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Kinetics of colour changes in dehydrated carrots
Journal of Food Engineering 78 (2007) 449–455 www.elsevier.com/locate/jfoodeng
Kinetics of colour changes in dehydrated carrots Nuray Koca, Hande Selen Burdurlu, Feryal Karadeniz
*
Department of Food Engineering, Ankara University, Faculty of Engineering, Campus of Agricultural Faculty, Dısßkapı, 06110 Ankara, Turkey Received 3 August 2005; accepted 17 October 2005 Available online 2 December 2005
Abstract Carotenoid degradation and colour loss in dehydrated blanched and unblanched carrot slices during storage at 27, 37, 47 and 57 °C were investigated. The degradation of b-carotene and colour loss followed a first-order reaction. A significant relationship was found between the colour loss and b-carotene degradation in blanched (r = 0.878–0.971, p < 0.05) and unblanched samples (r = 0.903– 0.998, p < 0.05). Degradation of b-carotene had an activation energy of 15.8 ± 0.81 kcal mol1 and 9.3 ± 2.07 kcal mol1 for blanched and unblanched carrots, respectively. For blanched samples, the activation energies of colour loss determined on the basis of a and b values were 15.7 ± 2.81 kcal mol1 and 21.7 ± 4.13 kcal mol1, while those for unblanched samples determined as 10.5 ± 0.62 kcal mol1 and 14.7 ± 1.30 kcal mol1, respectively. Nonenzymatic browning of dehydrated carrots was also investigated and this reaction fitted to zero-order kinetic model. Activation energies of nonenzymatic browning occurred in blanched (25.3 ± 2.30 kcal mol1) and unblanched (25.9 ± 2.61 kcal mol1) dehydrated carrots was found similar. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: b-carotene; Colour loss; Nonenzymatic browning; Kinetics
1. Introduction Carotenoids are widely known as provitamin A, while there is an increasing interest in their role as antioxidants (Bohm, Putpitasarı-Nienaber, Ferruzzi, & Schwartz, 2002; Elliot, 1999; Larson, 1988). Epidemiological evidence has shown inverse correlation between consumption of some carotenoid-containing fruits and vegetables and cancer incidence (Kalt, Forney, Martin, & Prior, 1999; Ziegler, 1989). In addition to the anti-cancer activity, other health benefits provided by b-carotene includes protection against cardiovascular disease, cataract prevention (Dietmar & Bamedi, 2001; Sulaeman et al., 2001) and enhanced immune responses (Kurilich et al., 1999). Carrot (Daucus carota L.) has the highest carotenoid content among foods and is consumed in large quantities (Desobry, Netto, & Labuza, 1998). Dehydrated carrots are used as an ingredi*
Corresponding author. Tel.: +90 312 596 10 00x1716/317 05 50; fax: +90 312 317 87 11. E-mail address: [email protected] (F. Karadeniz). 0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.10.014
ent in instant soups or meals and an excellent candidate for developing oil free, healthy snack food if the nutritional value can be well preserved (Lin, Durance, & Scaman, 1998). Dehydration is one of the oldest methods of food preservation and it represents a very important aspect of food processing (Lin et al., 1998). Longer shelf life, product diversity and volume reduction are the reasons for the popularity of dried fruits and vegetables (Prakash, Jha, & Data, 2004). The quality of dehydrated product does not only depend on the drying conditions but also on the other operations carried out before and after drying (Negi & Roy, 2001). For example, blanching is carried out prior to dehydration to inactivate the enzyme peroxidase, which may otherwise lead to formation of unacceptable colours and flavours (Baloch, Buckle, & Edwards, 1977; Mazza, 1983). Blanching before drying was also reported to enhance the stability of carotenoids during storage (Arya, Natesan, Premavalli, & Vijayaraghavan, 1982). Several deteriorative reactions that affect the colour, nutrient properties, texture and flavour of dehydrated
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products are initiated during processing and dehydration operations, and continue during storage at a rate that is influenced by storage conditions. An important aspect concerned with dehydrated food products is their stabilization to chemical and physical changes as well as microbiological degradation, which could occur during storage (Can˜ellas, Rossello, Simal, Soler, & Mulet, 1993). Degradation of carotenoids not only affects the attractive colour of foods but also their nutritive value and flavour (Pesek & Warthesen, 1990). Dehydrated vegetables lose colour due to the oxidation of highly unsaturated molecules upon exposure to air during storage (Park, 1987). Carotenoids are susceptible to oxidation when exposed to light (Pesek & Warthesen, 1990), oxygen and enzymes (Gregory, 1996). A rapid degradation of b-carotene associated with the development of an off flavour in dehydrated carrot was observed by Park (1987). Another important chemical change occurred in some dehydrated foods is nonenzymatic browning (Baloch, Buckle, & Edwards, 1973). The formation of dark-coloured pigments in foods during processing and storage is a very common phenomenon. Most of the browning occurring in foods during drying is due to Maillard reaction (Mcbean, Joslyn, & Nury, 1971). Browning can also appear during long storage, and is generally dependent upon product characteristics and storage conditions (Toribio & Lozano, 1984). The most common method for characterizing browning in dehydrated carrot is the measurement of absorbance at 420 nm (Baloch et al., 1973). In addition, tristimulus colourimetry was also used to evaluate colour changes in carrots (Chen, Peng, & Chen, 1995; Inazu & Makino, 1998; Zhao & Chang, 1995). The Hunter a and b values were used by Lin et al. (1998) for determining the colour changes of dried carrot slices. However, studies on determining reaction kinetic parameters by using colorimetric values are limited. The stability of carotenoids and the extent of browning during processing and storage is a very important objective to make the final product attractive and acceptable. Several researchers have studied the effect of drying on carrot quality (Doymaz, 2004; Lin et al., 1998; Mazza, 1983; Park, 1987; Prakash et al., 2004; Schadle, Burns, & Talley, ¨ ztop, 2005). However, the liter1983; Sumnu, Turabi, & O ature contains few references on the retention of b-carotene and colour stability in dehydrated carrots during storage (Baloch et al., 1973; Baloch, 1987; Negi & Roy, 2001). Kinetic parameters including reaction order, rate constant and activation energy are essential to predict the quality of foods. Therefore, kinetic studies are needed in order to minimize the undesired change and to optimize quality of dehydrated specific foods such as carrots. The objectives of this study were to determine the kinetics of carotenoid degradation and nonenzymatic browning and also to evaluate the correlation between total carotenoid content and colour loss in blanched and unblanched dehydrated carrot during storage. In addition, kinetic
