Food Chemistry 123 (2010) 92–98

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Food Chemistry

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Effect of osmotic pre-treatment and microwave heating on lycopenedegradation and isomerization in cherry tomato

A. Heredia *, I. Peinado, E. Rosa, A. AndrésInstitute of Food Engineering for Development, Polytechnic University of Valencia, Camino de Vera s/n, P.O. Box 46022, Valencia, Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 December 2009Received in revised form 29 March 2010Accepted 7 April 2010

Keywords:TomatoHot air-microwave dryingLycopeneHPLCColour

0308-8146/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.foodchem.2010.04.005

* Corresponding author. Tel.: +34 963877676; fax:E-mail address: anhegu@tal.upv.es (A. Heredia).

Cherry tomato were dehydrated by a combination of different techniques (osmotic dehydration, convec-tive drying, and microwaves assisted air drying) in order to evaluate the effect of the process variables onthe degradation and isomerization of lycopene, as well as on the optical properties. Specifically, the effectof prior osmotic treatment, air drying temperature (40, 55, and 80 �C) and level of microwave energy (0, 1,and 3 W/g) were studied. Obtained results showed that the osmotic pre-treatment limited the isomeri-zation during the later stage of drying, whereas both the loss of total lycopene and the trans–cis isomer-ization, mainly to the 13-cis form, were favored by an increase in temperature and the microwave power.Furthermore, a positive correlation between the degree of isomerization experienced by the samples dur-ing drying and the hue (h*) were obtained. This correlation was reflected in the colour of the sample withpredominantly more orange tones and less reddish ones.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Lycopene, the pigment responsible for the red colour of toma-toes, is an open chain hydrocarbon (C40). Hence, the rotation ofany of its 11 conjugated double bonds enables the formation ofgeometric cis isomers, which may have implications in thefunctional properties of this carotenoid. Although the potentiallybeneficial effects of lycopene on human health have been knownsince the 1950s (Lingen, Ernster, & Lindberg, 1959), the numberof scientific studies began to be significant when the consumptionof fresh or processed tomato started to be associated, through epi-demiological studies, to a reduction in the incidence of certaindegenerative diseases among them some types of cancer, and espe-cially the prostate one (Giovanucci et al., 1995; Cao, Sofic, & Prior,1996; Liu et al., 2000). In addition, studies carried out with humancells and animal tissue have enabled the identification of the generesponsible for extracellular communication, called connexin43gene. This gene is regulated by the concentration of lycopene inblood, causing a deficient cellular communication at a low concen-tration, thereby favoring the proliferation of carcinogenic cells(Heber & Lu, 2002; Zhang & Hamauzu, 2004).

Due to its high regular consumption, tomato is an importantsource of antioxidant compounds (Raffo, La Malfa, Fogliano,Maiani, & Quaglia, 2006), among which lycopene is particularlynoteworthy as this fruit is, along with the watermelon, the only

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major sources of this component in nature. In fresh tomato, 90%of the lycopene is found in its trans isomeric form. During indus-trial processing of tomato, oxidation and/or trans–cis isomerizationof the lycopene take place. The increase in the bioavailability oflycopene as a consequence of thermal processing is related to thebiochemical properties of the cis isomers. Compared to the transisomers, the cis ones have a lower tendency to aggregate and crys-tallize and greater solubility in lipid media (Boileau, Merchen,Wasson, Atkinson, & Erdman, 1999; Stahl & Sies, 1996). On theother hand, they are less stable when exposed to possible oxidationcaused by external agents such as the presence of light, oxygen,and oxidation reaction catalysts (Boskovic, 1979; Shi, Le Maguer,Kakuda, & Lipaty, 1999). Different epidemiological studies statethat after consuming processed tomato products (cis isomers),the lycopene levels in blood and in different organs (colon,prostate, etc.) increase more quickly than after consuming freshor minimally-processed tomato (Gartner, Stahl, & Sies, 1997;Giovanucci et al., 1995).

