Thermal Degradation Kinetics of Pigments and Visual Color in Watermelon Juice

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1 International Journal of Food Properties ISSN: (Print) (Online) Journal homepage: Thermal Degradation Kinetics of Pigments and Visual Color in Watermelon Juice Radhika Sharma, Devinder Kaur, D.P.S. Oberoi & D.S. Sogi To cite this article: Radhika Sharma, Devinder Kaur, D.P.S. Oberoi & D.S. Sogi (2008) Thermal Degradation Kinetics of Pigments and Visual Color in Watermelon Juice, International Journal of Food Properties, 11:2, , DOI: / To link to this article: Copyright Taylor and Francis Group, LLC Published online: 23 Apr Submit your article to this journal Article views: 573 Citing articles: 24 View citing articles Full Terms & Conditions of access and use can be found at

2 International Journal of Food Properties, 11: , 2008 Copyright Taylor & Francis Group, LLC ISSN: print / online DOI: / THERMAL DEGRADATION KINETICS OF PIGMENTS AND VISUAL COLOR IN WATERMELON JUICE Radhika Sharma, Devinder Kaur, D.P.S. Oberoi and D.S. Sogi Department of Food Science and Technology, Guru Nanak Dev University, Amritsar, India Thermal degradation of total carotenoids, lycopene and visual colour of watermelon juice was studied at C up to 5 h. Total carotenoids content in fresh watermelon juice was reduced from to mg/100 g, lycopene from to 0.82 mg/100 g and Hunter a b value from to when heated at 90 C for 5 h. First order model explained the degradation behaviour of total carotenoids, lycopene and Hunter a b value evident from correlation coefficient (R 2 ) higher than The dependence of degradation rate constant of total carotenoids, lycopene and Hunter a b value on temperature was adequately explained by Arrhenius equation. The activation energies for total carotenoids, lycopene and Hunter a b value were 24.19, 26.46, and kj/mol, respectively. Total carotenoids and lycopene were correlated with Hunter a b value with R 2 > 0.99 indicating that visual colour may be used to predict lycopene and total carotenoids contents in watermelon juice. Keywords: Watermelon juice, Total carotenoids, Lycopene, Hunter colour value, Degradation kinetics, Arrhenius model. INTRODUCTION The watermelon (Citrullus vulgaris), member of cucurbitaceae family, is a native plant of tropical Africa and cultivated throughout warmer parts of the world. The main pigments in red, orange and yellow-fleshed watermelons are lycopene, β-carotene and both β-carotene and xanthophylls respectively. [1,2] Lycopene is a highly effective antioxidant while lycopene and β-carotene have been positively correlated with reduced cancer incidences. [3] Carotenoid pigments are sensitive to light, heat, oxygen, and acid. Maintenance of natural pigments in thermally processed and stored foods has been a major challenge in food processing. [4] Reports on lycopene content of watermelon juice are available but information on pigments and visual color degradation during thermal processing is scanty. The visual color measurement has been accepted by food processing industry as an on-line quality control technique while measurement of pigment content quantifies the actual degradation during processing. Received 25 December 2006; accepted 23 June Address correspondence to D.S. Sogi, Department of Food Science and Technology, Guru Nanak Dev University, Amritsar, Punjab, India. sogids@gmail.com 439