parameters of colour loss in dehydrated carrots by using tristimulus colorimetry were also assessed. 2. Materials and methods 2.1. Materials Carrots (Daucus carota L. var. Nanco) were harvested from Beypazarı´, Turkey and stored at 1 °C and 97% humidity until drying. Prior to drying, samples were washed, peeled and sliced into circular discs (5 mm thickness) using a hand-operated slicer. The cut vegetables were separated into two parts. One part remained unblanched, and the other part was blanched in boiling water at 90 °C for 7 min to inactivate peroxidase (EC 1. 11. 1.7). Peroxidase test was made by using guaiacol and H2O2 as substrate according to the modified method of Jacobs (1958). The blanched samples were immediately cooled under tap water at 22 °C and then drained. 2.2. Drying Blanched and unblanched carrots were dried using a hot-air drier designed and fabricated in Department of Agricultural Machinery, Faculty of Agriculture, Ankara University, Ankara, Turkey. Drying conditions were temperature 60 ± 5 °C, air flow 1.5 m/s, RH 6–10% and tray load 3.0–3.4 kg/m2. Dehydrated samples (6–7% moisture content) were packed in polyethylene bags and stored at 27, 37, 47 and 57 °C. Drying experiments were carried out with two replicates. 2.3. Moisture analysis Moisture analysis was performed in a hot air oven at 105 ± 2 °C (AOAC, 1990). Triplicate samples were analyzed and average moisture content was recorded. 2.4. Determination of total carotenoids Total carotenoids were determined using the modified method described by Park (1987). Duplicate 0.5 g dehydrated carrot powder were extracted in a 25 mL 7:3 hexane:acetone mixture using a shaker. The extract was vacuum-filtered through a Buchner funnel with Whatman No. 1 filter paper. The residue was re-extracted until became colourless. The filtrates were combined in a separatory funnel and washed with 50 mL distilled water. The water phase was discarded and Na2SO4 (10 g) was added as desiccant. The hexane phase was transferred to a 50 mL volumetric flask and brought to volume with hexan. The concentration of carotene in the solution was determined by its absorbance at 450 nm with a Unicam UV/Vis spectrophotometer (Cambridge, United Kingdom). The total carotenoid content determined as b-carotene from the standard curve prepared b-carotene standard.
N. Koca et al. / Journal of Food Engineering 78 (2007) 449–455
2.5. Nonenzymatic browning Browning measurements were carried out using the method described by Baloch et al. (1973). Dehydrated carrot powder (2.5 g) was rehydrated for 10 min in 50 mL acetic acid–formaldehyde aqueous solution (2%–1%, v/v) and homogenized for 5 min. The slurry was filtered and 0.5% lead acetate was added to the filtrate. Then, the slurry was collected in a volumetric flask, made up to 100 mL with acetic acid–formaldehyde solution. After mixing, the extract was centrifuged at 2500 rpm for 5 min and the supernatant was mixed with an equal volume of ethyl alcohol. The mixture was centrifuged again and absorbance measurements were taken at 420 and 600 nm by using UV–VIS spectrophotometer. Browning was calculated from the difference between the two absorbance values.
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at 27, 37, 47 and 57 °C. The initial b-carotene content of dehydrated carrots was higher in blanched carrots compared to unblanched samples since blanching carried out prior to dehydration inactivates the enzyme peroxidase. Unblanched dehydrated carrots contain 606.1 mg b-carotene/kg, while 844.3 mg b-carotene/kg was determined for blanched dehydrated carrots. Carotenoid destruction in unblanched and blanched dehydrated carrots during storage is shown in Figs. 1 and 2, respectively. Blanched and unblanched dehydrated carrots were stored for 180 days. However, unblanched carrots at 47 °C were stored for 90 days, since no b-carotene was detected in these samples after 3 months. At 57 °C, the samples were stored for 25 days since 50% loss of initial b-carotene content
2.5
2.6. Colour The sample colour was also assessed by the chromaticity value a* (red colour) and b* (yellow colour) of the CIE colour system using a Minolta CR-300 colourimeter (Osaka, Japan). The means of four measurements were used. 2.7. Calculation of kinetic parameters The degradation of carotenoids and colour loss in dehydrated carrots were calculated by using the standard equation for a first-order reaction given below:
log (% retention of β-carotene)
2
27 ˚C
37 ˚C
47 ˚C
57 ˚C
1.5 1 0.5 0 0
50
100
150
200
-0.5
ln C ¼ ln C 0 kt where C, the concentration at time t; C0, the concentration at time zero; k, the first-order rate constant (days1); t, the storage time (day). Analysis of kinetic data from A420 values suggested that a zero-order reaction for nonenzymatic browning:
-1 Storage time (day) Fig. 1. b-carotene degradation of unblanched dehydrated carrot slices stored at different temperatures.
C ¼ C 0 kt
k ¼ k 0 eEa =RT where Ea, the activation energy (kcal mol1); k, the rate constant; k0, the pre-exponential factor; R; the universal gas constant (1.987 kcal mol1); T, the absolute temperature (°K). 3. Results and discussion
2.5 log (% retention of β-carotene)
where C, the concentration at time t; C0, the concentration at time zero; k, the zero-order rate constant (days1); t, the storage time (day). Temperature dependence of carotenoid degradation, browning (A420) and CIE a* and b* colour parameters were determined by Arrhenius equation:
2
1.5
1
0.5
37 ˚C
47 ˚C
57 ˚C
0 0
Carotenoid degradation and nonenzymatic browning reactions were investigated in both blanched and unblanced dehydrated carrot slices during storage temperatures
27 ˚C
50
100 Storage time (day)
150
200
Fig. 2. b-carotene degradation of blanched dehydrated carrot slices stored at different temperatures.
