Carotenes in processed tomato after thermal treatment

Food Control 48 (2015) 67e74

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Carotenes in processed tomato after thermal treatment Svjetlana Luterotti a, *, Dane Bicanic b, Ksenija Markovi
c c, Mladen Franko d University of Zagreb, Faculty of Pharmacy and Biochemistry, Ante Kovaci
ca 1, HR-10000 Zagreb, Croatia Wageningen University, Laboratory of Biophysics, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands c University of Zagreb, Faculty of Food Technology and Biotechnology, Pierottieva 6, HR-10000 Zagreb, Croatia d University of Nova Gorica, Laboratory for Environmental Research, Vipavska 13, 5000 Nova Gorica, Slovenia a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 November 2013 Received in revised form 2 June 2014 Accepted 4 June 2014 Available online 14 June 2014

This report adds to the ongoing vivid dispute on the fate of carotenes in tomato upon thermal processing. Although many papers dealing with changes in the raw tomatoes during industrial treatment have already appeared, data on the fate of finished, processed tomato products when they are additionally heated (e.g., when used in the household) is scarce. In this study, effects of heating and storage on a
e were examined spectrophotometrically. Our results commercial, double concentrated tomato pure
e to thermal treatindicate that upon exposing unopened cans with double concentrated tomato pure ments between 100 and 135
C during specific time intervals spectral profile of lycopene remained preserved. Likewise, a slight hypsochromic shift of lycopene peak III did not occur up to 135
C. However, significant (20%) initial loss of lycopene content was induced by thermal treatment for 20 min at 100
C. During the more intensive treatments that followed the lycopene content was first leveling off and then slightly increased. After storage of thermally treated samples at 18
C the content of lycopene was found to increase. All these results suggest simultaneous working of several mechanisms: possible autooxidation and isomerization processes of carotenes taking place, in addition to the Maillard reaction and enhanced extractability of carotenes at increased temperatures. Results acquired from hexane solutions of samples treated at temperatures of 120 and 135
C obtained at different time points, confirmed severe isomerization in organic solvent and/or photo-oxidative degradation of lycopene. © 2014 Elsevier Ltd. All rights reserved.

Keywords:
e Double concentrated tomato pure Thermal treatment Lycopene Beta-carotene Total carotenes Spectrophotometry

1. Introduction Carotenoids are naturally occurring pigments, polyenic chromophores, that impart yellow, orange and red colour to the commonly eaten fruits and vegetables (Astorg, 1997; Rock, 1997). There exists an evidence for their potential ability to decrease the risk for coronary heart disease, possible role in prevention of cancer and sight disorders; carotenoids with nine or more conjugated double bonds are capable of quenching singlet oxygen with lycopene being the most effective (e.g., Astorg, 1997; Agarwal & Rao, 2000; Khachik et al., 2002; Paiva, Russel, & Dutta, 1999; Rao & Rao, 2007; Rock, 1997; Singh & Goyal, 2008). Lycopene exhibits ability for single oxygen quenching that is twice that of betacarotene but has no provitamin-A activity (Shi & Le Maguer, 2000; Xianquan, Shi, Kakuda, & Yueming, 2005, 2008).

* Corresponding author. Tel.: þ385 1 63 94 433; fax: þ385 1 63 94 400. E-mail addresses: [email protected], [email protected] (S. Luterotti). http://dx.doi.org/10.1016/j.foodcont.2014.06.004 0956-7135/© 2014 Elsevier Ltd. All rights reserved.

Tomato fruit is a very important part of a human diet. It comprises about 94% water, 2% total fiber, 1% proteins, reducing sugars (about 2% total glucose and fructose), acids (ca 0.4%, mainly citric, malic and ascorbic acid), phenolics (caffeic acid and chlorogenic acid), flavonoids (quercetin and kaempferol), lipids, amino acids (glutamic, aspartic, g-aminobutyric acid and glutamine, making up to 80% total free amino acids; the optimum ratio of 4:1 of the first two possibly being essential to the genuine taste of tomato), and carotenoids (e.g., Koh, Charoenprasert, & Mitchell, 2012; Shi & Le Maguer, 2000). Among total carotenoids derived from thermally treated tomatoes, carotenes constitute ca 90% and the rest goes for xanthophylls (Gama, Tadiotti, & de Sylos, 2006). Tomato is the major source of lycopene which is the principal carotenoid in tomato (34e89%), followed by a-, b-, g- and z-carotenes (total 6e44%), colorless phytoene and phytofluene (0.4e30%), xanthophylls (total 1e9% including lutein), neurosporene (upto 11%) (e.g., Fraser, Truesdale, Bird, Schuch, & Bramley, 1994; Gama et al., 2006; Tonucci et al., 1995). Consequently, the ratio of lycopene-to-bcarotene content in ripe red tomato fruit varies widely between 1.5 and 40 (e.g., Darrigues, Schwartz, & Francis, 2008; Viskelis,

