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The effect of irradiation temperature on the non-enzymatic browning reaction in cooked rice
ARTICLE IN PRESS
Radiation Physics and Chemistry 76 (2007) 886–892 www.elsevier.com/locate/radphyschem
The effect of irradiation temperature on the non-enzymatic browning reaction in cooked rice Ju-Woon Leea, Sang-Hee Oha, Jae-Hun Kima, Eui-Hong Byuna, Mee Ree Kimb, Min Baekc, Myung-Woo Byuna, a
Radiation Application Research Division, Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, PO Box 1266, Jeongeup, Jeonbuk 580-185, Republic of Korea b Department of Food and Nutrition, Chungnam National University, Gung-Dong 220, Yuseong, Daejeon 305-764, Republic of Korea c Atomic Energy Policy Division, Ministry of Science and Technology, Government Complex-Gwacheon, Kyunggi 427-715, Republic of Korea Received 22 December 2005; accepted 22 July 2006
Abstract The effect of irradiation temperature on the non-enzymatic browning reaction in a sugar–glycine solution and cooked rice generated by gamma irradiation was evaluated in the present study. When the sugar–glycine solution and cooked rice were irradiated at room temperature, the browning reaction was dramatically increased during the post-irradiation period. In the case of irradiation at below the freezing point, the browning by irradiation was retarded during not only irradiation but also a post-irradiation period. The changes of the sugar profile, such as a sugar loss or reducing power of the irradiated sugar–glycine solution and the electron spin resonance signal intensity of the irradiated cooked rice were also decreased with lower irradiation temperature. The present results may suggest that the production of free radicals and a radiolysis product is inhibited during gamma irradiation in the frozen state and it may prevent the browning reaction generated by gamma irradiation from occurring. r 2006 Elsevier Ltd. All rights reserved. Keywords: Non-enzymatic browning reaction; Gamma irradiation; Cooked rice
1. Introduction The non-enzymatic browning reaction is a complex chemical reaction that produces brown pigments with a condensation of various derivatives during a food processing or storage (Hodge, 1953). Brown pigment formation is desirable during some types of food processing (baking, cocoa and coffee roasting, or cooking of meat), while it is undesirable for other Corresponding author. Tel.: +82 63 570 3200; fax: +82 63 570 3202. E-mail address: [email protected] (M.-W. Byun).
technologies (milk drying, thermal treatments for the stabilization of milk, fruit juices, and tomatoes) (Ferna´ndez-Artigas et al., 1999; Martins et al., 2001). Despite these investigations, the non-enzymatic browning reaction is notoriously difficult to control or prevent. Addition of sulfhydryl compounds, acetylation of the amino group, and anitioxidant treatment have been investigated to inhibit or prevent the non-enzymatic browning reaction, however, they have not been applied to food due to their toxicity or weak activity (Friedman, 1996). Irradiation processing of food is now recognized as another method of preserving food and ensuring its
0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2006.07.004
ARTICLE IN PRESS J.-W. Lee et al. / Radiation Physics and Chemistry 76 (2007) 886–892
wholesomeness by sterilization, and it has diverse applications worldwide (Olson, 1995). Some researchers have observed that the color intensity of irradiated foods was increased as the irradiation dose was increased (Roushdi et al., 1981; Kim et al., 2004; Lee et al., 2004). The mechanism of the color change in irradiated meat is clarified through many researches (Brewer, 2004; Taub et al., 1978). However, there have been no scientific publications concerning the mechanism of the nonenzymatic browning in irradiated starchy foods as yet. Nicoli et al. (1994) suggested that irradiation leads to non-enzymatic browning reactions similar to those induced in heat-treated food. They hypothesized that these observations were due to the formation of colored compounds by the Maillard reaction. Oh et al. (2005, 2006) reported on the non-enzymatic browning reaction in a gamma-irradiated aqueous model solution. They observed that the non-enzymatic browning reaction that resulted as a result of gamma irradiation was influenced by the conditions of a system such as the reactant type, pH or medium, and this was similar for the browning reaction during heat processing or storage. Gamma irradiation could be applied to chilled or frozen food at low temperatures. The product temperature during irradiation is important because the initial ionization, excitation events and the reactions of the active species are dependent on the temperature (Swallow, 1997). Especially, the free radical, which is a major factor for a chemical change, has a limited mobility in the frozen state. The present study was undertaken to evaluate the effect of temperature on the non-enzymatic browning reaction, which occurred as a result of gamma-irradiation in both an aqueous model solution (glucose/ sucrose–glycine solution) and food (cooked rice).