N. Koca et al. / Journal of Food Engineering 78 (2007) 449–455
was found at the end of this storage time The loss of bcarotene was found as 69% and 86% for blanched and unblanched samples, respectively. It is known that the reaction should be carried out beyond 50% change of the initial value so that the proper kinetic model can be selected (Labuza, 2000). A spontaneous loss of b-carotene was observed in both blanched and unblanched dehydrated carrot slices during storage. As seen from the rate constants of b-carotene degradation in Table 1, the breakdown of b-carotene in unblanched dehydrated carrots occurs easily compared to blanched dehydrated carrots. The high level of b-carotene oxidation in unblanched carrots might be attributed to the dehydration without blanching treatment which inhibits enzyme activity. It was determined that a first-order reaction best described the degradation of b-carotene in dehydrated carrot slices. Wagner and Warthesen (1995) also determined a first-order reaction for the degradation of both a-carotene and b-carotene. It was reported that carotene degradation due to heat, oxidation and light followed first-order or pseudo-first-order behaviour (Wagner & Warthesen, 1995). Pesek and Warthesen (1987) investigated the carotene degradation in a vegetable juice system and found a- and b-carotene degradation followed first-order kinetics. A similar conclusion was also reported by Chou and Brene (1972) and Stevanovich and Karel (1982) in b-carotene degradations in model dry systems. In addition, Baloch et al. (1977) found that carotenoid oxidation in dehydrated carrots followed a first-order reaction. On the basis of linear regression analysis of natural logarithms of rate constants against reciprocal absolute temperature 1/T in °K (Fig. 3), activation energies of bcarotene degradation were calculated as 9.3 ± 2.07 kcal mol1 and 15.8 ± 0.82 kcal mol1 for unblanched and blanched carrot slices, respectively (Table 1). It can be suggested that the degradation of b-carotene in blanched samples easily affected by temperature changes. The loss of b-carotene at 27, 37, 47 and 57 °C for blanched and unblanched carrots during storage were in the ranges of 52–72% and 87–99%, respectively. Blanching prior to dehydration of carrots enhanced the stability of
7.00 y = 7.95x - 20.649, R2 = 0.803
6.00 5.00
-ln k
452
4.00 3.00 y = 4.69x - 11.702, R2 = 0.9696
2.00
Unblanched
1.00
Blanched 0.00 3
3.1
3.2 1/T 10 3 (˚K)
3.3
3.4
Fig. 3. Arrhenius plots for b-carotene degradation in blanched and unblanched carrot slices.
carotenoids during storage. This is consistent with the findings of Arya et al. (1982), Baloch (1987) and Zhao and Chang (1995). Absorbance values (A420) at 420 nm were used as an index of brown pigment formation in dehydrated carrot slices. Nonenzymatic browning in dehydrated carrot slices followed a zero-order reaction kinetic model (Figs. 4 and 5), which was also determined by Baloch et al. (1977) in dehydrated carrot. During storage, unblanched slices showed higher browning than blanched samples as reported by Negi and Roy (2001). The differences between the initial and the final A420 values for unblanched samples at 27, 37, 47 and 57 °C were 0.018, 0.058, 0.197 and 0.138; while those for blanched samples were found as 0.014, 0.033, 0.149 and 0.093. The browning rate constants of dehydrated carrots increased with increasing storage time and temperature as
0.400 57 ˚C
0.350 0.300
Dehydrated carrots
Blanched
Unblanched
Temperature (°C)
Rate constant (k)
Determi nation coefficient (R2)
Activation energy (kcal mol1)
0.250
A 420
Table 1 Kinetic parameters of first-order b-carotene degradation in dehydrated carrot slices during storage at different temperatures
47 ˚C
0.200 0.150 37 ˚C
0.100
27 37 47 57
0.004 ± 0.0015 0.006 ± 0.0020 0.007 ± 0.0012 0.052 ± 0.0012
0.9543 0.9840 0.9836 0.8977
15.8 ± 0.82
27 37 47 57
0.022 ± 0.0001 0.027 ± 0.0028 0.052 ± 0.0118 0.085 ± 0.0173
0.9638 0.9833 0.9675 0.9680
9.3 ± 2.07
27 ˚C
0.050 0.000 0
50
100 150 Storage time (day)
200
Fig. 4. Nonenzymatic browning of unblanched dehydrated carrot slices stored at different temperatures.
N. Koca et al. / Journal of Food Engineering 78 (2007) 449–455
0.300
10 57 ˚C
y = 12.76x - 32.972, R 2 = 0.9839
9
0.250
8
0.200
47 ˚C
0.150
- ln k
A 420
453
0.100 37 ˚C
7 6 y = 13.03x - 34.078, R 2 = 0.9951
5
0.050 0.000
0
50
100 150 Storage time (day)
Blanched
4
27 ˚C
200
Unblanched
3 3
3.1
Fig. 5. Nonenzymatic browning of blanched dehydrated carrot slices stored at different temperatures.