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S. Luterotti et al. / Food Control 48 (2015) 67e74

Jankauskiene, & Bobinaite, 2008). All-trans-lycopene is a dominating geometrical isomer in fresh tomatoes and derived products (84e96%); the rest is accounted for by three cis-isomers (5-, 13-, 9-) (Gama et al., 2006; Schierle et al., 1997). This is in accordance with reports of Nguyen & Schwartz (1998) who found 90e95% transisomer in tomato sauce and tomato paste and Stahl & Sies (1992) who reported on 20e30% cis-isomers of lycopene when tomato juice was heated at 100
C for 1 h. Although a substantial amount of research has been devoted to the studies that focus on the effect of processing parameters on the content of lycopene in fresh tomato tissue and/or products derived from it, the information on what is happening to lycopene and beta-carotene during heating are still inconsistent. Despite the fact that processing of tomatoes by cooking, freezing or canning usually does not bring about significant changes in total lycopene content, it is widely assumed that, with increase in time, various parameters like heat, light, oxygen, acids, metallic ions (Cu2þ, Fe3þ, etc.) do cause trans-cis isomerization of lycopene (e.g., Shi & Le Maguer, 2000; Shi, Le Maguer, Kakuda, Liptay, & Niekamp, 1999). Also, the components of the food matrix, like lipids, influence the isomeric distribution of lycopene (Schierle et al., 1997). During production of
e the prolonged thermal processing and irradiation by tomato pure light give rise to degradation of lycopene by isomerization and oxidation and the lycopene content in concentrated tomato products is lowered. Sharma & Le Maguer (1996) and Nguyen & Schwartz (1998) found that in tomato products processed up to 100
C no isomerization occurred, but are opposing the results obtained by Shi, Dai, Kakuda, Mittal, & Jun Xue (2008). At higher temperature and heating time degradation dominated over isomerization (Xianquan et al., 2005, 2008). Higher loss of lycopene due to trans-cis isomerization was observed after heating tomato-based foods in oil, namely during preparation of meals (Gama et al., 2006; Schierle et al., 1997). Both, trans- and cis-lycopene undergo oxidation processes. During tomato processing the losses of both lycopene and betacarotene may occur due to isomerization (trans-cis), oxidation and co-oxidation by lipooxygenases and peroxidases (e.g., Biacs & Daood, 2000). According to Nguyen & Schwartz (1998) isomerization of lycopene during food processing is less pronounced than that of beta-carotene. For example, hot break (93
C, 5 min) leads to the losses of phenolics, ascorbic acid and carotenoids and deactivation of enzymes in tomato (Koh et al., 2012). After hot break the content of beta-carotene decreased by almost 30% while lycopene content was reduced only by 4%. However, during sterilization at ca 100
C (3e5 min) lycopene concentration decreased by 20%. Sharma & Le Maguer (1996) measured 24% loss after heating tomato pulp at 100
C for 2 h. Takeoka et al. (2001) did not observe consistent changes but found 9e28% loss of lycopene during the processing of tomato into final paste. Likewise, Shi, Le Maguer, Bryan, & Kakuda (2003) reported a loss of lycopene as high as 35% when the temperature was increased from 90 to 150
C. At increased temperatures Shi et al. (2008) found a zig-zag (oscillatory) trend of total lycopene content for an in a laboratory-made
e; this is, probably due to several factors acting tomato pure simultaneously. At temperatures <100
C lycopene is relatively stable, particularly in the tomato matrix, but at higher temperatures stable alltrans-lycopene isomerizes into less reddish and less stable, cisform. The level of cis-isomer increases with temperature and time of processing as long as the latter is not longer than 2 h (Shi et al., 2003). Higher temperatures (>100
C) and prolonged heating times may provoke irreversible auto-oxidative degradation of lycopene with fragmentation into smaller ketone and aldehyde compounds, which imparts an off-flavor and color fading to the products. The heating process of tomato products also initiates the Maillard