3 440 SHARMA ET AL. Degradation of lycopene during heating at 50, 100, or 150 C or illuminated at 25 C for varied lengths of time was found to fit a first order model and degradation rate constant (min 1 ) of lycopene rose with increasing temperature and the activation energy was calculated to be 61.0 kj/mol. [5] The degradation kinetics of carotenoids and visual color of papaya puree at different temperatures C followed first order model with coefficient of determination > The activation energies for carotenoids and visual color in papaya puree were and kj/mol respectively. [6] Thermal treatment of tomato pulp at 100 C in the presence of O 2, with/without light caused significant loss of lycopene. [7] Concentration of tomato pulp resulted in up to 57% loss of lycopene. [8] Degradation kinetics of lycopene and visual color in tomato peel isolated from pomace at C for 10 h resulted in decreased in lycopene content from mg/100 g and visual color values a b from , respectively. [9] Studies on lycopene degradation revealed that heating tomato pulp at 100 C for 120 min decreased lycopene content from to mg/100 g of total solids. The lycopene loss was highest in the presence of air and light at 25 C, and lowest under vacuum and dark. [10] The degradation rate of lycopene was higher than β-carotene when safflower oil was heated at 75, 85, or 95 C. [11] The lycopene content in tomatoes slightly decreased using different dehydration methods: vacuum-drying (55 C, 4 8 h), conventional airdrying (95ºC, 6 10 h), and osmotic-vacuum-drying (first dehydrated with an osmotic treatment at 25 C for 4 h, followed by vacuum-drying at 55 C for 4 8 h. The lycopene content in samples slightly decreased during the dehydration processes. During osmotic treatment, lycopene content remained constant. After osmotic-vacuum-drying, total lycopene retention in tomatoes was greater than that using vacuum-drying. Conventional air-drying decreased lycopene retention greatly in tomato samples, which was attributed to the influence of heat and oxygen. [12] The degradation kinetics of pigments and visual color in watermelon juice was generally observed to see the effect of processing conditions on the pigments degradation during the watermelon juice. Undesirable degradation of pigments and visual color affects the sensory attributes, health promoting ability and natural appearance of the watermelon juice. This study was conducted to investigate the degradation kinetics of lycopene, total carotenoids and visual color during heat treatment of watermelon juice and their correlation with each other. MATERIALS AND METHODS Materials Ripe watermelons (Citrullus vulgaris) were procured from local market of Amritsar. Fruits were washed, cut into quarters, deseeded, peeled, passed through screw juice extractor (Kalsi Industries Ltd., India) and filtered to get watermelon juice. All the chemicals used for analysis were of analytical grades. Physicochemical Analysis Total soluble solids were quantified by using Abbe Mat Refractometer (Milton Roy Co., USA) at 21 C. Titrable acidity (as anhydrous citric acid) was determined using a ph meter (Elico, India). Reducing and total sugars were determined according to Lane and Eyon method. [13]

4 THERMAL DEGRADATION KINETICS OF PIGMENTS IN WATERMELON 441 Thermal Treatment The watermelon juice was sealed in cultured tubes (19 mm internal diameter 930 mm length) and was immersed in a water-bath (Brookfield Laboratories, USA) for preset times (0, 1, 2, 3, 4, and 5 h) at 50, 60, 70, 80, and 90 C. The desired temperature was considered to have achieved when the temperature of water-bath reached the set value. The samples were transferred to an ice-water bath immediately after the treatment. Total Carotenoids Juice (20 g) was mixed with 100 ml tetrahydrofuran and 50 ml petroleum ether in a separating funnel, the aqueous phase was separated and the organic phase was washed with three aliquots of water (25 ml). The extraction was repeated with petroleum ether until aqueous phase became colorless. Ether extracts was dried on anhydrous sodium sulphate and volume was made to 250 ml. Carotenoids were determined spectrophotometrically (Shimadzu, Japan) by measuring absorbance at 450 nm and were expressed as mg/ 100 g using extinction coefficient of mol cm 1. [14] Lycopene Sample (1 g) was extracted with acetone in a pestle and mortar till residues became colorless. Lycopene was transferred into petroleum ether phase by diluting acetone extract in a separating funnel, passed through sodium sulfate, volume made to 50 ml and absorbance was measured at 503 nm using UV visible spectrophotometer (Shimadzu, Japan). [15] The extinction coefficient ( mol cm 1 ) was verified with standard lycopene solution (Sigma Chemical Co, St. Louis, Missouri, U.S.A.), and the lycopene in sample was calculated. Visual Color The Hunter Color Lab (Hunter Associates Laboratory, USA) was calibrated with standard white tile (L = 90.55, a = 0.71, b = 0.39). A sample handling dish was charged with samples, placed on the analyzing port and noted the L, a, b values. Kinetics of Pigments and Visual Color Degradation The kinetics of degradation of both pigments and visual color has been reported to follow first order reaction adequately. [16,17, 18, 6, 19] The first order kinetic model based on lycopene concentration is: ln( L/Lo ) = kt, (1) where, L is the concentration of lycopene at time t (mg/100 g), L o is the initial concentration of lycopene (mg/100 g), k is the degradation rate constant (h 1 ), and t is heating time (h). Similar model was used for total carotenoids and Hunter a b values by replacing lycopene concentration with total carotenoids concentration and Hunter a b values.