3.2
3.3
3.4
1/T 10 3 (˚K) Fig. 6. Arrhenius plots for nonenzymatic browning of blanched and unblanched dehydrated carrot slices.
previously reported by Legault, Talburt, Mylne, and Bryan (1947) and Baloch et al. (1973). Rate constants of browning reactions occured in blanched and unblanched dehydrated carrots were found similar at storage temperatures of 27, 37 and 47 °C (Table 2). However, it is clearly seen that a remarkable increase in the rate of brown pigment formation was observed in unblanched carrots compared to blanched ones at 57 °C. The nonenzymatic browning reaction had an activation energy of 25.9 ± 1.71 kcal mol1 and 25.3 ± 0.98 kcal mol1 for unblanched and blanched dehydrated carrot slices, respectively. Arrhenius plots for nonenzymatic browning reaction in dehydrated carrot slices is shown in Fig. 6. The oxidation of b-carotene in dehydrated carrot slices stored at different temperatures was also confirmed by the change of a and b values. The a and b values decreased with storage time and temperatures, suggesting that the colour of dehydrated carrot slices changed from red to lighter colour. It was noted that red and yellow colour of unblanched dehydrated carrot slices disappeared more rapidly relative to blanched samples (Table 3). The decrease in a and b values in carrot samples correlates with the loss of b-carotene. A significant relationship was found between
the colour loss and b-carotene destruction in blanched (r = 0.878–0.971; p < 0.05) and unblanched samples (r = 0.903–0.998; p < 0.05) stored at all temperatures. The surface colour of dried powder of carrots was also monitored during storage using Hunter colour ratio a/b by Inazu and Makino (1998). They observed that a/b value of dried powder of carrots decreased with time, indicating the colour changed from orange to white. Arrhenius plots for red and yellow colour loss in dehydrated carrot slices are shown in Fig. 7. It was pointed out that temperature dependence of colour loss as observed for b-carotene destruction was found higher in blanched samples. The lower activation energies for colour loss determined on the basis of a (10.5 ± 0.62 kcal mol1) and b (14.7 ± 1.30 kcal mol1) values in unblanched samples indicating that unblanched carrots are more susceptible to colour degradation than blanched samples. For blanched samples, activation energies of colour loss represented with the a and b values were found as 15.7 ± 2.81 kcal mol1 and 21.7 ± 4.13 kcal mol1, respectively.
Table 2 Kinetic parameters of zero-order nonenzymatic browning reaction in dehydrated carrot slices during storage at different temperatures Dehydrated carrots
Temperature (°C)
Rate constant (k)
Determination coefficient (R2)
Activation energy (kcal mol1)
Blanched
27 37 47 57
0.0001 ± 0.00 0.0002 ± 0.00 0.001 ± 0.0001 0.004 ± 0.0003
0.8286 0.8780 0.9784 0.9768
25.3 ± 0.98
Unblanched
27 37 47 57
0.0001 ± 0.00 0.0003 ± 0.00 0.001 ± 0.00 0.005 ± 0.0001
0.8926 0.9616 0.9700 0.9353
25.9 ± 1.71
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Table 3 Regression equations for a first-order colour loss in dehydrated carrot slices stored at different temperatures Dehydrated carrots
Temperature (°C)
a
b
Blanched
27 37 47 57
y = 0.0004x + 1.3642 y = 0.0013x + 1.3367 y = 0.0020x + 1.2868 y = 0.0048x + 1.3196
(R = 0.907) (R2 = 0.9481) (R2 = 0.9004) (R2 = 0.9335)
y = 0.0001x + 1.5830 y = 0.0005x + 1.5856 y = 0.0005x + 1.5474 y = 0.0038x + 1.5455
(R2 = 0.9438)a (R2 = 0.9301) (R2 = 0.8469) (R2 = 0.9280)
Unblanched
27 37 47 57
y = 0.0031x + 1.3054 y = 0.0048x + 1.2007 y = 0.0050x + 0.9650 y = 0.0178x + 1.2527
(R2 = 0.9808) (R2 = 0.9806) (R2 = 0.8136) (R2 = 0.8850)
y = 0.0006x + 1.5863 y = 0.0012x + 1.5300 y = 0.0012x + 1.4791 y = 0.0071x + 1.5627
(R2 = 0.9702) (R2 = 0.9485) (R2 = 0.7479) (R2 = 0.9532)
a
2
Numbers in brackets gives determination coefficients.
9.50
References
a, unblanched a, blanched
8.50
b, unblanched
- ln k
7.50
y = 10.92x - 28.06 R2 = 0.894
b, blanched
y = 7.9x - 19.452 R2 = 0.9706
6.50 y = 7.41x - 17.946
5.50
R2 = 0.8201
4.50 y = 5.29x - 12.55
3.50
R2 = 0.821
2.50 3
3.1
3.2 1/T 10 3 (˚K)
3.3
3.4
Fig. 7. Arrhenius plots for red and yellow colour loss in blanched and unblanched dehydrated carrot slices.
4. Conclusion Kinetics of colour degradation and nonenzymatic browning was investigated in blanched and unblanched dehydrated carrots in the storage temperatures of 27– 57 °C. The first-order kinetic model was best fitted to the degradation of b-carotene, while nonenzymatic browning followed a zero-order reaction. Blanching prior to dehydration enhances the retention of b-carotene in dehydrated carrots. The decrease in a and b values in dehydrated carrot slices correlates with the loss of b-carotene. Objective colour measurements can be suggested instead of chemical analyses for studying the degradation of b-carotene, since a significant relationship was found between the colour loss and b-carotene degradation in dehydrated carrot slices. Acknowledgements This work was supported by research grant 2005-0745009 from Scientific Research Projects at Ankara University. We also thank Dr. Kamil Sacılık in the Department of Agricultural Machinery, Faculty of Agriculture at Ankara University for providing hot-air drier facilities.