reaction including formation of Amadori compounds, which produces important organoleptic changes (Zanoni, Pagliarini, Giovanelli, & Lavelli, 2003) and exhibit antioxidant activity (Yilmaz & Toledo, 2005). As far as b-carotene (BC) is concerned, a marked isomerization from trans-to-cis form is expected during thermal processing (Lemmens, Tchuenche, Van Loey, & Hendrickx, 2013; Seybold, €chlich, Bitsch, Otto, & Bo € hm, 2004) and significant decrease of Fro BC is observed even by hot break (Koh et al., 2012); mechanism of trans-isomer oxidative degradation is also suggested (Marty & Berset, 1990). Despite the abundance of the data dealing with processes during production of tomato concentrates from fresh tomato, the information on the behavior of finished tomato products during an additional thermal processing is still lacking, or is not directly comparable (Schr€ ader & Eichner, 1996). It can be presumed that in products such as, for example, double concentrated tomato products, a high percentage of lycopene is already present in the form of cis-isomers. This research, which is a continuation of our previous project (Luterotti et al., 2013), aims at answering two questions: (i) what are the changes happening in a double concentrated tomato
e exposed to additional thermal treatments?, (ii) is it possible pure to reliably monitor such processes using a conventional spectrophotometry? 2. Materials and methods 2.1. Chemicals and samples Solvents used for extraction were of p. a. purity (Merck, Germany). Hexane with dissolved 2,6-di-tert-butyl-4-methylphenol purum (Fluka, Switzerland), (BHT, 0.025%), was used throughout.
e (purchased at the grocery Double concentrated tomato pure store in the Netherlands) was thermally treated prior to extraction. The temperature/time scheme was defined as 100, 120 and 135
C each during 20, 60 and 120 min. Thermal treatment (autoclave) of
es in original, sealed cans was performed in the laboratory. pure 2.2. Extraction of carotenes from tomato concentrate Following the heating in autoclave and cooling down spontaneously, the cans were opened and the material homogenized for first analyses. The remaining quantity of sample was stored protected from light at 18
C for further analyses. Prior to these analyses the samples were equilibrated overnight at room temperature, homogenized 1 h on a shaker and then manually. Modified extraction procedure as described by Sadler, Davis, & Dezman (1990) was used. The clear orange hexane layer obtained upon extraction was diluted with hexane (10e25-fold) and measured against hexane. The lower polar layer changed from almost colorless and light yellowish through yellow, to dark orange and red-brown and the remaining solid was colorless to grayish-brown, both following the severity of the thermal treatment and the appearance of brown pigments. After thermal treatment samples were grainy making the homogenization and extraction more laborious. Two to twelve independent analyses were carried out. All operations were performed under light protected conditions. Measurements were performed at 502, 471, 444 and 365 nm and at respective minima. This data was used to estimate carotenes contents and to get an insight into their spectral fine structure. To evaluate lycopene content the absorption coefficient of 3150 dL g1 cm1 in hexane at 502 nm was used. The measurements performed at 471 nm were used to calculate the content of

S. Luterotti et al. / Food Control 48 (2015) 67e74

69

Table 1
e. Cumulative spectrophotometric (SPe) data for thermally treated double concentrated tomato pure Lycopene [mean ± SEM, (n), g kg1]a,b

Treatment

Remark (appearance, flavor)

t-test (p, unpaired) Treated vs. nontreated sample

None 100
C

120
C

135
C

20 min 60 min 120 min 20 min 60 min 120 min 20 min 60 min 120 min

0.39 ± 0.003 0.31 ± 0.01 0.29 ± 0.01 0.29 ± 0.01 0.28 ± 0.02 0.30 ± 0.01 0.29 ± 0.01 0.31 ± 0.02 0.33 ± 0.01 0.32 ± 0.01

e 9.49 5.53 1.49 1.04 7.27 4.33 6.53 5.84 7.27

(2) (12) (8) (10) (8) (8) (8) (9) (10) (10)