Studies published on digestion, absorption, and excretion ofdifferent lycopene isomers have not clarified the mechanismsresponsible for these physiological functions. Nevertheless, certainstudies do coincide in that when the lycopene is ingested in cisform, this is released from the matrix, goes onto the cell mucosaand is absorbed by the lymphatic system more easily than whenit is consumed in trans form (Schierle et al., 1996; Boileau et al.,1999; Gartner et al., 1997). Moreover, there is an hypothesis that,since the lycopene in its trans form is more stable, it is stored bythe organism where the isomerization occurs to balance the trans

A. Heredia et al. / Food Chemistry 123 (2010) 92–98 93

and cis forms in the organs, blood, etc. (Re, Fraser, Long, Brameley,& Rice-Evans, 2001).

Tomato is processed in different ways, dried in halves, slices, orquarters (structured food), or transformed into purée (unstruc-tured food). It is therefore an ingredient for prepared or formulatedfoods such as pizzas, sauces, soups, and juices (Toor & Savage,2006). The effect of both temperature and processing time on thechanges undergone by the lycopene has been widely studied, withvarying results. Whereas Zanoni, Pagliarini, and Foschino (2000)and Takeoka et al. (2001) published significant losses of lycopeneduring tomato drying at 110 �C and during the production ofconcentrated purée respectively, other studies emphasized theefficiency of thermal treatment as a method for obtaining stableprocessed tomato, increasing the bioavailability of the lycopenecompound by fostering its trans–cis isomerization (Dewanto, Wu,Adom, & Liu, 2002; Kerkhofs, 2003; Mayeaux, Xu, King, &Prinyawiwatkul, 2006). In this sense, Toor and Savage (2006)pointed out the possibility of preserving the lycopene by meansof applying hot air drying at 42 �C with the aim of obtaining aproduct with a 70% moisture content.

On the other hand, colour is one of the main factors of quality intomato since it significantly affects consumers’ decision to buy(Gould, 1992). During drying, the typical red colour of fresh tomatogradually changes towards more brownish hues or intense reddepending on the mechanisms involved. This change occurs mainlyas a result of the changes that lycopene undergoes, the oxidation ofthe ascorbic acid and enzymatic and non-enzymatic browningreactions (Gould, 1992).

Due to the changes suffered by the product as a consequence ofbeing exposed to high temperatures and long processing timesmentioned above, the food industry tends to use microwaveenergy in order to reduce drying times. Whilst traditional dryingexposes the product to high temperatures and long processingtimes, the combination of hot air and microwaves could reducedegradation in the nutritional compounds (Vadivambal & Jayas,2007). Moreover, submitting the product to an osmotic dehydra-tion prior to the drying process may help to preserve its organolep-tic characteristics such as colour and nutritional properties, as wellas to reduce the subsequent drying time. Thus, it would be inter-esting to study in more detail the influence of process variableson dried tomato products through a combination of osmotic dehy-dration pre-treatment followed by a microwave assisted air dryingprocess.

The aim of this work was to study the effect of different processvariables (the type of the osmotic pre-treatment, drying air tem-perature, and microwave power) on lycopene changes as well ason the optical properties of dry cherry tomato.

2. Materials and methods

2.1. Raw material

Cherry tomato (Solanum lycopersicum L. var. cerasiform cv.Cocktail) was used as the raw material in this study. The raw mate-rial was acquired in a local supermarket but always from the samesupplier that has three controlled production areas in differentareas of Spain. After carrying out a visual selection by colour, size,and absence of physical damage to ensure maximum homogeneity,the tomatoes were cleaned and cut in halves along the equatorialzone. The raw material was not processed whole due to the waxynature of its epidermis, which constitutes a barrier to mass transferprocesses (water and solutes) provided that it was not subjected toany pre-treatment to increase its permeability (physical or chemi-cal peeling) (Shi et al., 1999). On the other hand, since the greater

lycopene content is located near the skin (Shi et al., 1999), peelingthe fruit was not deemed suitable.