5 442 SHARMA ET AL. Temperature Dependence of Degradation Rate Constant The Arrhenius model was applied to describe the temperature dependence of pigments and visual color degradation. k = k exp( E / RT), (2) o where, k o is Frequency factor (h 1 ); E a = Activation energy (J/mol); R is the universal gas constant (8.314 J/mol K); and T is absolute temperature (K). a Relationship Between Pigments Content and Visual Color The Hunter a b values were correlated with pigments concentration of watermelon juice. The relationship between visual color and pigment concentration has been found to be well described using the linear equation [6] : a b = k + k L, (3) a where k a and k b are the coefficients, and L is lycopene concentration (mg/100g). To determine relationship between visual color and total carotenoids, C replaced L where C represents total carotenoid concentration (mg/100 g). b Statistical Analysis The experimental data was analyzed employing given models [Eqs. (1 3)], and adequacy of fit was evaluated by comparing the regression coefficients (R 2 ) and standard error values that were computed using Excel (Microsoft Inc., USA). RESULTS AND DISCUSSION Physicochemical Properties Total soluble solids of watermelon juice varied from B. Similar results for total soluble solids were found for bottled Indo-American hybrid watermelon juice i.e B [20]. The acidity of watermelon juice varied from %. Earlier studies on watermelon juice reported acidity in the range of %. [20 22] Reducing sugars and total sugars of watermelon juice varied from % and %, respectively. Earlier studies reported higher values for reducing sugars and total sugars for Indo- American hybrid watermelon juice i.e. 5.52% and 7.76% which may be due differences in cultivar or processing conditions. [20] Degradation Kinetics of Total Carotenoids Total carotenoids content of fresh watermelon juice was found to be mg/100 g. When the juice was heated at selected temperatures, its total carotenoids content was decreased. Total carotenoids decreased to mg/100 g after 5 h of heating at 50 C (Figure 1). The degradation of total carotenoids increased with increase in temperature and it

6 THERMAL DEGRADATION KINETICS OF PIGMENTS IN WATERMELON Total carotenoids (mg/100g) Temperature C Model Time (h) Figure 1 Degradation kinetics of total carotenoids in watermelon juice. reduced to mg/100 g after 5 h of heating at 90 C. The first order model [Eq. (1)] was applied to the data and the degradation parameters were obtained (Table 1). The degradation rate constant was 0.09 h 1 at 50ºC, which increased progressively to h 1 with increase in temperature to 90ºC. The predicted values of the total carotenoids were obtained from the model and shown as solid line in Figure 1. The correlation coefficients (R 2 ) ranged from to , whereas standard error varied from to that adequately explained the degradation behavior. Total carotenoids are highly unsaturated compounds and therefore, susceptible to oxidation, isomerisaton, and other chemical changes during processing and storage. [23, 24] Present study also gave similar trend in watermelon juice. Degradation Kinetics of Lycopene Lycopene content of watermelon juice was found to be mg/100 g, which was very near to that of earlier reported value of 4.87 mg/100 g. [25] When the juice was heated, the lycopene content decreased to 2.8 mg/100g after 5 h at 50 C accounting for approximately Table 1 The kinetic parameters of pigments and visual color degradation with time (Eq 1) at selected temperatures (n = 3). Temperature ( C) Parameters Total carotenoids (mg/100g) k(h 1 ) R S.E Lycopene (mg/100g) k (h 1 ) R S.E Hunter a b value k (h 1 ) R S.E k: Degradation rate constant. S.E.: Standard Error.