        

103, 103, 102, 102, 105, 105, 102, 104, 104,

ho, ho, ho, ho, ho, ho, ho, ho, ho,

Between successive treatments e e 0.415, 0.682, 0.637, 0.184, 0.364, 0.386, 0.314, 0.297,

S S S S S S NS S S

Dark red Dark red, grainy ho, NS ho, NS ho, NS he, NS ho, NS he, NS he, NS ho, NS

Darker red, grainy Brown nuance, grainy Red-brown, grainy Brown, caramelized, grainy Brown-black, caramelized, grainy

n e number of independent analyses, SPe e spectrophotometry based on specific absorption coefficient, SEM e standard error of the mean, statistical inference: S, NS e significant, non-significant difference, he, ho e heteroscedastic, homoscedastic data sets; p e significance level. a Estimates upon measurements at 502 nm. b Data collected during 3 months (l1t1þl1t2þl1t3þl2t3).

the rest of carotenes as b-carotene equivalents (BCeq), finally giving total carotenes content (specific absorption coefficient of 3450 dL g1 cm1 for lycopene at 472 nm, and of 2049 dL g1 cm1 for b-carotene at 475 nm, Goodwin, 1954; Sharpless, Thomas, Sander, & Wise, 1996).

2.3. Data analysis Experiments were performed in three laboratories (l1, l2, l3) at four time instants (t1, t2, t3, t4). The correlations were expressed through correlation coefficient, R, slope of the regression line,

Table 2
e. Interlaboratory comparison of SPe data for lycopene, b-carotene equivalents and total carotenes in thermally treated double concentrated tomato pure Analyte

Lycopene

Temperature
( C)

Content (g kg1)

RSD (%)

l2t3 100

0.23e0.37

10.6e19.0

Comparison parameters 7.3 0.991e0.994a,b

120

0.24e0.36

4.1e20.2

8.0

135

0.24e0.38

3.7e21.8

0.986a,b a,b

0.924e0.927

0.23e0.36

2.3e18.6

l2t3 100

0.02e0.03

9.6e15.2

120

0.02e0.03

5.1e19.1

9.7

135 l1t3 100

0.23e0.35

11.2e18.1

120

0.22e0.33

6.1e19.7

135 Total carotenes

Slope

135

120

equivalents

4.8

R

Comparison parameters 4.9 0.917e0.934a 0.935b 41.6 e, 0.305a 0.492b 23.4 0.661, ea 0.666b Comparison parameters 6.2 0.993a 0.994b 34.3 0.235, ea 0.461b 18.0 0.485e0.510a 0.624b Comparison parameters 24.0 0.203a,b,c

100

b-carotene

Mean bias (±%)

0.03e0.04

3.2e8.6

21.4

0.793e0.856a,b a,b

0.645e0.786 a,b

100

0.26e0.40

9.8e18.3

4.9

0.992

120

0.26e0.38

4.1e20.1

6.5

0.985e0.986a,b

135

0.27e0.41

3.2e20.7

2.0

0.990a,b

t-test (2-tail, unpaired)

Content (g kg1)

RSD (%)

p

Statistical inference

0.928e1.077a 0.928e1.111b 0.923e1.083a 0.916e1.062b 0.966-1.033a 0.887e0.969b

0.222

ho, NS

l1t3 0.23e0.35

11.2e18.1

0.132

ho, NS

0.22e0.33

6.1e19.7

0.485

ho, NS

0.23e0.36

2.3e18.6

0.969e1.024a 0.813e1.075b 0.662e1.463a 0.373e0.650b 0.785e1.262a 0.393e1.128b

0.825

ho, NS

l3t4 0.23e0.37

6.2e18.8

1.74  104

ho, S

0.17e0.28

16.4e22.3

4

he, S

0.22e0.31

6.9e13.9

0.946e1.056a 0.974e1.013b 0.716e1.351a 0.387e0.548b 0.833e1.191a 0.513e0.757b

0.317

0.786e1.251a 0.193e0.215b 0.922e1.074a 0.781e0.922b 0.811e1.226a 0.783e0.788b 0.951e1.051a 0.921e1.069b 0.936e1.068a 0.903e1.077b 0.981e1.019a 0.985e0.996b

