Journal of the Science of Food and Agriculture J Sci Food Agric 79 :663–670 (1999)

Effects of microwave heating on pigmentcomposition and colour of fruit purees¹

Begon8 a de Ancos , M Pilar Cano,* Almudena Hernandez and MarianneMonrealPlant Foods Science & Technology Department , Ins tituto del (CSIC), Ciudad Univers itaria , 28040-Madrid , SpainFr •�o

Abstract : Microwave energy was applied to inactivate the oxydoreductases peroxidase (POD, EC

1.11.1.7) and polyphenol oxidase (PPO, EC 1.14.18.1) in processed fruit products. Microwave blanching

of papaya, strawberry and kiwi purees at various conditions of power and time produced inactivation

of PPO and POD activities depending on the fruit and the heating conditions. Treatment at 850 W/60 s

produced about 60% of POD inactivation for papaya and kiwi fruit. POD activity in strawberry,

however, seemed to be more resistant to microwave inactivation; treatment at 850 W/60 s only

achieved a loss of POD activity near 8% . Papaya oxidoreductases showed lower stability in the micro-

wave treatments tested. Microwave blanching at 475 W/45 s produced about 75% inactivation of POD

activity and nearly complete PPO inactivation. Kiwi fruit and strawberry purees exhibited similar

inactivation of PPO – 32% at 475 W/30 s and 70% at 475 W/60 s. The decrease of PPO activity in both

products was almost linear at constant power. This thermal treatment, however, directly aþ ects the

colour of the fruit pulps. Papaya, kiwi and strawberry purees suþ ered slight colour (CIE L*a*b*)

changes. Carotenoid, chlorophyll and anthocyanin changes were evaluated by HPLC and related to

objective colour. Microwave treatments produced small modiücations of the quantitative and qualit-

ative composition of carotenoids (in papaya) and anthocyanins (in strawberry). Chlorophylls (kiwi)

showed signiücant degradation as a consequence of microwave heating.

1999 Society of Chemical Industry(

Keywords: microwave; polyphenol oxidase; peroxidase; pigments ; kiwi fruit ; papaya; strawberry ; purees ;colour

INTRODUCTION

Variability in colour and lack of colour stability aremajor problems experienced with processed fruitproducts. Undesirable sensory and biochemicalchanges during handling, processing and storage offruit products are a result of enzymatic browning1 orof non-enzymatic reactions (Maillard mechanisms).2Development of browning (or discoloration), oþ-ýavours and nutritional damage were attributed tothe action of enzymes polyphenol oxidase (PPO, EC1.14.18.1) and peroxidase (POD, EC 1.11.1.7).3h6The use of microwave energy to inactivate enzymesprior to processing fruits and vegetables is not acommon practice. The advantages of the use ofmicrowave energy when compared with conventionalheat-blanching are: (1) in-depth heating in theabsence of a temperature gradient ; (2) inactivation ofenzyme complexes and (3) avoidance of the leachingof vitamins, ýavours, pigments, carbohydrates andother water-soluble components.

Microwave blanching operations have beenapplied successfully in the enzymatic inactivation inwhole tomato fruits and whole soybeans.7,8 Eþects ofwater- and microwave-blanching methods on activi-ties of peroxidases and lipoxygenases in green beans,peas and carrots were reported by Guenes and Bayin-dirli,9 who concluded that the less severe heat treat-ment required to inactivate the enzymes, when themicrowave treatment was applied, should result inimproved product ýavour, colour, texture and nutri-tional value.

A preliminary study of microwave blanching offruit tissues was made by Cano et al10 in banana pro-ducts. In this work the eþects of steam and micro-wave blanching on PPO and POD enzymes weredescribed. More recently Cano11 reported that theoptimal quality in frozen banana was obtained withmicrowave pretreatment. Also, Giami12 reportedbeneücial eþects of microwave treatment appliedprior to freezing plantain (Musa paradisiaca) slices in

¹ This work was partially pres ented as a pos ter to the IX World

Congres s of Food Science & Technology, held in Budapes t

(Hungary) 31 July–4 Augus t, 1995

* Corres pondence to : Pilar Plant Foods Science &M Cano,

Technology Department, Ins tituto del Fri�o (CSIC), Ciudad

Univers itaria, 28040-Madrid, Spain

Contract/grant s pons or : Comis io� n Interminis terial de Ciercia yTecnologia

Contract/grant number : ALI 94-1442, ALI 95-0105

(Received 9 June 1997; revis ed vers ion received 5 February

1998; accepted 12 Augus t 1998)