7 444 SHARMA ET AL Lycopene (mg/100g) Temperature C Model Time (h) Figure 2 Degradation kinetics of lycopene in watermelon juice. 36% loss (Figure 2). Similar trend was observed during heating of the juice at higher temperatures. After 5 h of heating, the lycopene content decreased to 2.01, 1.54, 1.2, and 0.82 mg/ 100 g at 60, 70, 80 and 90ºC, respectively. The lycopene content decreased in tomato pulp as the temperature was increased from 29 to 40ºC. [26] Degradation takes place due to various reasons such as high temperature, long processing time, light and oxygen. Cole and Kapur, [7] reported that decline in lycopene was due to destruction by heat and oxidation resulting in fragment products like acetone, methyl heptenone, laevulinic aldehyde and glyoxal. First order model [Eq. (1)] for degradation of lycopene during heating was fitted and the degradation rate constants were found to vary from to h 1 (Table 1). It was observed that as the temperature increased, the degradation rate constant increased. The predicted values of the lycopene using first order model are shown by solid line and best fit was found in the plot of lycopene degradation at 50 C (Figure 2). The R 2 value ranged from to , while standard error values ranged from to , which validated the first order model for explaining degradation changes. Studies on the stability of lycopene during heating and illumination also concluded that the degradation of lycopene fitted the first order model with degradation rate and increased with increasing temperature. [5] Ax et al. [27] reported that in oil-in-water emulsion, the degradation rate constant for total and all trans lycopene increased with increase in temperature. Hackett et al. [28] concluded that lycopene degradation followed first order model and degradation rate constant values followed similar trends. The present findings concluded that degradation of lycopene followed the first order model and degradation rate constant increased with increase in heating temperature and these results were similar with earlier reported observations. Degradation Kinetics of Hunter Color Value Visual color of thermally processed juice was observed on Hunter color Lab in the form of L, a, and b values. It was desired to find out the variable that varied in linear fashion and could be adequately used to describe first order degradation kinetics,

8 THERMAL DEGRADATION KINETICS OF PIGMENTS IN WATERMELON 445 Table 2 Correlation coefficients (R 2 ) of change in Hunter values with time (Eq. 1) at selected temperatures (n = 3) Temperature, ( C) Hunter values Average a b L L a L b L a b a b a/b b/a (a/b) a/(b+L) which can be correlated with pigment degradation of watermelon juice. Thus different combinations of Hunter values L, a, b were plotted against time and R 2 were computed (Table 2). The a b values showed maximum R 2 value (0.9944) and therefore, selected to describe the visual color change in watermelon juice. In previous studies (a/b) 2 values were correlated with lycopene content in tomato juice with R 2 of 0.77 [29] and the a values were correlated with lycopene content of grapefruit juice with R 2 of [30] Perkins-Veazie et al., [25] found that the a and 1000a/ (b+l) were best correlated with lycopene content of melons. The present study has shown different results since a b values gave best correlation but it was closely followed by a [17, 6] value. However a b values have been found best suited for broccoli and papaya. Davis et al. [31] reported on use of xenon flash colorimeter/spectrophotometer to correlate its absorption with lycopene content of watermelon puree. Hunter a b value for fresh watermelon juice was observed to be and it decreased to at 50 C and at 90 C after 5 h of heating. The first order model [Eq. (1)] was applied and its degradation rate constant was found to vary from h 1 at C (Table 1). The predicted values computed using the models have been given as a solid line (Figure 3). The R 2 values varied from to and standard error values varied from to (Table 2). The best relationship was found when watermelon juice was heated at 50 C for 5 h. Ahmed et al., [6] showed that in case of papaya puree thermal degradation of Hunter a b value followed first order model with R 2 values of Reports on the watermelon were not traceable but present findings are in accordance with earlier studies on papaya puree. Temperature Dependence of Degradation Rate Constant Effect of temperature on the degradation rate constants has been shown in Figure 4. Arrhenius model [Eq. (2)] was employed to explain the change in degradation rate constant with temperature. The K o values for total carotenoids, lycopene and Hunter a b value were , 806.9, and h 1 while the activation energies were found to be 55.46, 26.46, and 24.2 kj/mol, respectively. Higher activation energy for visual color signified greater heat sensitiveness during thermal processing. The R 2 values for total carotenoids, lycopene and Hunter a b value were , , and ,