7.0e12.3

2.77  10

ho, NS

1.12  103

ho, S

4

ho, S

4.20  105

ho, S

l1t3 0.02e0.03

0.255

ho, NS

0.02e0.03

1.2e15.7

ho, S

0.03e0.05

2.6e15.0

0.383

ho, NS

0.25e0.38

11.1e17.6

0.203

ho, NS

0.25e0.36

3.5e19.1

0.705

ho, NS

0.26e0.41

2.0e18.2

3.92  10

1.30  10

3

Statistical inference: S, NS e significant, non-significant difference, ho, he e homoscedastic, heteroscedastic data sets, p e significance level, mean bias (±%) e mean percentage bias between results, slope e slope of the correlation line, R e coefficient of correlation between results. Number of samples: 100
C: N ¼ 11e13, 120
C: N ¼ 9e10, 135
C: N ¼ 11e12. a Ideal line. b Non-ideal line. c No correlation.

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S. Luterotti et al. / Food Control 48 (2015) 67e74

average bias between results and p-value (t-test). Level of significance was set at p < 0.05. 2.4. Instruments Pye Unicam SP6-500 UV spectrophotometer (Pye Unicam Ltd, UK) was used in lab l1 while measurements in labs l2 and l3 were conducted with Agilent 8453 UV/Vis spectrophotometers equipped with the photodiode detector (HewlettePackard, Germany). 3. Results and discussion Our data, still not directly comparable, are in line with the literature reports (e.g., Benjakul, Lertittikul, & Bauer, 2005; Koh et al., 2012; Nguyen & Schwartz, 1998; Sharma & Le Maguer, 1996; Shi et al., 2003, 2008; Takeoka et al., 2001). Tables 1 and 2 show the outcome of measurements at 502 nm; these results are expressed as the total lycopene content. Likewise, the measurements carried out at 471 nm were used to express the rest of carotenes as BC equivalents and finally the TC content. 3.1. Absorbance and appearance A severe drop of absorbance measured at 502 nm was observed after heating the sample at 100
C for 20 min; respective loss of lycopene amounted to 20.3%. Table 1 shows that compared to nontreated sample, significant losses of lycopene (upto 28.8% maximum loss after 20 min at 120
C) occurred in all sample subgroups. The initial lycopene loss was followed by equilibration and slow but steady increase of lycopene content, from 1 h at 120
C onwards (Table 1 and Figs. 1 and 2). No significant changes were found when comparing durations of treatment at a constant temperature; similarly, differences of 3e9% were observed in all successive thermally treated sub-groups. The same trend is also evident for total carotenes. Spectra from Fig. 1 show the decrease of absorbance for all three major lycopene peaks in visible (I at 444 nm, II at 471 nm, and III at 502 nm) upto 135
C, and continuous increase of cis-peak at 365 nm. At 135
C slight but steady increase of absorbance of peak I, followed by II and III, together with marked increase of cis-peak, were observed. This resulted in the sharp increase of AB/AII peaks ratio (AB and AII stand for absorbance of cis-lycopene peak at 365 nm and of peak II, resp.) and pronounced decrease of III/II peaks ratio, for lycopene, after 2-h heating at 135
C (see Figs. 3 and 4).