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Pesek, C. A., & Warthesen, J. J. (1990). Kinetic model for photoisomerization and concominant photodegradation of b -carotene. Journal of Agricultural and Food Chemistry, 38, 1313–1315. Prakash, S., Jha, S. K., & Data, N. (2004). Performance evaluation of blanched carrots dried by three different driers. Journal of Food Engineering, 62, 305–313. Schadle, E. R., Burns, E. E., & Talley, L. J. (1983). Forced air drying of partially freeze-dried compressed carrot bars. Journal of Food Science, 48, 193–196. Stevanovich, A. F., & Karel, M. (1982). Kinetics of b-carotene degradation at temperature typical of air drying of foods. Journal of Food Processing and Preservation, 6, 227–242. Sulaeman, A., Keeler, L., Gı´raud, D. W., Taylor, S. L., Wehling, R. L., & Driskell, J. A. (2001). Carotenoid content and physicochemical and sensory characteristics of carrot chips deep fried in different oils at several temperatures. Food and Chemical Toxicology, 66, 1257–1264. ¨ ztop, M. (2005). Drying of carrots in Sumnu, G., Turabi, E., & O microwave and halogen lamp-microwave combination ovens. Lebensmittel Wissenschaftliche und Technologie, 38, 548–553. Toribio, J. L., & Lozano, J. E. (1984). Nonenzymatic browning in apple juice concentrate during storage. Journal of Food Science, 49, 889–892. Wagner, L. A., & Warthesen, J. J. (1995). Stability of spray-dried encapsulated carrot carotenes. Journal of Food Science, 60, 1048–1053. Zhao, Y. P., & Chang, K. C. (1995). Sulfite and starch affect colour and carotenoids of dehydrated carrots during storage. Journal of Food Science, 60, 324–326. Ziegler, R. G. (1989). A review of the epidemiological evidence that carotenoids reduce the risk of cancer. Journal of Nutrition, 119, 116–122.
Kinetics of colour changes in dehydrated carrots Nuray Koca, Hande Selen Burdurlu, Feryal Karadeniz
*
Department of Food Engineering, Ankara University, Faculty of Engineering, Campus of Agricultural Faculty, Dısßkapı, 06110 Ankara, Turkey Received 3 August 2005; accepted 17 October 2005 Available online 2 December 2005
Abstract Carotenoid degradation and colour loss in dehydrated blanched and unblanched carrot slices during storage at 27, 37, 47 and 57 °C were investigated. The degradation of b-carotene and colour loss followed a first-order reaction. A significant relationship was found between the colour loss and b-carotene degradation in blanched (r = 0.878–0.971, p < 0.05) and unblanched samples (r = 0.903– 0.998, p < 0.05). Degradation of b-carotene had an activation energy of 15.8 ± 0.81 kcal mol1 and 9.3 ± 2.07 kcal mol1 for blanched and unblanched carrots, respectively. For blanched samples, the activation energies of colour loss determined on the basis of a and b values were 15.7 ± 2.81 kcal mol1 and 21.7 ± 4.13 kcal mol1, while those for unblanched samples determined as 10.5 ± 0.62 kcal mol1 and 14.7 ± 1.30 kcal mol1, respectively. Nonenzymatic browning of dehydrated carrots was also investigated and this reaction fitted to zero-order kinetic model. Activation energies of nonenzymatic browning occurred in blanched (25.3 ± 2.30 kcal mol1) and unblanched (25.9 ± 2.61 kcal mol1) dehydrated carrots was found similar. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: b-carotene; Colour loss; Nonenzymatic browning; Kinetics
1. Introduction Carotenoids are widely known as provitamin A, while there is an increasing interest in their role as antioxidants (Bohm, Putpitasarı-Nienaber, Ferruzzi, & Schwartz, 2002; Elliot, 1999; Larson, 1988). Epidemiological evidence has shown inverse correlation between consumption of some carotenoid-containing fruits and vegetables and cancer incidence (Kalt, Forney, Martin, & Prior, 1999; Ziegler, 1989). In addition to the anti-cancer activity, other health benefits provided by b-carotene includes protection against cardiovascular disease, cataract prevention (Dietmar & Bamedi, 2001; Sulaeman et al., 2001) and enhanced immune responses (Kurilich et al., 1999). Carrot (Daucus carota L.) has the highest carotenoid content among foods and is consumed in large quantities (Desobry, Netto, & Labuza, 1998). Dehydrated carrots are used as an ingredi*
Corresponding author. Tel.: +90 312 596 10 00x1716/317 05 50; fax: +90 312 317 87 11. E-mail address: [email protected] (F. Karadeniz). 0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.10.014
ent in instant soups or meals and an excellent candidate for developing oil free, healthy snack food if the nutritional value can be well preserved (Lin, Durance, & Scaman, 1998). Dehydration is one of the oldest methods of food preservation and it represents a very important aspect of food processing (Lin et al., 1998). Longer shelf life, product diversity and volume reduction are the reasons for the popularity of dried fruits and vegetables (Prakash, Jha, & Data, 2004). The quality of dehydrated product does not only depend on the drying conditions but also on the other operations carried out before and after drying (Negi & Roy, 2001). For example, blanching is carried out prior to dehydration to inactivate the enzyme peroxidase, which may otherwise lead to formation of unacceptable colours and flavours (Baloch, Buckle, & Edwards, 1977; Mazza, 1983). Blanching before drying was also reported to enhance the stability of carotenoids during storage (Arya, Natesan, Premavalli, & Vijayaraghavan, 1982). Several deteriorative reactions that affect the colour, nutrient properties, texture and flavour of dehydrated
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products are initiated during processing and dehydration operations, and continue during storage at a rate that is influenced by storage conditions. An important aspect concerned with dehydrated food products is their stabilization to chemical and physical changes as well as microbiological degradation, which could occur during storage (Can˜ellas, Rossello, Simal, Soler, & Mulet, 1993). Degradation of carotenoids not only affects the attractive colour of foods but also their nutritive value and flavour (Pesek & Warthesen, 1990). Dehydrated vegetables lose colour due to the oxidation of highly unsaturated molecules upon exposure to air during storage (Park, 1987). Carotenoids are susceptible to oxidation when exposed to light (Pesek & Warthesen, 1990), oxygen and enzymes (Gregory, 1996). A rapid degradation of b-carotene associated with the development of an off flavour in dehydrated carrot was observed by Park (1987). Another important chemical change occurred in some dehydrated foods is nonenzymatic browning (Baloch, Buckle, & Edwards, 1973). The formation of dark-coloured pigments in foods during processing and storage is a very common phenomenon. Most of the browning