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temperature for 20 min. The rice soaked in water was cooked by a rice cooker (SR-0611, Cuckoo homesys. co., Ltd., Yangsan, Kyungnam-Do, Korea). The cooked rice was stored at room temperature (2573 1C) after irradiation. 2.3. Gamma irradiation The aqueous solutions and cooked rice were prepared for four treatment batches (25, 4, 20, and 70 1C). Each batch was adjusted to each temperature before irradiation in a refrigerator or a deep-freezer. The samples were irradiated in a cobalt-60 irradiator (IR-7P, MDS Nordion Intl., Ottawa, Ont., Canada) at a chilled or a frozen state by using ice or dry ice. The source strength was about 100 kCi with a dose rate of 10 kGy/h at 1570.5 1C. Dosimetry was performed by using 5-mm-dia alanine dosimeters (Bruker Instruments, Rheinstetten, Germany), and the free radical signal was measured by using a Bruker EMS 104 EPR Analyzer. The absorbed doses in this study were 0, 10, 20, and 30 kGy and the actual doses were typically within 2% of the target doses. 2.4. Determination of the color changes The browning of the irradiated sugar–glycine solutions was determined via an absorbance at 420 nm as a index of the brown polymers formed in the more advanced stages of a non-enzymatic browning reaction in a spectrophotometer (UV-1601 PC, Shimadzu Co., Tokyo, Japan) by using an untreated solution as a reference (Hodge, 1953). The color of the cooked rice was measured at the surface using a Color Difference Meter (Spectrophotometer CM-3500d, Minolta Co, Ltd., Osaka, Japan).
2. Materials and methods
2.5. Determination of the reducing level of the sugar
2.1. Preparation and storage of the aqueous solutions
Reducing sugars were determined by the 3,5-dinitrosalicylic acid (DNSA) method (Miller, 1959). One milliliter of the sample was transferred into 15 ml glass tubes and 2 ml of the modified DNSA reagent (0.5 g dinitrosalicylic acid, 8 g sodium hydrate, and 150 g rochell salt in distilled water up to 500 ml) was added. The solution was mixed with a vortex mixer for 5 s and it was boiled for 10 min, and then cooled in ice. The reducing sugar level of the sample was analyzed by the spectrophotometer in reference to the glucose standard.
The sugar–glycine solutions were prepared as follows: each sugar (glucose or sucrose) and glycine was dissolved in deionized water at a concentration of 0.4 M. The sugar–glycine solutions were prepared as equimolar (0.2 M) mixtures. The suagr–glycine solution was stored at room temperature (2573 1C) after irradiation. 2.2. Preparation and storage of the cooked rice
2.6. Electron spin resonance (ESR) analysis Milled, non-waxy and medium grain rice (Oryza sativa L. cv Dongjinbyeo, harvested in Dangjin, Chungnam-Do, Korea) was purchased at a local supermarket in Daejeon, Korea in 2005. Raw grains (300 g) were soaked in 450 ml of deionized water at room
The cooked rice was dried in a freeze dry system (SFDSF12, Samwon freezing co., Seoul, Korea) to remove moisture and short-life ESR signals, and ground into powder (ca. 20 mesh) and about 0.2 g was placed in
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a ESR quartz tube. The ESR spectrum was measured at a microwave frequency of 9.18 GHz, a magnetic field of 328.2070.5 mT, a microwave power of 0.4 mW, modulation of 100 kHz, a time constant of 0.03 s, and a sweep time of 30 s by using an ESR spectrometer (JESTE300, Jeol Co., Tokyo, Japan). At this time, the spectra of the samples were scanned to record the signal intensity (peak-to-peak height). 2.7. Statistical analysis For the statistical analysis of the samples Windows SPSS 10.0 (SPSS, 1999) was used. The general linear model procedure was processed and Student–Newman– Keul’s multiple range test was used to compare the differences in the mean values. Mean values and pooled standard errors of the mean (SEM) were reported, and the significance was defined at Po0:05.