9 446 SHARMA ET AL Hunter 'axb' values Temperature C Model Time (h) Figure 3 Degradation kinetics of Hunter a b values in watermelon juice. ln k (h 1) total carotenoids lycopene Hunter 'axb' values /T (K 1 ) Figure 4 Dependence of degradation rate constant for total carotenoids, lycopene and Hunter a b values of watermelon juice. respectively, indicating that Arrhenius model explained well the dependence of visual color and pigment degradation on temperature. Sharma & Maguer, [10] found the activation energy for lycopene degradation in tomato pulp ranging from 19.9 to 27.7 kj/mol while Ahmed et al., [6] reported the activation energies for carotenoids and visual color as and kj/mol respectively in papaya puree. Ax et al., [27] reported that the activation energy of total and all trans lycopene degradation in water emulsion varied from kj/mol. Hackett et al., [28] reported the activation energy of 11.5 to 15 kj/mol for lycopene degradation in tomato oleoresins. In the present study, the activation energy for pigments and visual color degradation in watermelon juice ranged between kj/mol. Relationship Between Visual Color and Pigments Concentration The Hunter a b values were correlated with total carotenoids and lycopene contents of watermelon juice using the model [Eq. (3)] and its coefficients have been

10 THERMAL DEGRADATION KINETICS OF PIGMENTS IN WATERMELON 447 Table 3 Regression parameters for the correlation of visual color with total carotenoids and lycopene [Eq. (3)] content (n = 3). Temperature ( C) k a k b R 2 error Standard Total Carotenoids Lycopene given in Table 3. The k a value for total carotenoids varied from to where as k b value ranged between to It was observed that k a value increased while k b value decreased, with increase in temperature. Total carotenoids were found to be highly related with Hunter a b values with R 2 values ranging from and standard error varying from to Similarly, when lycopene was correlated to visual color, k a increased from to while k b decreased from to with increase in temperature (Table 3). The correlation coefficient (R 2 ) ranged between , and standard error varied from These results suggested that as the total carotenoids and lycopene degradation occurred with processing, the visual color of the product got affected. This might be because lycopene acts as a polyenic chromophore with H-conjugated double bonds, which absorbs and reflects light [32] and thus imparts red color to watermelon. As the pigment degraded due to oxidation and isomerisation, so did the visual color. In previous study absorption on xenon flash colorimeter/spectrophotometer was correlated to lycopene content of watermelon puree and correlation coefficient of was reported. [31] Thus there is a direct relationship between visual color and pigments content in watermelon juice and could be used interchangeably. CONCLUSION The thermal degradation kinetics of total carotenoids, lycopene and visual color of watermelon juice were studied at different temperatures C for 0 5 h. The total carotenoids content, lycopene content and Hunter a b value of watermelon juice was reduced from mg/100 g, mg/100 g and , respectively, when heated at C for 5 h. The degradation of the total carotenoids content, lycopene, and Hunter a b values followed first order kinetic model. Total carotenoids degradation, lycopene degradation and color loss followed Arrhenius Model and the activation energies for total carotenoids, lycopene and Hunter a b value were 24.19, 26.46, and kj/mol respectively indicating the greater temperature sensitivity of visual color parameters. The total carotenoids and lycopene were found well correlated to

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