2.2. Experimental methodology

Fresh tomato cherry halves were osmotically treated before themicrowave assisted air drying step. Three different osmotic pre-treatments were tested: 55% sucrose solution (w/w) at 30 �C,120 min (OD1); ternary solution of 27.5% sucrose + 10% NaCl(w/w), 40 �C, 60 min (OD2); and 42.1% sucrose + 5% NaCl (w/w),30 �C, 210 min (OD3). The conditions of the different osmotictreatments were selected according to a previous study on theinfluence of the osmotic dehydration variables on the lycopeneand b-carotene content in tomato cherry halves (Heredia, Peinado,Barrera, & Andrés, 2009).

The drying experiments were performed in a specially designedhot air-microwave oven equipped with continuous output-powermicrowave energy as described by Andrés, Bilbao, and Fito(2004). This equipment allowed microwave power, oven tempera-ture, and air velocity to be controlled. Microwave power levels wereset at 0, 1, and 3 W/g combined with hot air (40, 55, and 80 �C) atthe velocity of 1.6 ± 0.2 m/s. For each experiment, samples of halvescherry tomato were distributed in a horizontal tray avoiding anycontact among them. The drying process took as long as necessaryfor the samples to reach a final moisture content of 60% (w/w). Thismoisture level was established based on the moisture found incommercial products of tinned dry tomato in oil. At the end of eachexperiment, moisture content was determined gravimetrically bymeans of a vacuum oven at 60 �C according to the AOAC methodno. 934.06 (AOAC, 1990). The analysis of the lycopene isomersand the colour measurements were carried out on fresh, osmoti-cally dehydrated samples and at the end of the drying stage.

2.3. HPLC analysis of lycopene

2.3.1. Lycopene extractionThe lycopene was extracted following the modified protocol of

Mayeaux et al. (2006), according to which 6 ml of methanol, ace-tone, and hexane (1:1:1 (v/v/v)) were added to a glass tube con-taining a sample of 0.5 g of fresh or reconstituted dehydratedtomato. The tomato cherry samples obtained after applying thedrying stage were reconstituted by adding 1 ml of bidistilled waterto 1 g of mass of the sample. After adding the dissolvent mixture,each tube was shaken in a vortex mixer for 30 s and immediatelyafterwards they continued to be shaken horizontally for 30 min.During these 30 min, the tubes were shaken every 10 min for1 min in the vortex mixer in order to encourage even more extrac-tion and obtain a colourless residue. After this, 2 ml of bidistilledwater was added to each tube, and these were shaken for 1 minin the vortex mixer in order to separate the hydrosoluble and lypo-soluble phases adequately. Next, 1 ml of the non-polar phase thatcontained the carotenoid pigments, and therefore the lycopene,was transferred to the HPLC vials after being filtered with0.22 lm nylon filters. After filtering, the sample (20 ll) was in-jected into the HPLC apparatus in order to begin determining thelycopene isomers present in the fresh and processed cherry tomatosamples. Lycopene extractions were carried out in darkness.

2.3.2. EquipmentLycopene isomers in the fresh and processed tomato cherry

samples were determined by means of high performance liquidchromatography (HPLC) with a C30 reversed phase column, follow-ing the determination protocol published by Qiu, Jiang, Wang, andGao (2006). The high performance liquid chromatography equip-ment consists of a separations module made up of the pump andinjector (Waters 2695 mod. Alliance) and a photodiode array

Table 1Percentage of residual trans isomer and residual total lycopene in cherry tomatosamples after osmotic dehydration and hot-air-microwave drying (n = 2).

Osmoticdehydration

Hot airT (�C)

Power(W/g)

Residual trans-lycopene (%)

Residual totallycopene (%)

OD1 – – 82 (0.9) 93.3 (0.2)OD2 – – 149.322 (0.102) 169.2 (0.9)OD3 – – 78.8 (0.8) 90.2 (2.3)– 40 0 74 (13) 90 (17)OD1 145 (10) 155 (10)– 55 0 105.9 (0.4) 122.6 (3.8)OD1 52 (3) 63 (5)OD2 75 (3) 89 (6)OD3 54 (3) 64 (23)– 80 0 49 (4) 68.8 (0.2)OD1 116 (2) 130.1 (1.6)– 40 1 90.5 (0.5) 113 (6)OD1 61.41 (0.02) 69.3 (0.3)OD2 104.1 (3) 119 (4)OD3 32.2 (0.8) 42.1 (1.3)– 55 1 112 (6) 136 (5)OD1 32 (2) 36 (2)OD2 110 (6) 156 (3)OD3 114 (7) 128 (11)– 80 1 62 (17) 71 (10)OD1 112 (8) 125 (10)OD2 81.7 (1.7) 98 (4)OD3 24.28 (0.027) 36.50 (0.06)– 40 3 60 (9) 74 (9)OD1 23 (3) 36 (4)– 55 3 67 (8) 85 (10)OD1 33 (2) 41.1 (1.6)OD2 82 (2) 98.3 (3)