occurring in foods during drying is due to Maillard reaction (Mcbean, Joslyn, & Nury, 1971). Browning can also appear during long storage, and is generally dependent upon product characteristics and storage conditions (Toribio & Lozano, 1984). The most common method for characterizing browning in dehydrated carrot is the measurement of absorbance at 420 nm (Baloch et al., 1973). In addition, tristimulus colourimetry was also used to evaluate colour changes in carrots (Chen, Peng, & Chen, 1995; Inazu & Makino, 1998; Zhao & Chang, 1995). The Hunter a and b values were used by Lin et al. (1998) for determining the colour changes of dried carrot slices. However, studies on determining reaction kinetic parameters by using colorimetric values are limited. The stability of carotenoids and the extent of browning during processing and storage is a very important objective to make the final product attractive and acceptable. Several researchers have studied the effect of drying on carrot quality (Doymaz, 2004; Lin et al., 1998; Mazza, 1983; Park, 1987; Prakash et al., 2004; Schadle, Burns, & Talley, ¨ ztop, 2005). However, the liter1983; Sumnu, Turabi, & O ature contains few references on the retention of b-carotene and colour stability in dehydrated carrots during storage (Baloch et al., 1973; Baloch, 1987; Negi & Roy, 2001). Kinetic parameters including reaction order, rate constant and activation energy are essential to predict the quality of foods. Therefore, kinetic studies are needed in order to minimize the undesired change and to optimize quality of dehydrated specific foods such as carrots. The objectives of this study were to determine the kinetics of carotenoid degradation and nonenzymatic browning and also to evaluate the correlation between total carotenoid content and colour loss in blanched and unblanched dehydrated carrot during storage. In addition, kinetic
parameters of colour loss in dehydrated carrots by using tristimulus colorimetry were also assessed. 2. Materials and methods 2.1. Materials Carrots (Daucus carota L. var. Nanco) were harvested from Beypazarı´, Turkey and stored at 1 °C and 97% humidity until drying. Prior to drying, samples were washed, peeled and sliced into circular discs (5 mm thickness) using a hand-operated slicer. The cut vegetables were separated into two parts. One part remained unblanched, and the other part was blanched in boiling water at 90 °C for 7 min to inactivate peroxidase (EC 1. 11. 1.7). Peroxidase test was made by using guaiacol and H2O2 as substrate according to the modified method of Jacobs (1958). The blanched samples were immediately cooled under tap water at 22 °C and then drained. 2.2. Drying Blanched and unblanched carrots were dried using a hot-air drier designed and fabricated in Department of Agricultural Machinery, Faculty of Agriculture, Ankara University, Ankara, Turkey. Drying conditions were temperature 60 ± 5 °C, air flow 1.5 m/s, RH 6–10% and tray load 3.0–3.4 kg/m2. Dehydrated samples (6–7% moisture content) were packed in polyethylene bags and stored at 27, 37, 47 and 57 °C. Drying experiments were carried out with two replicates. 2.3. Moisture analysis Moisture analysis was performed in a hot air oven at 105 ± 2 °C (AOAC, 1990). Triplicate samples were analyzed and average moisture content was recorded. 2.4. Determination of total carotenoids Total carotenoids were determined using the modified method described by Park (1987). Duplicate 0.5 g dehydrated carrot powder were extracted in a 25 mL 7:3 hexane:acetone mixture using a shaker. The extract was vacuum-filtered through a Buchner funnel with Whatman No. 1 filter paper. The residue was re-extracted until became colourless. The filtrates were combined in a separatory funnel and washed with 50 mL distilled water. The water phase was discarded and Na2SO4 (10 g) was added as desiccant. The hexane phase was transferred to a 50 mL volumetric flask and brought to volume with hexan. The concentration of carotene in the solution was determined by its absorbance at 450 nm with a Unicam UV/Vis spectrophotometer (Cambridge, United Kingdom). The total carotenoid content determined as b-carotene from the standard curve prepared b-carotene standard.
N. Koca et al. / Journal of Food Engineering 78 (2007) 449–455
2.5. Nonenzymatic browning Browning measurements were carried out using the method described by Baloch et al. (1973). Dehydrated carrot powder (2.5 g) was rehydrated for 10 min in 50 mL acetic acid–formaldehyde aqueous solution (2%–1%, v/v) and homogenized for 5 min. The slurry was filtered and 0.5% lead acetate was added to the filtrate. Then, the slurry was collected in a volumetric flask, made up to 100 mL with acetic acid–formaldehyde solution. After mixing, the extract was centrifuged at 2500 rpm for 5 min and the supernatant was mixed with an equal volume of ethyl alcohol. The mixture was centrifuged again and absorbance measurements were taken at 420 and 600 nm by using UV–VIS spectrophotometer. Browning was calculated from the difference between the two absorbance values.
451
at 27, 37, 47 and 57 °C. The initial b-carotene content of dehydrated carrots was higher in blanched carrots compared to unblanched samples since blanching carried out prior to dehydration inactivates the enzyme peroxidase. Unblanched dehydrated carrots contain 606.1 mg b-carotene/kg, while 844.3 mg b-carotene/kg was determined for blanched dehydrated carrots. Carotenoid destruction in unblanched and blanched dehydrated carrots during storage is shown in Figs. 1 and 2, respectively. Blanched and unblanched dehydrated carrots were stored for 180 days. However, unblanched carrots at 47 °C were stored for 90 days, since no b-carotene was detected in these samples after 3 months. At 57 °C, the samples were stored for 25 days since 50% loss of initial b-carotene content
2.5
2.6. Colour The sample colour was also assessed by the chromaticity value a* (red colour) and b* (yellow colour) of the CIE colour system using a Minolta CR-300 colourimeter (Osaka, Japan). The means of four measurements were used. 2.7. Calculation of kinetic parameters The degradation of carotenoids and colour loss in dehydrated carrots were calculated by using the standard equation for a first-order reaction given below:
log (% retention of β-carotene)
2
27 ˚C
37 ˚C
47 ˚C
57 ˚C
1.5 1 0.5 0 0
50
100
150
200
-0.5
ln C ¼ ln C 0 kt where C, the concentration at time t; C0, the concentration at time zero; k, the first-order rate constant (days1); t, the storage time (day). Analysis of kinetic data from A420 values suggested that a zero-order reaction for nonenzymatic browning:
-1 Storage time (day) Fig. 1. b-carotene degradation of unblanched dehydrated carrot slices stored at different temperatures.