( 1999 Society of Chemical Industry. J Sci Food Agric 0022-5142/99/$17.50 663

B de Ancos et al

that the microwaved product had a texture similar tothat obtained with calcium chloride treatment.Application of microwave energy in the enzyme inac-tivation of tropical fruit pulps (guava, papaya andmango) were reported by Abd-El-Al-Mg et al ;13they found that the exposure times required forenzyme inactivation in pulps varied from 20 to 80sdepending on enzyme and pulp type.

There is, however, little information about theapplication of microwave blanching to other fruits.Colour and pigment degradation in strawberry pro-ducts have been well documented.14h16 Heat pro-cessing and subsequent storage of fruits in whole,sliced, pureed and concentrated form have beenaccompanied by discoloration due to the degradationof the anthocyanins. Wrolstad et al17 studied theeþect of microwave blanching on the colour andcomposition of strawberry concentrate and juice;they found that this treatment protected pigmentcomposition and improved colour stability.

The most important sensory and nutritionalchanges that aþect kiwi fruit pulp during processingare losses of bright green colour and ascorbic acidcontent. Cano et al18 studied changes in the majorpigment constituents of frozen kiwi fruit slicesduring prolonged storage at [ 18¡C and the degra-dation of chlorophylls and formation of pheophytinsand metallochlorphyll complexes (Zn-pheophytin aand Zn-pyropheophytin a) in canned products due tothermal treatments,19 but information about micro-wave treatment on kiwi fruit products was notreported.

The major purpose of this study is to investigatethe eþect of microwave energy on the PPO and PODactivities of strawberry, papaya and kiwi fruit purees,and to study the eþects of microwave blanching onthe CIE L*a*b* colour values of processed fruit pro-ducts. In addition, the HPLC analysis of pigmentcomposition of the fruit purees after treatments isundertaken in order to compare heat pretreatmentwith other traditional methods for the preservationof fruit purees.

MATERIALS AND METHODS

Fruit purees

Strawberries (Fragaria ananassa ] Duchesne, cultivarChandler), papayas (Carica papaya, cultivar Sunrise)and kiwi fruits (Actinida chinensis, Planch, cultivar

Hayward) were purchased from local supermarkets.Ten (kiwi and papaya) and 50 (strawberry) fruitswere peeled and inedible parts removed. Fruit pureeswere obtained by homogenisation using a blender(Osterizer, Proctor-Siles, Inc, North Caroline,USA). Table 1 shows the initial characteristics ofthese fruit purees.

Microwave treatment

Samples (50g) of each fruit puree were placed in10cm diameter glass dishes to a depth of 2cm. Threesample dishes (50g) were heated in a 850W house-hold microwave oven (Toshiba ER-6860w, Japan).The power generator for this unit operates at a fre-quency of 2450MHz. Treatments applied to the fruitpuree were adjustable to the following values :285W, 570W and 850W for constant time (30s), orconstant power (475W) for 15s, 30s, 45 s and 60s.After treatment, the purees were placed in a suitablevessel and cooled in ice tap water for 5min andanalyses were carried out immediately. All experi-ments were carried out in triplicate.

Determination of colour data

Colour of treated fruit purees was measured in acylindrical sample cup, 5cm diameter ] 2cm highülled to the top, using the Tristimulus reýectancecolorimeter model HunterLab, model D25 (HunterAssociates Laboratory mod. 25-9, Reston, VA, USA)calibrated with a white tile No. C2-19913(X \ 82.51, Y \ 84.46, Z \ 101.44). Colour wasmeasured using the CIE L*a*b* colour values, whereL* value is a measure of lightness from completelyopaque (0) to completely transparent (100), a* is ameasure of redness ([a* greenness) and b* ofyellowness ([b* blueness). Hue angle (h) was calcu-lated from h \ arctan b*/a*(¡) and chroma (C) fromC \ [(a*)2] (b*)2]0.5. All measurements were donein triplicate (three diþerent dishes).

