, Spain) and enumerating the viablecells in plate count agar (PCA, Scharlab Chemie S.A., Barcelona, Spain) and potato dextrose agar(PDA, Scharlab Chemie S.A., B arcelona, Spain) acidied with tartaric acid (10%) (Sigma-Aldrich,Germany) by adding 1 mL of tartaric acid per 10 mL of PDA, respectively. The selected dilutionswere incubated at 30° C for 48 h for TMB and at 25° C for 5 days for Y&M. Additionally, the vita-min C content, tota l phenols and avonoids, and antioxidant activity were determined as previouslydescribed (see Section 8.6.3).

The shelf life of the treated products was determined, taking into account the acceptable limitestablished by EU legislation ( L. monocytogenes ≤ 2.0 log10 CFU/g, and TMB and Y&M≤ 3.0 log10 CFU/g) (EU, 2005). On this basis, the shelf life of the microwaved and conventionally pasteurizedpurees was found to be 123 and 81 days, respectively(Figure 8.9). These results are in the range

of those published by other authors for various fruits subjected to conventional thermal processes.The shelf life of heat-pasteurized orange and carrot juice (98°C for 21 s) stored at 2° C, thermallypasteurized pomegranate (90° C for 5 s) stored at 5° C, and conventionally heat-pasteurized orangejuice (90°C for 50 s) stored at 4° C was found to be 70, 120, and 105 days, respectively (Leizersonand Shimoni, 2005; Rivas et al., 2006; Vegara et al., 2013). Picouet et al. (2009) reported that anapple puree preserved by gentle microwave heating (652 W–35 s) had a shelf life of at least 14 daysunder refrigeration conditions.

The nutritional and functional value of the microwaved and conventionally heat-treated kiwi-fruit purees at the beginning and end of their shelf life is presented in Table 8.3. Variations in thecomponents due to the combined effects of processing and storage were calculated as the differencein each compound in the treated puree at the end of its shelf life in relation to the fresh puree, withreference to 100 g of fresh puree. Losses of 43%, 23%, and 62%in vitamin C, total phenols, andtotal avonoids were found for the microwave-pasteurized sample (123 days at 4° C), while losses of61%, 58%, and 56%in vitamin C, total phenols, and total avonoids were observed for the conven-tionally thermally pasteurized puree (81 days at 4°C), respectively. However, antioxidant activity


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was reduced by 62%in both cases. These results clearly indicate the superiority of microwaves topreserve the nutritional and functional value of the product. Igual et al. (2010, 2011) found lossesof similar magnitude and reported that microwave-pasteurized grapefruit juices stored at − 18°Cpreserved total phenols, individual avonoids, and antioxidant capacity better when compared withfresh or conventionally pasteurized ones.

TABLE 8.3Mean Values and Standard Deviation of the Vitamin C Content (mg/100 g), Total Phenols(mg GAE/100 g), Total Flavonoids (mg RE/100 g), and Antioxidant Activity (mM Trolox/100 g)of Microwaved and Conventionally Heated Kiwifruit Puree, at the Beginning and at theEnd of Their Shelf Life at 4°C

Beginning of Shelf-Life End of Shelf-Life

0 days 123 days 81 days

MicrowavedConventionally

heated MicrowavedConventionally

heatedVitamin C 64.2 ± 0.7a 62.3 ± 0.7a 37.2 ± 0.6b 25.4 ± 1.5cTotal phenols 25.50 ± 0.07a 22.2 ± 0.3b 13.92 ± 0.08c 9.3 ± 0.3dTotal avonoids 0.825 ± 0.004a 0.67 ± 0.02b 0.437 ± 0.013c 0.505 ± 0.010dAntioxidant activity 1211 ± 37a 1117 ± 27b 478 ± 35c 463 ± 41c

Note: In rows, different letters (a, b, c or d) denote signicant differences ( p < 0.05) according to the Tukey test.

00

1

2

34

5

6

7

8

25 50 75 100Days

l o g

( N

)

125 150 175 200

00

1

2

3

4

5

6

7

8

25 50 75 100Days

l o g

( N

)

125 150 175 200

(a)

(b) (c)0

0

1

2

3

4

5

6

7

8

25 50 75 100Days

l o g

( N

)

125 150 175 200

FMW

C

FIGURE 8.9 Survival of (a) L. monocytogenes , (b) total mesophilic bacteria, (c) and Y&M in the kiwifruitpuree (F, fresh; MW, microwaved; C, conventionally heated) during storage at 4° C.


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In view of the results obtained in the present study, microwave heating not only seems to pro-vide greater microbial stability than conventional heat processing, allowing longer preservation ofkiwif ruit puree, but also maintains the bioactive compound contents and antioxidant activity of theproduct to an equal or greater extent.

8.7 CONCLUSIONS AND FUTURE TRENDSMicrowave heating is an interesting technique for processing fruit-based products and preserv-ing their safety and quality during storage. The use of this technology represents a good alter-native to conventional heating methods, a llowing ef cient inactivat ion of both microorganismsand enzymes to be achieved, while it does not greatly affect product quality. Considering theincreased demand for high-quality foods as well as cost competitiveness, microwave technol-ogy might be taken as an innovative tool to help consumer expectations to be addressed by themarketing of safe, high-quality, minimally processed fruit-based products. C urrently, with therapidly changing scenario, the future prospects of microwaves in food processing are bright.

However, the use of microwave technology is still limited at present to selected categoriesof high-value food products. Some of the key issues that should be adequately addressed tomake microwave processing more attractive are improvement in equipment design, reduction inequipment cost, and improvement in process control. Moreover, in general terms, there is a needfor further studies to bridge the gap between laboratory research and industrial applications.

ACKNOWLEDGMENTSThe authors thank the Ministerio de Educación y Ciencia for the nancial support given throughProjects AGL 2010-22176 and AGL2013-48993-C2-2-R and the Generalitat Valenciana for the

nancia l support given through Project ACOMP/2012/161 and the grant awarded to the authorMaría B enlloch.

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