3. Results Fig. 1 shows the brown color development, measured at 420 nm, of the sugar–glycine solutions after gamma irradiation at various temperatures. After gamma irradiation, the browning of the irradiated solution was continued to develop during storage at room temperature in both the glucose–glycine solution and the sucrose–glycine solution. However, the brown color development differed with the temperature condition during irradiation. In the case of irradiation at room temperature, the browning was dramatically increased during the post-irradiation period. However, the browning developed more slowly at low irradiation temperatures. When the sugar–glycine solutions were irradiated
below the freezing point in a container within dry-ice, the browning was controlled not only during irradiation but also during the post-irradiation period. The irradiation temperature also influenced the sugar loss (Fig. 2) and the reducing power production (Fig. 3). When the sugar–glycine solution was irradiated in a frozen state, the sugar loss is much less than in the unfrozen state, in both the glucose–glycine and the sucrose–glycine solution. The reducing power of the sucrose–glycine solution was produced by irradiation. However, the reducing power production was decreased at low temperatures. In the present study, there were no further changes of the sugar profile (contents and reducing power) during the post-irradiation periods (data is not shown). The effect of irradiation temperature on the browning reaction was also investigated in rice. Table 1 shows the hunter color values of the cooked rice irradiated at various temperatures. The yellowness (b-value) of the cooked rice was significantly (Po0:05) increased with the irradiation dose after irradiation at room temperature. When the cooked rice was irradiated in the frozen state, the browning was effectively controlled during irradiation as well as storage at room temperature. Fig. 4 shows the ESR spectrum and the signal intensity of the irradiated cooked rice at various conditions. As the irradiation temperature was lowered, the ESR signal intensity was decreased.
4. Discussion The non-enzymatic browning reaction generated by gamma irradiation has been hypothesized to be the result of the breakdown of the glycosidic and peptidic linkages being promoted during irradiation and then the
0.30
1.0
0.8 Absorbance at 420 nm
Absorbance at 420 nm
0.25 0.20 0.15 0.10
0.4
0.2
0.05 0.00
0.0 0
(A)
0.6
20 40 60 80 100 Post-irradiation period (hr)
0 (B)
20 40 60 80 100 Post-irradiation period (hr)
Fig. 1. Effects of the irradiation conditions on a brown color development of a sugar–glycine solution after gamma-irradiation at 30 kGy: (A) glucose–glycine; (B) sucrose–glycine. Room temperature (J), 4 1C (’), 20 1C (,), 70 1C (&).
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8
16
7
14
6
Loss of sucrose (%)
Loss of glucose (%)
J.-W. Lee et al. / Radiation Physics and Chemistry 76 (2007) 886–892
12 10 8 6
889
5 4 3
4
2
2
1 0
0 0 (A)
10 20 Dose (kGy)
30
0 (B)
10 20 Dose (kGy)
30
Fig. 2. Effects of the irradiation conditions on a sugar loss (%) of a sugar–glycine solution during gamma-irradiation: (A) glucose–glycine; (B) sucrose–glycine. Room temperature (J), 4 1C (’), 20 1C (,), 70 1C (&).