Biochemical analysis

The enzyme extracts for determination of POD andPPO were made by homogenisation of 10g of freshweight of each fruit puree sample with 20ml (or10ml for kiwi fruit samples) of 0.2M sodium phos-phate buþer (pH 7.0 for papaya and kiwi fruit or pH6.5 for strawberry) containing 40g litre~1 insolublepolyvinylpolypyrolidone (PVPP) (or 10g litre~1 forkiwi fruit) and 10ml litre~1 Triton X-100, in ultra-

Table 1. Initial characteris tics of fruits purees a

Characteris tic Kiwi Strawberry Papaya

Mois ture (g kgÉ1 FW) 810.00^ 0.10 925.50^ 0.15 859.60^ 0.17Total acidity (g anhydrous citric acid kgÉ1 FW) 14.50^ 0.12 9.40^ 0.18 1.00^ 0.14pH 3.39^ 0.16 3.40^ 0.22 6.41^ 0.34Soluble s olids (¡Brix) 17.10^ 0.23 7.73^ 0.51 12.80^ 0.18

a Values are average of three independent determinations ^ s tandard deviation h. FW fres h weight

664 J Sci Food Agric 79 :663–670 (1999)

Eþ ect of microwave heating on fruit colour

homogeniser (Omnimixer, mod ES-207, Omni Inter-national, Inc, Gainsville, VA, USA) with externalcooling, for 3min with stop intervals each 30s. Thehomogenate was centrifuged at 16000] g at 4¡C for15min. The supernatant was ültered through a nyloncloth and the volume of the ültrate measured.

POD activity was assayed spectrophotometricallyusing aliquots (0.025ml) of enzyme extract and areaction mixture composed by 2.7ml of 0.05M

sodium phosphate buþer (pH 6.5 for papaya andstrawberry and pH 7 for kiwi fruit) with 0.2ml, 10glitre~1 p-phenylenediamine as H-donor and 0.1ml15g litre~1 hydrogen peroxide as oxidant. The oxi-dation of p-phenylenediamine was measured with adouble beam spectrophotometer (Perkin Elmer, modLambda 15, Bodenseewerk, FRG) at 485nm and25¡C.

PPO activity was assayed using aliquots (0.075ml)of enzyme extract and 3.0ml of a solution of 0.07M

catechol (papaya and strawberry) or a solution of0.15M catechol (kiwi fruit) in 0.05M sodium phos-phate buþer (pH 6.5). The reaction was measure-ment at 420nm at 25¡C.

HPLC pigment analysis

ApparatusA Hewlett-Packard 1050 quaternary solvent deliveryunit equipped with a Hewlett-Packard 1040A rapidscanning UV-visible photodiode array detector wasemployed. The data were stored and processed witha Hewlett-Packard Model 9000/300 computingsystem and Colour Pro-plotter. The absorptionspectra of the pigments were recorded between 300and 600nm at the rate of 12 spectra min~1. TheHP-9000 computer with a built-in integrationprogram was used to evaluate the peak area and peakheight. Absorption spectra of isolated components invarious solvents were recorded on a Perkin ElmerLambda 15 UV-visible spectrophotometer(Bodenssewerk, FRG).

Separation was performed on a stainless steel(250] 4min id) Hypersil ODS (5lm sphericalparticles) column (Hewlett-Packard) protected withguard column cartridge (2cm length ] 4.0mm id)packed with ODS-Hypersil C18 (5lm particle size).

Chromatographic proceduresThe analytical separations of kiwi fruit and papayapigments were carried out according to the pro-cedure of Cano18 with minor modiücations forpapaya pigments.20 A gradient mixture of methanol/water (75 : 25v/v), eluent A, and ethyl acetate, eluentB, was used, beginning at time zero until time 10minwith a penultimate composition of eluent B (70% v).The gradient eluent composition was followed attime 10min until time 14min with the ünal composi-tion of eluent B (100% v). The ýow rate employedwas 1.7ml min~1 and the chromatographic runs weremonitored at 430nm (450nm for carotenoids). At theend of the gradient the column was re-equilibrated

under the initial conditions by new gradient condi-tions beginning at time 14min until time 20min witha ünal composition of eluent B (0% v) at the sameýow rate (1.7ml min~1).