C ¼ C 0 kt
k ¼ k 0 eEa =RT where Ea, the activation energy (kcal mol1); k, the rate constant; k0, the pre-exponential factor; R; the universal gas constant (1.987 kcal mol1); T, the absolute temperature (°K). 3. Results and discussion
2.5 log (% retention of β-carotene)
where C, the concentration at time t; C0, the concentration at time zero; k, the zero-order rate constant (days1); t, the storage time (day). Temperature dependence of carotenoid degradation, browning (A420) and CIE a* and b* colour parameters were determined by Arrhenius equation:
2
1.5
1
0.5
37 ˚C
47 ˚C
57 ˚C
0 0
Carotenoid degradation and nonenzymatic browning reactions were investigated in both blanched and unblanced dehydrated carrot slices during storage temperatures
27 ˚C
50
100 Storage time (day)
150
200
Fig. 2. b-carotene degradation of blanched dehydrated carrot slices stored at different temperatures.
N. Koca et al. / Journal of Food Engineering 78 (2007) 449–455
was found at the end of this storage time The loss of bcarotene was found as 69% and 86% for blanched and unblanched samples, respectively. It is known that the reaction should be carried out beyond 50% change of the initial value so that the proper kinetic model can be selected (Labuza, 2000). A spontaneous loss of b-carotene was observed in both blanched and unblanched dehydrated carrot slices during storage. As seen from the rate constants of b-carotene degradation in Table 1, the breakdown of b-carotene in unblanched dehydrated carrots occurs easily compared to blanched dehydrated carrots. The high level of b-carotene oxidation in unblanched carrots might be attributed to the dehydration without blanching treatment which inhibits enzyme activity. It was determined that a first-order reaction best described the degradation of b-carotene in dehydrated carrot slices. Wagner and Warthesen (1995) also determined a first-order reaction for the degradation of both a-carotene and b-carotene. It was reported that carotene degradation due to heat, oxidation and light followed first-order or pseudo-first-order behaviour (Wagner & Warthesen, 1995). Pesek and Warthesen (1987) investigated the carotene degradation in a vegetable juice system and found a- and b-carotene degradation followed first-order kinetics. A similar conclusion was also reported by Chou and Brene (1972) and Stevanovich and Karel (1982) in b-carotene degradations in model dry systems. In addition, Baloch et al. (1977) found that carotenoid oxidation in dehydrated carrots followed a first-order reaction. On the basis of linear regression analysis of natural logarithms of rate constants against reciprocal absolute temperature 1/T in °K (Fig. 3), activation energies of bcarotene degradation were calculated as 9.3 ± 2.07 kcal mol1 and 15.8 ± 0.82 kcal mol1 for unblanched and blanched carrot slices, respectively (Table 1). It can be suggested that the degradation of b-carotene in blanched samples easily affected by temperature changes. The loss of b-carotene at 27, 37, 47 and 57 °C for blanched and unblanched carrots during storage were in the ranges of 52–72% and 87–99%, respectively. Blanching prior to dehydration of carrots enhanced the stability of
7.00 y = 7.95x - 20.649, R2 = 0.803
6.00 5.00
-ln k
452
4.00 3.00 y = 4.69x - 11.702, R2 = 0.9696
2.00
Unblanched
1.00
Blanched 0.00 3
3.1
3.2 1/T 10 3 (˚K)
3.3
3.4
Fig. 3. Arrhenius plots for b-carotene degradation in blanched and unblanched carrot slices.
carotenoids during storage. This is consistent with the findings of Arya et al. (1982), Baloch (1987) and Zhao and Chang (1995). Absorbance values (A420) at 420 nm were used as an index of brown pigment formation in dehydrated carrot slices. Nonenzymatic browning in dehydrated carrot slices followed a zero-order reaction kinetic model (Figs. 4 and 5), which was also determined by Baloch et al. (1977) in dehydrated carrot. During storage, unblanched slices showed higher browning than blanched samples as reported by Negi and Roy (2001). The differences between the initial and the final A420 values for unblanched samples at 27, 37, 47 and 57 °C were 0.018, 0.058, 0.197 and 0.138; while those for blanched samples were found as 0.014, 0.033, 0.149 and 0.093. The browning rate constants of dehydrated carrots increased with increasing storage time and temperature as
0.400 57 ˚C
0.350 0.300
Dehydrated carrots
Blanched
Unblanched
Temperature (°C)
Rate constant (k)
Determi nation coefficient (R2)
Activation energy (kcal mol1)
0.250
A 420
Table 1 Kinetic parameters of first-order b-carotene degradation in dehydrated carrot slices during storage at different temperatures
47 ˚C
0.200 0.150 37 ˚C
0.100
27 37 47 57
0.004 ± 0.0015 0.006 ± 0.0020 0.007 ± 0.0012 0.052 ± 0.0012
0.9543 0.9840 0.9836 0.8977
15.8 ± 0.82
27 37 47 57
0.022 ± 0.0001 0.027 ± 0.0028 0.052 ± 0.0118 0.085 ± 0.0173
0.9638 0.9833 0.9675 0.9680
9.3 ± 2.07
27 ˚C
0.050 0.000 0
50
100 150 Storage time (day)
200
Fig. 4. Nonenzymatic browning of unblanched dehydrated carrot slices stored at different temperatures.
N. Koca et al. / Journal of Food Engineering 78 (2007) 449–455
0.300
10 57 ˚C
y = 12.76x - 32.972, R 2 = 0.9839
9
0.250
8
0.200
47 ˚C
0.150
- ln k
A 420
453
0.100 37 ˚C
7 6 y = 13.03x - 34.078, R 2 = 0.9951
5
0.050 0.000
0
50
100 150 Storage time (day)
Blanched
4
27 ˚C
200
Unblanched
3 3
3.1
Fig. 5. Nonenzymatic browning of blanched dehydrated carrot slices stored at different temperatures.