The similar trends but of lower intensity were observed at less drastic conditions. This pattern coincided with the alteration of sample's appearance. At 120
C, we observed darkening and browning of the sample (see Table 1). This finally resulted in the loss of characteristic tomato-product smell and appearance of caramel-like odor accompanied by dark brown color of the sample at the highest temperature (135
C). Here, the non-enzymatic browning might have contributed to the changes of color. Higher absorbances correlated well with these organoleptic changes, but may also, at least in part, be ascribed to protective antioxidant role of Maillard reaction products (MRPs) giving rise to the increase of lycopene/carotenes, as well as to their increased extractability from the matrix, analogously to the observations of Anese, Manzocco, Nicoli, & Lerici (1999), and Shi et al. (2008). During the experiment, TC content decreased at 120
C compared to 100
C but increased again at 135
C. However, the portion of lycopene within total carotenes decreased slightly but steadily both by the higher temperature and prolonged duration of thermal treatment (Fig. 5). On an average, relative lycopene content dropped from 92.1 ± 1.2 through 90.7 ± 0.9 to 89.7 ± 1.5% (n ¼ 6) of TC content when temperature was increased from 100 through 120 up to 135
C. In the case of other carotenes, expressed as betacarotene, the trend was just the opposite. In both cases significant change being clearly evidenced only at 135
C versus 100
C. The observed effects may be explained by the higher extractability of carotenes and/or preserving antioxidant action of MRPs as well as the formation of less absorbing cis-isomers of lycopene during the experiment and/or degradation by the auto-oxidation. Both phenomena (slow AB/AII ratio increase and III/II ratio decrease) may also be accounted for slow gradual cis-isomerization processes during thermal treatments. However, after 2 h at 135
C large jumps were observed. Interesting to note is the fact that throughout the whole experimental scheme, III/II peaks ratio changed from 72.0 ± 0.3% at 100
C through 69.3 ± 0.5% at 120
C to 66.4 ± 2.3% at 135
C (n ¼ 3e4). The lowest value of 63.8% was achieved at 135
C after 120 min. The lowering trend of III/II peaks ratio may also be accounted for by the increased portion of cisisomer(s) of lycopene (Hengartner, Bernhard, Meyer, Englert, & Glinz, 1992). The same may be accounted for from cis-lyco/AII peaks ratio which slowly increased from 12 to 13% throughout the experiment, except for 135
C after 120 min when it jumped to 15% (see Figs. 3 and 4). All this data may point out in favor of gradual increase in cis-isomer(s) portion in the mixture with trans-isomer of lycopene in the sample during thermal treatment with


e during thermal treatment. Fig. 1. Typical spectra from hexane extracts (diluted 25 times) of double concentrated tomato pure

S. Luterotti et al. / Food Control 48 (2015) 67e74

pronounced increase at 135
C after 2 h (Britton, 1995; Britton, Liaaen-Jensen, & Pfander, 2004; Hengartner et al., 1992). 3.2. Spectral evidence We found that the typical spectral profile of lycopene remained preserved after thermal treatments, even after the most drastic treatment condition (2 h at 135
C, see Fig. 1). Also, no marked shifts of peaks were measured upto 135
C; only at 135
C a slight (502 / 501 nm) but steady hypsochromic shift was detected. This
e almost no additional cismay indicate that in double tomato pure isomerization took place until 135
C and that the loss of lycopene

71

discussed so far was probably the consequence of oxidative lycopene degradation. The loss of characteristic tomato-like odor of the samples is in line with such reasoning. The likely cis-isomerization is additionally confirmed by a slight but consistent increase of cislycopene peak from 1 h at 120
C upwards. Our results harmonize well with the results of Sharma & Le Maguer (1996) and Nguyen & Schwartz (1998) but oppose the data obtained by Shi et al. (2008). Additional experiments are needed to fully clarify the observed phenomena. Our results rely on several factors that affect the fate of lycopene in different way(s) and are combined together; i.e., the character
e) and istics of the sample itself (double concentrated tomato pure


e. Measurements performed in Fig. 2. Spectrophotometric data for: a) lycopene, b) b-carotene equivalents, c) total carotenes, in thermally treated double concentrated tomato pure different laboratories (l1el3) at different time points (t2et4) (l1t2 e , l1t3 e , l2t3 e , l3t4 e ). Mean ± SD, n ¼ 2e4 independent analyses.

72

S. Luterotti et al. / Food Control 48 (2015) 67e74

Fig. 4. III/II peaks ratio (502 nm/471 nm) as a function of: a) time; b) temperature (fat connecting line added). Measured in laboratory l2 (l2t3); mean ± SD, n ¼ 3e4.

Fig. 3. Trendlines of AB (cis-lycopene peak at 365 nm)/AII (peak at 471 nm) ratio as a function of: a) time; b) temperature (fat connecting line added). Measured in laboratory l2 (l2t3); mean ± SD, n ¼ 3e4.