Chromatographic conditions for anthocyaninanalysis were according to the procedure describedby Hong and Wrolstad.21 Solvent A was 4g litre~1phosphoric acid and solvent B was 100% acetonitrileand the program began with isocratic elution with6% v B from 0 to 10min, and then a linear gradientto 20% v of B from 10 to 55min, and ünally an iso-cratic elution at 20% v of B from 55 to 65min. Flowrate employed was 1.0ml min~1 and the runs weremonitored at 520nm.

Extraction of kiwi fruit pigmentsThe general method for extraction was described in aprevious study of kiwi fruit pigments.18 The pro-cedure consists of a chilled acetone extraction(200ml) of 50g kiwi fruit puree (adjusted to pH 8–9with 1–2g of sodium carbonate to prevent conversionof chlorophyll to pheophytin). After mixing in ahomogeniser (Omnimixer, mod ES-207, Onmi Inter-national, Inc, Gainsville, VA, USA) and centrifugingat 4000] g for 10min (0–5¡C), the supernatantswere collected and transferred to a separatory funneland diethyl ether (75ml) and cold deionised water(100ml) added. After vigorous shaking and standing,the aqueous layer was discarded. The washing pro-cedure was repeated üve times to remove acetone.The diethyl ether layer was dehydrated (anhydroussodium sulphate). The extracts were evaporatedunder a stream of and the residue dissolved inN25ml of chromatographic grade acetone. Duplicate20ml samples of each extract were injected forHPLC analysis. The separation and identiücation ofpigments were based on the chromatographic behav-iour with HPLC and TLC, visible absorptionspectra and speciüc chemical reactions, as reportedpreviously.18 The pigments were quantiüed in agiven sample by means of a calibration curve thatincluded all of the chlorophylls and carotenoids to beassessed in that sample using the HP-9000 computersystem. The curves were prepared daily by dilutingportions of the starting solutions to the appropriateproportions for the samples being analysed.

Extraction of strawberry pigmentsSample preparation and HPLC separation and iden-tiücation of anthocyanins was carried out accordingto the procedure described by Hong and Wrolstad.21Strawberry puree samples (10g) were homogenisedwith 100ml of 10g litre~1 HCl in methanol. Theslurry was ültered, and the solids were washed withan additional 100ml of 10g litre~1 HCl in methanol.The methanol extracts were combined and concen-trated to about 10ml in a rotary evaporator (30¡C).The aqueous extract was placed in a volumetric ýaskand made up to 50ml with 0.1g litre~1 HCl solution.The aqueous solution of anthocyanins (5ml) was

J Sci Food Agric 79 :663–670 (1999) 665

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adsorbed onto an activated C18 Sep-pack cartridge(Water Associates, Milford, MA)(activated with 3mlof methanol and 3ml of 0.01M HCl solution). Thepigments adsorbed onto the cartridge were elutedwith 0.1g litre~1 HCl in HPLC grade methanol.The solution was evaporated to dryness in a rotaryevaporator and the extractor was dissolved in 40glitre~1 phosphoric acid and ültered through a0.45mm ülter and 20ll injected into the HPLCsystem. All the analyses were performed in duplicate.The separation of anthocyanins was carried out byHPLC and the identiücation was performed by theanalysis of spectral properties by an on-linephotodiode-array detector.

Total pigment concentration was assessed usingpelargonidin-3-glucoside, previously separated, as anexternal standard. A standard curve was prepared byplotting diþerent concentrations of pelargonidin-3-glucoside versus area measurement in HPLC.

Extraction of papaya pigmentsThe extraction was carried out following the pro-cedure described by Cano et al.20 A mix of fruitsample (30g) and sodium sulphate and magnesiumcarbonate (200% and 10% of the weight of fruit,respectively), and tetrahydrofuran (THF) (100mlstabilised with BHT) were homogenised at 0¡C, intotal darkness and under nitrogen atmosphere. Aftera rigorous clean up, the extracts were evaporatedunder nitrogen and the residue dissolved in 0.3ml ofdichloromethane. Duplicate 25ll samples wereinjected for HPLC analysis.