94 A. Heredia et al. / Food Chemistry 123 (2010) 92–98

detector (Waters 2996). The stationary phase was a Waters C30

packed column of 3 lm diameter particles (YMCTM modelCarotenoid S-5 4.6 � 250 mm). Isocratic working conditionswere used with a mobile phase consisting of methanol, methyl-tert-butyl ether and ethyl acetate (50:40:10 (v/v/v)) at a flow rateof 1.5 ml/min. The injection volume was 20 ll and the reading wascarried out at 472 nm after confirming maximum absorbance ofthe all-trans-lycopene standard (Sigma Chemicals) in the workingconditions. The determinations were made at controlled tempera-ture for the column of 24 ± 1 �C and in darkness.

2.3.3. Identification and quantification of lycopene isomersThe identification of trans and cis isomers of lycopene was carried

out based on the retention times of these compounds obtained byother authors who used the same column and mobile phase andaccording to the Q-ratio appearing for each isomer (Lee & Chen,2001; Qiu et al., 2006; Schierle et al., 1996). Trans-lycopene quanti-fication in samples was achieved by calibration curve (from 4.75 to60 mg/l) obtained with authentic standard of lycopene (all-trans,purity > 90%) from Sigma Chemicals. Trans-lycopene content offresh and treated samples was referred to 1 g of fresh tomato, asmg/fresh g, in order to avoid the effect of solids concentration duringdrying in lycopene results. For these calculations, the total mass lossresulting from water outflow from the tomato tissue to the osmoticsolution and soluble solids inflow from the osmotic solution to thetomato tissue during osmotic dehydration and from water outflowduring hot air-microwave drying air was taken into account.

Cis isomers were estimated from peak area value variation ofeach isomer in the samples referred to 1 g of fresh tomato (cis iso-mer relative area/fresh).

Percentage of residual total or trans-lycopene were calculatedas follows (Eq. (I)):

Residual trans or total lycopene ð%Þ ¼ ðxf =x0Þ � 100 ðIÞ

being:x0 = content at initial time (mg of trans-lycopene/fresh g);

xf = content after treatment (mg of trans-lycopene/fresh g) forresidual trans-lycopene estimation; and x0 =

Ppeak area value of

isomers at initial time/fresh g; xf = peak area value of isomers aftertreatment/fresh g, for residual total lycopene estimation.

Also, the isomerization from trans to cis experienced by thesamples during the processing operations and the percentage ofeach isomer in the processed samples was calculated.

2.4. Determination of the coordinates of the CIEL*a*b* colour-space

The coordinates of the CIEL*a*b* colour-space wereobtained from the absorption spectrum provided by Perkin–ElmerUV–visible emission spectrophotometer (mod. Lambda 25)between 380 and 770 nm for reflectance with a reference system,D65 illuminant and observer angle of 10�, and with a 7 mm lens.Due to the absence of translucidity of the samples, the analysiswas carried out on the epithelial zone on black background.

The chroma (C*) (Eq. (II)) related to the saturation or purity ofthe colour was calculated, and the hue (h*) (Eq. (III)) related tothe predominance of a* (red-green) or b* (yellow-green).

C� ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiða�Þ2 þ ðb�Þ2

qðIIÞ

h� ¼ arctgb�

a�ðIIIÞ

OD3 33.6 (1.4) 50.3 (0.6)– 80 3 14.4 (0.6) 33 (4)OD1 17 (4) 26 (6)

Values in parentheses: standard deviation; OD1: 55% sucrose (w/w), 30 �C,120 min; OD2: 27.5% sucrose + 10% NaCl (w/w), 40 �C, 60 min; OD3: 42.1%sucrose + 5% NaCl (w/w), 30 �C, 210 min.