3.2
3.3
3.4
1/T 10 3 (˚K) Fig. 6. Arrhenius plots for nonenzymatic browning of blanched and unblanched dehydrated carrot slices.
previously reported by Legault, Talburt, Mylne, and Bryan (1947) and Baloch et al. (1973). Rate constants of browning reactions occured in blanched and unblanched dehydrated carrots were found similar at storage temperatures of 27, 37 and 47 °C (Table 2). However, it is clearly seen that a remarkable increase in the rate of brown pigment formation was observed in unblanched carrots compared to blanched ones at 57 °C. The nonenzymatic browning reaction had an activation energy of 25.9 ± 1.71 kcal mol1 and 25.3 ± 0.98 kcal mol1 for unblanched and blanched dehydrated carrot slices, respectively. Arrhenius plots for nonenzymatic browning reaction in dehydrated carrot slices is shown in Fig. 6. The oxidation of b-carotene in dehydrated carrot slices stored at different temperatures was also confirmed by the change of a and b values. The a and b values decreased with storage time and temperatures, suggesting that the colour of dehydrated carrot slices changed from red to lighter colour. It was noted that red and yellow colour of unblanched dehydrated carrot slices disappeared more rapidly relative to blanched samples (Table 3). The decrease in a and b values in carrot samples correlates with the loss of b-carotene. A significant relationship was found between
the colour loss and b-carotene destruction in blanched (r = 0.878–0.971; p < 0.05) and unblanched samples (r = 0.903–0.998; p < 0.05) stored at all temperatures. The surface colour of dried powder of carrots was also monitored during storage using Hunter colour ratio a/b by Inazu and Makino (1998). They observed that a/b value of dried powder of carrots decreased with time, indicating the colour changed from orange to white. Arrhenius plots for red and yellow colour loss in dehydrated carrot slices are shown in Fig. 7. It was pointed out that temperature dependence of colour loss as observed for b-carotene destruction was found higher in blanched samples. The lower activation energies for colour loss determined on the basis of a (10.5 ± 0.62 kcal mol1) and b (14.7 ± 1.30 kcal mol1) values in unblanched samples indicating that unblanched carrots are more susceptible to colour degradation than blanched samples. For blanched samples, activation energies of colour loss represented with the a and b values were found as 15.7 ± 2.81 kcal mol1 and 21.7 ± 4.13 kcal mol1, respectively.
Table 2 Kinetic parameters of zero-order nonenzymatic browning reaction in dehydrated carrot slices during storage at different temperatures Dehydrated carrots
Temperature (°C)
Rate constant (k)
Determination coefficient (R2)
Activation energy (kcal mol1)
Blanched
27 37 47 57
0.0001 ± 0.00 0.0002 ± 0.00 0.001 ± 0.0001 0.004 ± 0.0003
0.8286 0.8780 0.9784 0.9768
25.3 ± 0.98
Unblanched
27 37 47 57
0.0001 ± 0.00 0.0003 ± 0.00 0.001 ± 0.00 0.005 ± 0.0001
0.8926 0.9616 0.9700 0.9353
25.9 ± 1.71
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Table 3 Regression equations for a first-order colour loss in dehydrated carrot slices stored at different temperatures Dehydrated carrots
Temperature (°C)
a
b
Blanched
27 37 47 57
y = 0.0004x + 1.3642 y = 0.0013x + 1.3367 y = 0.0020x + 1.2868 y = 0.0048x + 1.3196
(R = 0.907) (R2 = 0.9481) (R2 = 0.9004) (R2 = 0.9335)
y = 0.0001x + 1.5830 y = 0.0005x + 1.5856 y = 0.0005x + 1.5474 y = 0.0038x + 1.5455
(R2 = 0.9438)a (R2 = 0.9301) (R2 = 0.8469) (R2 = 0.9280)
Unblanched
27 37 47 57
y = 0.0031x + 1.3054 y = 0.0048x + 1.2007 y = 0.0050x + 0.9650 y = 0.0178x + 1.2527
(R2 = 0.9808) (R2 = 0.9806) (R2 = 0.8136) (R2 = 0.8850)
y = 0.0006x + 1.5863 y = 0.0012x + 1.5300 y = 0.0012x + 1.4791 y = 0.0071x + 1.5627
(R2 = 0.9702) (R2 = 0.9485) (R2 = 0.7479) (R2 = 0.9532)
a
2
Numbers in brackets gives determination coefficients.
9.50
References
a, unblanched a, blanched
8.50
b, unblanched
- ln k
7.50
y = 10.92x - 28.06 R2 = 0.894
b, blanched
y = 7.9x - 19.452 R2 = 0.9706
6.50 y = 7.41x - 17.946
5.50
R2 = 0.8201
4.50 y = 5.29x - 12.55
3.50
R2 = 0.821
2.50 3
3.1
3.2 1/T 10 3 (˚K)
3.3
3.4
Fig. 7. Arrhenius plots for red and yellow colour loss in blanched and unblanched dehydrated carrot slices.
4. Conclusion Kinetics of colour degradation and nonenzymatic browning was investigated in blanched and unblanched dehydrated carrots in the storage temperatures of 27– 57 °C. The first-order kinetic model was best fitted to the degradation of b-carotene, while nonenzymatic browning followed a zero-order reaction. Blanching prior to dehydration enhances the retention of b-carotene in dehydrated carrots. The decrease in a and b values in dehydrated carrot slices correlates with the loss of b-carotene. Objective colour measurements can be suggested instead of chemical analyses for studying the degradation of b-carotene, since a significant relationship was found between the colour loss and b-carotene degradation in dehydrated carrot slices. Acknowledgements This work was supported by research grant 2005-0745009 from Scientific Research Projects at Ankara University. We also thank Dr. Kamil Sacılık in the Department of Agricultural Machinery, Faculty of Agriculture at Ankara University for providing hot-air drier facilities.
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