The separation and identiücation were carried outfollowing the methodology previously reported19 inkiwi fruit. The carotenoids were separated byreversed-phased HPLC and peak purity was deter-mined by electronic spectra using the HP-9000 com-puter system. Carotenoids were identiüed accordingto their chromatographic behaviour on HPLC, TLCand UV-visible absorption spectra, by comparingboth their retention time and the absorption spectraobtained with those of authentic carotenoids pre-viously purchased or separated from fruits.18 Pig-ments functional group examination was carried outby speciüc chemical reactions. The carotenoidsseparated were quantiüed by HPLC using Sudan 1as internal standard and calculated as b-caroteneequivalent using a response factor of 0.0791, in aprocedure similar to that of Philip and Chen.22 Theresponse factor, f, was determined by injectingknown mixtures of b-carotene and Sudan 1 into theHPLC and measuring the integrator area responsesat 450nm ( f \ conc Sudan 1] area b-carotene/areaSudan 1] conc b-carotene).

Data analysis

Statistical analyses were performed using the soft-ware InStatTM. Data were analysed statistically byanalysis of variance (ANOVA) and using Student’st-test.

RESULTS AND DISCUSSION

Effect of microwave treatment on peroxidase activity

Figs 1 and 2 show the residual POD activityobtained from microwave treated fruit puree. Micro-wave heating for 30s produced almost linear inac-tivation of POD in kiwi fruit puree. In this product,application of a microwave treatment at 850W pro-duced near to 60% inactivation of POD if the treat-ment was performed during 30s. POD activity instrawberry, however, seemed to be more resistant tomicrowave inactivation and only a heat treatment at850W/30s rendered a signiücant (p ¹ 0.05) loss ofPOD activity (B 8%), Fig 1. Papaya puree alsoexhibited a signiücant loss of POD activity when amicrowave treatment at 570W/30s was applied, butno better results were obtained by increasing thepower output to 850W.

When the microwave treatments were applied atüxed power (475W) with various times of exposure,similar results could be obtained for POD activity(Fig 2). Papaya POD activity was most eþectivelyinactivated when treatment time was increased.Application of microwaves at 475W/45s produced aB 75% inactivation of peroxidase activity and,

Figure 1. Effect of microwave treatments at fixed time (30 s ) on

peroxidas e (POD) activity of fruit purees .

Figure 2. Effect of microwave treatments at fixed power (475W)

on peroxidas e (POD) of fruit purees .

666 J Sci Food Agric 79 :663–670 (1999)

Eþ ect of microwave heating on fruit colour

beyond this exposure time, no signiücant increase ofenzyme inactivation was observed. Peroxidase activ-ity in kiwi fruit puree was eþectively lost by micro-wave treatments at 475W from 45s of exposure time.Prolonged treatments (60s) did not increase thePOD inactivation.

Effects of microwave treatments on polyphenol

oxidase activity

PPO activity changes in microwaved fruit purees areshown in Figs 3 and 4. PPO activity of papaya pureewas most eþectively inactivated by microwaveheating. Power of 570W for 30s was enough toproduce a 90% loss of PPO activity. This result wassimilar to that for POD activity in this product (Fig1), but an evident and greater microwave resistanceof peroxidase could be observed when the same con-ditions of treatment were applied.

Kiwi fruit PPO activity was most resistant tomicrowave heating. In this product, a slight decreaseof PPO activity could be obtained (18%, 850W/30s,Fig 3), but POD activity in kiwi fruit puree was effi-ciently inactivated by treatments from 570W/30s.

Strawberry PPO showed a 50% loss of activityusing a mild microwave treatment (285W/30s), but

Figure 3. Effect of microwave treatments at fixed time (30 s ) on

polyphenol oxidas e (PPO) activity of fruit purees .

Figure 4. Effect of microwave treatments at fixed power (475W)

on polyphenol oxidas e (PPO) activity of fruit purees .

this eþectiveness was not maintained when thepower was raised to 570W/30s. This fact cannot beeasily explained by a simple protein denaturationmechanism; other eþects such as thermal rupture ofcell organules, which liberates enzyme fractionslinked to celular membranes, could result in thishigher value of residual PPO activity in strawberrypuree treated at intermediate power.