2.5. Statistical analysis

The statistical analyses of the variance (factorial ANOVA) with aconfidence level of 95% (p-value 6 0.05) were carried out by means

of the software package Statgraphics Plus 5.1 to estimate thesignificant effect of the different variables of the process (osmoticpre-treatment, drying air temperature, and microwaves power).Furthermore, a principal component analysis (PCA) was applied.

3. Results and discussion

3.1. Influence of the process variables on lycopene changes

The chromatogram of cherry tomato samples showed the pres-ence of all-trans, 13-cis, 9-cis, 5-cis isomers in the samples. Peak 1(RT 5.5 min) corresponded to the 13-cis isomer, peak 2 (RT 12 min)to the 9-cis, peak 3 (RT 20 min) to the 5-cis, and peak 4 (RT 32 min)to the all-trans-lycopene.

Fresh cherry tomato presented trans-lycopene content of 3.3(0.5) mg/fresh g. Cis isomers were also found to be present at11%, with 9.5% corresponding to the 13-cis isomer (Nguyen &Schwartz, 1998; Shi et al., 1999).

After the osmotic dehydration (Table 1), samples treated withternary solution (10% NaCl and 27.5% sucrose at 40 �C (OD2)showed greater lycopene content than the fresh cherry tomato.These conditions were reported to be especially favorable topreserve or increase the lycopene content. This fact has beenpreviously pointed out in a previous research carried out by thesame authors (Heredia et al., 2009). In this paper, lycopene andb-carotene changes during osmotic dehydration were studied indepth. It was observed an increase in the lycopene content whenthe 55% sucrose (w/w) solution or the ternary solution of 10% NaCland 27.5% sucrose at 40 �C (w/w) at 30 �C and 40 �C during the ini-tial period of the process (until 90 min). This phenomenon could beattributed to the optimal temperature of synthesis of lycopene

Table 2Degree of trans–cis isomerization (%) experienced by cherry tomato samples during the process and lycopene isomers distribution (%) (n = 2).

Osmotic dehydration Hot air T (�C) Power (W/g) trans–cis isomerization (%) trans isomer (%) 13-cis isomer (%) Other cis isomers (%)

Fresh – – – 89.1 (1.3) 9.2 (1.3) 1.65 (0.02)OD1 – – 11.2 (0.7) 77.9 (0.7) 14.9 (0.4) 7.24 (1.16)OD2 – – 10.5 (0.4) 78.6 (0.4) 13.32 (0.08) 8.1 (0.4)OD3 – – 11.4 (1.6) 77.7 (1.6) 20.2 (1.3) 2.1 (3)– 40 0 15.6 (0.4) 73.47 (1.14) 21.7 (0.3) 4.8 (1.5)OD1 5.3 (0.3) 83.8 (0.3) 10.5 (0.2) 5.72 (0.05)– 55 0 12 (3) 77 (3) 21.22 (0.09) 3.873 (–)OD1 14.9 (1.4) 74.2 (1.4) 19.3 (0.9) 6.4 (2.3)OD2 14 (2) 75 (2) 22.5 (1.5) 5.472 (–)OD3 14.3 (0.4) 74.8 (0.4) 25.2 (0.4) 0– 80 0 27 (4) 62 (4) 34.8 (0.3) 6.001 (–)OD1 9.5 (0.7) 79.6 (0.7) 13.9 (0.5) 6.5 (0.2)– 40 1 17.6 (1.4) 81 (16) 20.44 (0.17) 8.0 (1.6)OD1 10.58 (0.35) 78.5 (0.3) 17.4 (0.6) 4.1 (0.2)OD2 11.42 (0.69) 77.7 (0.7) 17.3 (0.7) 5.0 (1.4)OD3 22. 3 (0.3) 66.8 (0.3) 33.1 (0.3) 0– 55 1 16.2 (1.2) 82 (16) 15 (0.2) 11.8 (1.4)OD1 11.01 (1.04) 78.09 (1.03) 21.91 (1.04) 0OD2 2.06 (0.43) 87.0 (0.4) 12.9 (0.4) 0OD3 9.6 (1.8) 79.5 (1.7) 15.2 (0.7) 5 (2)– 80 1 13.06 (1.02) 76.04 (1.02) 23.95 (1.02) 0OD1 8.7 (1.5) 80.4 (1.5) 11.1 (0.7) 8 (2)OD2 16.56 (1.08) 72.55 (1.08) 16.7 (0.7) 10.7 (1.8)OD3 31.8 (0.3) 57.2 (0.3) 42.8 (0.3) 0– 40 3 18 (3) 71 (3) 23 (2) 6,0 (0.7)OD1 19.08 (0.16) 70.02 (0.16) 22.1 (1.8) 7.8 (1.9)– 55 3 19.08 (0.16) 70.02 (0.16) 22.1 (1.8) 7.8 (1.9)OD1 21 (2) 68 (2) 32 (6) 0OD2 14.9 (0.6) 74.2 (0.6) 15.6 (0.5) 10.21 (0.09)OD3 30 (3) 59 (3) 31.16 (0.04) 10 (3)– 80 3 52 (3) 37 (3) 58 (4) 9.891 (–)OD1 30.8 (0.9) 58.3 (0.9) 38 (4) 7.686 (–)