the processing conditions. The latter imply inactivated enzymes (probably also lipooxygenase), the absence of ascorbic acid (as a preserving antioxidant) and the absence of oxygen and light during heating. Possible catalysis of oxidative reaction by the metal ions from the can may not be ruled out. 3.3. Storage effects and data correlations The lycopene content after storing samples at 18
C in dark over a period of one month was also measured (see Fig. 2a). Measurements performed in the same laboratory at two different time points, l1t2 and l1t3, resulted in the increase (0.2e9.9%) of calculated lycopene content. This can possibly be ascribed to cis-trans reisomerization of lycopene during this storage. Comparison of the data for lycopene, b-carotene and total carotenes among three laboratories shows that the higher is the treatment temperature the lower is R between results obtained by these laboratories. As an example, an excellent correlation obtained for samples heated at 100
C reduces to a weak one for these treated at 120 and 135
C (see Table 2). If all test groups are concerned, for example at R > 0.9, mean bias between data sets amounted to 5e8% and 2e7% for lycopene and total carotenes, respectively, and neither was recognized as significant by unpaired t-test. At lower R values (0.2e0.9) mean bias went up to 42% and the lowest value recognized as significant by unpaired t-test was 18%, namely, even the mean bias of 10% was not recognized as significant. This was additionally confirmed by more detailed analysis (data not shown): both paired and unpaired t-test recognized as

significant mean bias of 18% at R 0.2e0.8. From this R value onwards unpaired test failed to indicate significance of mean bias of 10, 5 and 2% as paired test did. This is in agreement with our previous findings that both tests work almost equally up to R 0.8e0.9 (Luterotti et al., 2013). At R 0.92e0.99 paired test indicated as significant as low as 2% for mean bias, while unpaired t-test failed to indicate significance of 8 and 7% for lycopene and total carotenes, respectively. This discussion is fully documented in Table 2. When results for lycopene between lab l3 and either lab l1 or lab l2 were compared at different time points, low to moderate values for R and large deviations between all but samples treated at 100
C (see also Fig. 2) were found. Therefore, for the samples heated at 100
C correlations were still excellent; however they decreased to low and medium for samples treated at 120 and 135
C, resp. The differences in lycopene concentration (on average 5 and 6%, resp.) between values obtained in lab l3 and lab l1 or lab l2 for 100
Ctreated samples were not recognized as significant. As far as samples treated at temperatures of 120 and 135
C were concerned, correlations between results were characterized by low R values and high losses of lycopene were found. Here, actually hexane solutions were measured after a 2-month storage at room temperature in lab l3. As expected, lycopene was probably subject to further trans-to-cis isomerization and/or photo-oxidative degradation in organic solvent with the effects increasing with the intensity of thermal treatment. Our data is in agreement with observations of other authors (Nguyen & Schwartz, 1998). 4. Conclusions By virtue of its appearance and based on spectral data obtained
e after thermal treatment, from a double concentrated tomato pure it follows that lycopene was probably subject to both degradation

S. Luterotti et al. / Food Control 48 (2015) 67e74

73

References

Fig. 5. Percentage fraction of: a) lycopene and b) b-carotene equivalents, out of total
e. Measured carotenes, during thermal treatment of double concentrated tomato pure in laboratories l1 (l1t3) and l2 (l2t3); mean ± SD, n ¼ 3e4.

and isomerization processes. Processing at temperatures 120
C provided the evidence that contribution of non-enzymatic browning reaction products with antioxidant activity (Maillard's products) should be taken into account. The more efficient extractability of carotenes should be ruled out neither.
e under controlled Storage of double concentrated tomato pure temperature- and light-conditions has possibly led to cis-trans reisomerization of lycopene, whereas the storage of hexane solutions resulted in severe trans-cis isomerization. The change of color and flavor of tomato product which takes place during processing and storage should expectedly be accompanied by reduced nutritional value of proteins. Moreover, whether or not the advanced glycosylation end-products (AGEs) formed during non-enzymatic glycation between proteins and carbohydrates play a role as uremic toxins is still under investigation (e.g., Glorieux, Schepers, & Vanholder, 2007; Henle, 2003; Vanholder, et al., 2003). One of the questions posed above, i.e., whether or not spectrophotometry may provide useful data about the fate of finished tomato products when exposed to the additional thermal treatment can now be answered. We believe that spectrophotometry is indeed capable of providing a quick and reliable insight about the possible deterioration of nutritional value of such products. Since minimizing the loss of sensory and nutritional quality and maximizing the safety of meals ultimately to be served on plate are all a necessity, the risk of additional heating of the processed tomato foods when manipulated in the household/kitchen should be fully understood and quickly assessed.

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