Fig 4 represents the residual PPO activity ofmicrowave treated purees at preüxed power (475W).Papaya PPO appeared to be the most labile in theseconditions. This product lost all PPO activity bytreatment at 475W for 45s or more. Again, the lowerstability of oxidoreductases, PPO and POD, wasfound in papaya tissues.

Kiwi fruit and strawberry purees exhibited thesame inactivation of PPO, B 32% at 475W/30s, andB 70% at 475W/60s. The reduction of PPO activityin these products by microwave heating was almostlinear in these experiments, indicating that theprocess was better controlled by pre-üxing the powerthan the time of exposure.

Effects of microwave treatment on fruit pigment

composition

Changes of total pigment concentration duringmicrowave blanching are shown in Table 2. Thecarotenoid composition of Spanish papaya (cvSunrise) had previously been studied.20 The majorcarotenoids found in papaya extracts were lycopeneand the fatty acid esters of b-cryptoxanthin and b-cryptoxanthin-5,6-epoxide. Qualitative carotenoidcomposition analysed by HPLC was unchanged aftermicrowave treatment, however, microwave treatmentinduced the degradative loss of total carotenoidcontent (Table 2). Treatment at 475W for 45sshowed the greatest loss of the total carotenoid (57%)in this fruit puree.

In contrast, during microwave heating there wasno signiücant change in total anthocyanin concentra-tion (Table 2); the anthocyanin pattern of strawberrywas almost unchanged by microwave heating. Otherauthors had reported an increase of the total antho-cyanin contents in microwaved strawberry productscompared with controls.17 They attributed this phe-nomenon to enzymatic action during the process.The increase in anthocyanins content observed insome microwaved treated samples could be explainedby a more efficient extraction of these compoundsfrom the tissue due to cellular disruption producedby microwave heating.

Microwave heating of kiwi purees produced a sig-niücant decrease of chlorophyll a and b concentra-tion due to chlorophyll degradation throughtransformation to chlorophyllide a, pheophorbide aand pheophytin b (Table 2). The more drastic treat-ments, 850W for 30s and 474W for 60s, tested inour work produced an almost total loss of chlo-rophyll b, which is the least heat-stable chlorophyll.The kiwi puree blanched at 475W for 60s showed

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B de Ancos et al

Table 2. Effects of microwave treatment on fruit puree pigment concentration (mg kgÉ1 puree)

Pigments * Microwave treatment

Control t\ 30s P\ 475W

285W 570W 850W 15 s 30 s 45 s 60 s

Papaya

Total carotenoids ¹ 10.5^ 0.2a 7.4^ 0.3a 10.8^ 0.8a 6.0^ 1.2a 8.7^ 0.4a 7.4^ 0.5a 4.5^ 0.6a 8.9^ 0.6a

Strawberry

Total anthocyaninº 297^ 0.5a 282^ 0.3a 301^ 0.5a 303^ 0.5a 321^ 0.6a 312^ 0.4a 332^ 0.6a 314^ 0.2a

Kiwi

Chlorophyll a 14.0^ 0.3a 3.5^ 0.2b 10.3^ 0.5c 13.6^ 0.3a 8.9^ 0.4c 4.0^ 0.4b 5.9^ 0.1b 5.2^ 0.5bChlorophyll b 7.6^ 0.2a 1.1^ 0.1b 1.7^ 0.1b 0.1^ 0.2c 0.8^ 0.3b 2.8^ 0.3d 1.1^ 0.2b 0.5^ 0.4bXanthophylls 6.5^ 0.3a 5.6^ 0.4a 4.1^ 0.7a 6.1^ 0.4a 3.9^ 0.6ab 2.5^ 0.4b 2.0^ 0.2b 1.8^ 0.1bPheophytin a 4.8^ 0.2a 3.1^ 0.2a 2.7^ 0.5a 3.1^ 0.2a 2.7^ 0.4a 1.2^ 0.7ab 0.3^ 0.5b 0.3^ 0.4ab-carotene 0.4^ 0.1a 0.2^ 0.3a 0.3^ 0.4a 0.2^ 0.4a 0.1^ 0.6a 0.1^ 0.4a 0.1^ 0.5a 0.1^ 0.5aChlorophyllida a nd 0.1^ 0.4a 0.3^ 1.1a 0.5^ 0.4a 1.0^ 0.6a 0.4^ 0.3a 0.2^ 0.3a 0.5^ 0.4aPheophorbide a nd 0.1^ 0.5a 0.1^ 0.9a 0.2^ 0.2a 0.3^ 0.1a 0.2^ 0.5a 0.1^ 0.2a nd