Values in parentheses: standard deviation; OD1: 55% sucrose (w/w), 30 �C, 120 min; OD2: 27.5% sucrose + 10% NaCl (w/w), 40 �C, 60 min; OD3: 42.1% sucrose + 5% NaCl(w/w), 30 �C, 210 min.

Table 3Colorimetric ordinates L*, a*, b*, chroma (C*) and hue (h*) of fresh and osmoticallydehydrated cherry tomato (n = 3).

Fresh OD1 OD2 OD3

L* 30 (3)a 27 (3) b 26 (2) b 22.6 (0.8) ca* 17 (2)a 16.9 (0.4) a 17,9 (0.8) a 14.08 (1.14) bb* 22 (3)a 24.9 (0.6) a 27 (2) a 24 (4) aC* 28 (3)a,b 30.2 (0.5) ab 33 (2) a 26 (4) bh* 50 (5)a 56.0 (1.4) b 55.6 (0.9) b 64.8 (1.7) c

Values in parentheses: standard deviation; similar letters indicate statisticallyhomogenous groups; OD1: 55: 55% sucrose (w/w), 30 �C, 120 min; OD2: 27.5%sucrose + 10% NaCl (w/w), 40 �C, 60 min; OD3: 42.1% sucrose + 5% NaCl (w/w),30 �C, 210 min.

A. Heredia et al. / Food Chemistry 123 (2010) 92–98 95

(from 12 to 40 �C), to the effect of osmotic stress that encouragedphytochemicals generation and the presence of a precursor like su-crose. On the other hand, in the osmodehydrated samples with theother two treatments (OD1 and OD2), a total lycopene loss wasregistered of around 10%, and an isomerization of 10% from transto cis, mainly to the 13-cis isomer (Tables 1 and 2). The rest ofthe cis isomers present in the samples were, in all cases, 9-cisand 5-cis isomers.

Tables 1 and 2 also show the residual percentage of trans-lycopene (%) and of total lycopene, as well as the degree ofisomerization (%) and the distribution of the lycopene isomers(%) in the tomato samples after the osmotic and drying operations,respectively.

Based on the results obtained, it could be considered that trans–cis isomerization seemed to be favored during the drying stage. Inthis sense, the percentage of residual total lycopene was greaterthan the percentage of residual trans-lycopene, which implies thatpart of the lycopene in the trans form underwent isomerization tothe cis forms. In most of the conditions tested in this study, the val-ues of total residual lycopene were found to be below 100%, indi-cating a loss of this carotenoid in comparison with fresh tomato.

Moreover, the isomerization from trans to cis oscillated between5% and 20% in most cases (Table 3), with the cis isomers represent-ing around 30% of total lycopene isomers in the processed cherrytomato. The formation of the 13-cis isomer (60–100% of the totalcis isomers), to a greater extent than the other cis isomers (in thiscase, 9-cis and 5-cis), coincides with the results obtained by otherauthors with tomato processed by means of thermal treatment(Nguyen & Schwartz, 1998; Qiu et al., 2006; Stahl & Sies, 1996).

Since the data about the level of isomerization shown inTable 2 refers to the whole process (osmotic dehydration + hot air-microwave drying), when evaluating the effect of the osmotic pre-

treatment on the changes occurred in the following drying stage,one must consider that the samples treated osmotically showed10% more cis isomers than the fresh samples. Hence, taking this intoaccount, the results showed that the osmotic pre-treatment reducesisomerization during the subsequent drying stage.

In relation to the residual total lycopene, the content found inthe samples after the osmotic dehydration stage was also reflectedafter the drying stage. The non-osmotically pre-treated samplesand the osmotically pre-treated ones in the OD2 conditions(27.5% sucrose + 10% NaCl (w/w), 40 �C, 60 min) showed a greatercontent than the rest.

Concerning air temperature and microwave power level, it canbe observed that upon raising both process variables, the isomeri-zation was encouraged but also the oxidation responsible for theloss of total lycopene. Specifically, as it has been mentioned before,the samples dehydrated at 80 �C and 3 W/g were the ones thatshowed a greater cis isomer content but also the ones that retainedless total lycopene. Particularly noteworthy are the samples

96 A. Heredia et al. / Food Chemistry 123 (2010) 92–98

dehydrated at 80 �C and a level of power of 1 or 3 W/g, in which thephenomenon of isomerization was especially significant, reachingvalues of 30% and 50%, respectively.

This could be explained by the fact that when drying withmicrowaves, the drying rate does not depend only on the heattransfer through the surface, but also the generation of volumetricheating produces a rapid transfer of energy to the sample (Andréset al., 2004). Therefore, the temperature of the sample is not lim-ited by the temperature of the air drying. The thermal energy thatdissipates from the sample increases its temperature and thevelocity of evaporation and favouring isomerisation.These resultsare in contrast with some studies that found a percentage of cisisomers lower than 10% during thermal processing (Nguyen &Schwartz, 1998).

0

40

80

120

160

40 ºC 55 ºC

0 W/g 1 W

80 ºC

Res

idua

l to

tal l

ycop

ene

(%)

Fig. 1. Temperature–power interaction graph obtained from the multifactor A

10

15

20

25

30

35

40

10 15 20 25 30 35 40

a*

b* L

30

35

40

45

50

55

60

20 25 3

h*

Fresh 40 ºC-040 ºC-3 W/g 55 ºC-055 ºC-3 W/g 80 ºC-080 ºC-3 W/g

Fig. 2. Distribution of the colour coordinates of the dehydrated ch

In all cases, it was seen that isomerization and oxidation phe-nomena are found to be closely related. So, on reaching a level ofisomerization of approximately 20% or more, the total lycopenecontent becomes lower and lower. This fact is related to the lowstability of the cis forms. Some scientific studies affirm that the for-mation of cis isomers basically takes place at the very beginning ofthe process becoming oxidized from that moment (Mayer-Mie-bach, Behsnilian, Regier, & Schuchmann, 2005; Shi et al., 1999),giving rise to a decrease in the final lycopene content.

As some authors have already pointed out (Dewanto et al.,2002; Kerkhofs, 2003; Mayeaux et al., 2006; Tonucci et al., 1995),under certain process conditions, the final lycopene content ofthe samples may even be above that of the fresh tomato. This isthe case of the samples that have not been pre-treated and those

/g 3 W/g

0

10

20

30

40

50

40 ºC 55 ºC 80 ºC

Isom

eriz

atio

n trans-cis

(%)

NOVA for residual total lycopene and level of trans–cis isomerization (%).

20

25

30

35

40

10 15 20 25

a*

*

0 35 40

C*

W/g 40 ºC-1 W/g W/g 55 ºC-1 W/g W/g 80 ºC-1 W/g

erry tomato samples in the chromatic planes b*, a* and L*, a*.

A. Heredia et al. / Food Chemistry 123 (2010) 92–98 97

that have been pre-treated with OD2 at 40 �C and 0–1 W/g, and insome cases at 55 �C.