Pheophytin b nd nd nd nd nd 0.3^ 0.2a 0.2^ 0.4a 0.15^ 0.6a

* Means and s tandard deviations of triplicate determinations . Values in the s ame row with the s ame letter are not s tatis tically different(p\ 0.05)¹ Concentration calculated by HPLC as b-carotene equivalentsº Concentration calculated by HPLC as pelargonidin-3-glucos ide equivalents

the most notable pigment losses. This sample suf-fered losses of 72% of xanthophylls, 84% of chlo-rophyll a and 75% of b-carotene. Coincident withthe thermal degradation of pigments, other mecha-nisms of chlorophyll modiücation could haveoccurred in the kiwi fruit purees, such as enzymaticreactions that result in bleaching of fruit tissue. Infact, kiwi puree blanching at 475W for 60s retainsapproximately 60% of the peroxidase activity andB 35% of the polyphenol oxidase activity comparedwith the control.

Effect of microwave treatment on fruit puree

objective colour

Eþects of microwave blanching on fruit puree objec-tive colour are shown in Tables 3, 4 and 5. Micro-

wave blanching at 285W for 30s produced thelowest total colour diþerence (*E*) in the three fruitpurees. Total colour diþerence parameter combinesL* (luminosity), a* (greenness-redness) and b*(blueness-yellowness) parameters by integratingthese three characteristics in order to compare thecolour of control and puree samples microwaved atdiþerent conditions. The *E* value increases whenmicrowave power increases. Microwave blanchingdid not signiücantly aþect the papaya CIE L*a*b*colour values (Table 3). Only a minor decrease of a*value and slight increase of b* value were observed,showing that papaya treated purees tend to be lessred and slightly more yellow than untreated ones.

Total colour diþerence (*E*) calculated for micro-waved strawberry puree showed signiücant diþer-

Table 3. Effects of microwave treatment on papaya puree colour (CIE) meas urements

Colour values * Microwave treatment

Control t\ 30s P\ 475W

285W 570W 850W 15 s 30 s 45 s 60 s

L* 32.30a 33.43a 35.04a 33.43a 33.77a 33.99a 33.65a 32.31a

a* 13.52a 13.81a 12.68a 11.16a 12.61a 11.27a 12.13a 10.97a

b* 37.33a 39.42a 41.10a 38.58a 40.01a 39.57a 37.58a 35.01a

Hue angle 70.09a 70.69a 72.85a 73.87a 72.51a 74.10a 72.11a 72.60a

Chroma 39.70a 41.77a 43.01b 40.16a 41.95a 41.14a 39.49a 36.69a

*E* 2.39a 4.73b 2.90a 3.18a 3.59a 1.95a 3.45a

* Values in the s ame row with the s ame letter are not s tatis tically different (p¹0.05)h\ arctan (b*/a*) (hue angle). (total colour difference)*E* \ [(L

1* [L

2*)2 ] (a

1* [a

2*)2 ] (b

1* [b

2*)2]1@2

Chroma\ (a*2 ]b*2)1@2

668 J Sci Food Agric 79 :663–670 (1999)

Eþ ect of microwave heating on fruit colour

Table 4. Effects of microwave treatment on s trawberry puree colour (CIE) meas urements

Colour values * Microwave treatment

Control t\ 30s P\ 475W

285W 570W 850W 15 s 30 s 45 s 60 s

L* 30.15a 30.77a 32.77a 35.31b 33.46a 33.50a 36.05b 36.45b

a* 29.13a 29.70a 29.28a 28.89a 28.70a 29.81a 29.48a 29.21a

b* 21.20a 22.08a 22.07a 21.86a 20.17a 21.89a 22.35a 21.05a

Hue angle 36.02a 36.63a 37.01a 37.11a 35.06a 36.28a 37.17a 35.79a

Chroma 26.13a 27.87a 27.34a 26.54a 27.69a 29.25b 27.92a 26.99a

*E 1.26a 2.84a 5.22b 3.73b 3.48b 6.03b 6.30b

* Values in the s ame row with the s ame letter are not s tatis tically different (p¹ 0.05)

h\ arctan (b*/a*) (hue angle). (total colour difference)*E* \ [(L1* [L

2*)] (a

1* [a

2*)] (b

1* [b

2*)]1@2

Chroma\ (a*2 ]b*2)1@2

ences as the severity of heating increased (Table 4).There were no signiücant diþerences between a* andb* values of treated strawberry puree compared withthe untreated one. L* values ranged from 30.15 forthe untreated sample to 35.31, 36.05 and 36.45 forthe more drastically heated samples (850W/30s,475W/45s and 475W/60s, respectively); the CIEL*a*b* colour values underwent more modiücation.There were no good correlation between the antho-cyanin concentration of the blanched fruit pureesand the a* and b* colour values ; although the L*value expresses the darkness of the samples, therewas only a moderate correlation between the concen-tration of anthocyanins and L* value (r2\ 0.595).The slight browning observing with strawberrypuree could be due to slight inactivation of peroxi-dase and polyphenol oxidase activity during themicrowave treatments tested (Table 2).

The changes in objective colour parameters of kiwipurees after the various microwaved treatments areshowed in Table 5. The total colour diþerence (*E*)continuously increased when microwave powerincreased. This change could be related to the lossesof chlorophylls and total xanthophylls during themicrowave blanching. Also, the changes in other

objective colour parameters, such as the angle h, a*and b* values, after microwave treatment couldexplain the losses or breakdown of these two kinds ofpigments. The colour parameters a* (greenness) andL* (lightness) increased after almost all the assayedmicrowaved treatments and this fact was related tothe less green and lower lightness of the resultantkiwi purees, when compared with the control. Corre-lation coefficients of r2\ 0.8913 (treatment during30s/ diþerent powers) and r2\ 0.9620 (treatment at475W/ diþerent times) were obtained for the ratiochlorophyll a*/ Chroma. The highest correlationbetween the most important chemical class of pig-ments in this fruit and CIE L*a*b* colour parameter(Chroma) reýected the greater exterior browning ofkiwi fruit products compared with the control.

CONCLUSION

Microwave heating could be an eþective treatment toinactivate oxidoreductases enzymes (PPO and POD)in papaya, strawberry and kiwi fruit purees prior tostorage or use in various industrial processes. Micro-wave heating produced small modiücations of thequalitative and quantitative composition of carot-enoids (papaya) and anthocyanins (strawberry)

Table 5. Effects of microwave treatment on kiwi fruit puree colour (CIE) meas urements

Colour values * Microwave treatment

Control t\ 30s P\ 475W

285W 570W 850W 15 s 30 s 45 s 60 s

L* 36.01a 36.39a 37.77a 40.07b 40.23b 38.52a 41.04b 40.84b

a* [ 12.35a [ 13.31a [ 12.85a [ 11.99a [ 12.19a [ 11.72a [ 10.78a [ 11.17b* 23.03a 24.48a 24.13a 23.68a 24.84a 26.80a 25.76a 24.55a

Hue angle [ 61.77a [ 61.45a [ 61.96a [ 63.13a [ 63.75a [ 66.38b [ 67.28b [ 65.59bChroma 26.13a 27.88a 27.34a 26.54a 27.69a 29.25a 27.93a 26.99a

*E 1.81a 2.28a 4.13b 5.02b 4.60b 5.94b 5.32b

* Values in the s ame row with the s ame letter are not s tatis tically different (p¹ 0.05)

h\ arctan (b*/a*) (hue angle). (total colour difference)*E* \ [(L1* [L

2*)2 ] (a

1* [a

2*)2 ] (b

1* [b

2*)2]1@2

Chroma\ (a*2 ]b*2)1@2

J Sci Food Agric 79 :663–670 (1999) 669

B de Ancos et al

without important changes of the original colour ofthe fruit purees. By selecting the microwave blanch-ing conditions it is possible to produce a moderatedegradation of chlorophylls and a slight loss of thebright green colour of the kiwi fruit puree.

ACKNOWLEDGEMENT

This research was supported by the Comisio� n Inter-ministerial de Ciencia y Tecnolog•�a through theproject numbers ALI94-1442 and ALI95-0105.

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