The interaction graphs obtained from the factorial analysis AN-OVA carried out for the experimental data of residual total lyco-pene and the level of isomerization provide a summary of theinfluence of the different processing variables on these parameters(Fig. 1).

3.2. Influence of the process variables on the optical properties

Results showed a preservation of colour during osmotic dehy-dration. Nevertheless, samples submitted to OD3, with a longertreatment duration than the rest, showed statistically significantdifferences in the L*, a* and chroma (C*) parameters as comparedto the fresh product. On the other hand, the average value of thehue (h*) after treatment was in all cases significantly higher, whichimplies a greater increase in the coordinate b* than in the coordi-nate a*.

In Fig. 2 the samples have been grouped according to the micro-wave power level and drying temperature in the chromatic planesb*, a* and L* in order to analyze the effect of the drying stage on thecolour changes. As it can be seen, there was an increase in the coor-dinate a* related to non-enzymatic darkening that may take placeduring hot air drying in fruit and vegetables (Gould, 1992; Porretta& Sandei, 1991). On the other hand, coordinate b* increased moresharply on samples submitted to microwave energy (1 and 3 W/g), whereas it decreased when the drying was carried out exclu-sively with hot air (0 W/g). With regard to luminosity, the dryingstage did not imply significant changes as compared with the freshtomato values. After drying, the samples processed at 40 �C under-went a drop in C* and h*, which indicates that at low temperaturethe samples had a less reddish and less pure tone than the freshsamples. On the other hand, as the temperature and power appliedincreased, so did the C* and h*, which means a greater purity incolour as a consequence of less enzymatic darkening, and moreorange tones (an increase in the coordinate b*). According to aprevious study, the more orange tone of these samples could berelated to a greater formation of cis isomers on applying micro-wave energy (Shi, Dai, Kakuda, Mittal, & Xue, 2008).

h* C*

L*

Isomerization trans-cis (%) Residual

trans lycopene (%)

Residual total lycopene (%)

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1PC1 (41.51 %)

PC2

(31.

82 %

)

Fig. 3. Correlation loading plot for colour parameters and lycopene changesobtained by means of Principal Component Analysis (PCA).

3.3. Lycopene changes and optical properties correlation

A principal component analysis (PCA) was carried out on thedata in order to find the possible relationships between the colourand lycopene changes. The two principal components (PC1 andPC2) describe 73.33% of the total variability of the data (Fig. 3).The statistical analysis of the data showed a positive correlationbetween the C* and the h* of the samples as well as between theresidual total lycopene content and the residual trans-lycopene,whereas there was a negative correlation between the degree ofisomerization seen in the samples and the residual trans-lycopeneor the residual total lycopene content. Furthermore, the data seemsto indicate a slight, though not conclusive, relation between thechroma and the hue of the samples and the presence of cis isomers.

4. Conclusions

During the cherry tomato processing with combined methods,the mechanisms of lycopene oxidization and trans–cis isomeriza-tion were favored, principally into the 13-cis form. When the os-motic pre-treatment was applied, a 10% rate of isomerizationwas registered, although it was the non-osmotically dehydratedsamples that underwent the greatest degree of isomerization inthe subsequent drying stage. Regarding to the microwave energylevel, this turned out to be the most relevant parameter for thechanges experienced by the lycopene. In fact, the samples dehy-drated at 3 W/g, and especially at 80 �C, showed a greater levelof isomerization but a lower percentage of residual total lycopene.

The cherry tomato products showed different optical character-istics depending on the processing conditions applied. When con-vective drying was carried out at 40 �C exclusively with hot air,the samples showed more reddish tones and less luminosity incomparison with fresh tomato. On the other hand, when micro-wave energy was applied there was an increase in luminosity,and the colour of the samples changed towards more orange tones,which is related to a greater presence of lycopene cis isomers inthese samples.

Acknowledgements

The authors would like to thank the Ministry of Science andTechnology’s General Directorate of Research (AGL2003-00753)for the financial support given to this investigation and to thePolytechnic University of Valencia’s linguistic correction service.

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