0.4
Reducing power (%)
0.3
0.2
0.1
0.0 0
10
20
30
Dose (kGy) Fig. 3. Effects of the irradiation conditions on a reducing power (%) production of a sucrose–glycine solution during gamma-irradiation: (A) glucose–glycine; (B) sucrose–glycine. Room temperature (J), 4 1C(’), 20 1C(,), 70 1C (&).
breakdown products, such as the carbonyl and amino compounds, react forming colored compounds (Liggett et al., 1959). In the present study, the browning of a sugar–glycine solution was developed not only during gamma-irradiation but also during the post-irradiation period (Fig. 1). This observation may suggest that the breakdown products which were produced by the water radicals or due to direct effects of the ionizing radiation
during irradiation were condensed to colored copolymers during the post-irradiation period as previous hypothesis (Liggett et al., 1959). However, a little browning was observed when the sugar–solution was irradiated in the frozen state not only during irradiation but also during the post-irradiation periods (Fig. 1). The water state is important to the chemical changes induced by ionizing radiation because the changes are caused mostly by the products of water radiolysis. When pure water is irradiated, a number of highly reactive entities such as aqueous electrons (e aq), hydroxyl radical (dOH), hydroperoxyl radical (dH2O) and superoxide anion radical (dO 2 ) are formed. In the frozen state, the reactive intermediates of water radiolysis are trapped, and hence, not free to interact with each other or with other components (Furuta et al., 1992). In fact, little changes of the sugar profiles, such as the content and reducing power, in a solution irradiated at the frozen state were observed (Figs. 2 and 3). In a non-enzymatic browning reaction, the reducing power production is important because it has a reactive site (Hodge, 1953). Irradiation induced scission of a glycosidic linkage leads to the generation of the free radical at the C1 position on the glucose molecule in starch or disaccharide in the presence of water and, consequently, these radiolytic end-products have a reducing power (Stewart, 2001). These results may suggest that when the solution or food was irradiated in the frozen state, the reactive intermediates of water radiolysis are trapped, therefore the radiolysis of a component such as sugar and amino acid is inhibited and finally the browning reaction is prevented. This hypothesis was supported by the ESR spectrum of the irradiated cooked rice at various temperatures (Fig. 4). The ESR results in the present work on cooked rice are in agreement with those reported for starch (Korkmaz and Polat, 2000; Bertolini et al., 2001). The ESR pattern of irradiated starch can
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Table 1 Effects of the irradiation conditions on the hunter color values of gamma-irradiated cooked rice during a storage at room temperaturea Storage time (days)
L-value 0
11
a-value 0
11
b-value 0
11
Dose (kGy)
Irradiation conditionsb
SEMd
RTc
4 1C
20 1C
70 1C
0 10 20 30
50.37a 48.36a 47.85b 47.31b
49.09a 48.76ab 48.83ab
50.25a 50.67a 50.06a
50.26a 50.83a 50.94a
1.202 0.941 0.805
0 10 20 30
46.11a 47.58a 47.27a 46.42a
47.44a 47.11a 47.67a
50.17b 50.26b 50.43b
50.44b 50.23b 50.30b
1.081 1.172 0.762
0 10 20 30
1.03a 0.92a 0.90b 0.90a
0.97a 0.93ab 0.92a
1.03a 1.01ab 1.02a
1.03a 1.03ab 1.01a
0.082 0.060 0.061
0 10 20 30
1.04a 0.80c 0.79b 0.75b
0.89bc 0.89ab 0.88ab
1.04ab 1.02a 1.05a
1.00a 1.04a 1.10a
0.062 0.073 0.081
0 10 20 30
2.00a 1.19b 0.98b 0.90c
1.27b 0.99b 0.98c
1.52ab 1.39b 1.27bc
1.98a 1.99a 1.74ab
0.305 0.221 0.232
0 10 20 30
1.75a 0.74c 0.07c 0.35c
0.67c 0.31c 0.12c
1.05bc 1.07b 1.00b
1.39ab 1.02b 1.03b
0.251 0.251 0.182
a
Values with different letters (a–c) within a row and control (0 kGy irradiated sample) differ significantly (Po0:05). Temperature during gamma-irradiation. c Room temperature. d Standard error of the mean (n ¼ 50). b
present two main signals, AA0 and BB0 : the AA0 signal has been tentatively assigned to an R OR radical at C1 of anhydroglucose which turns into an ROOd radical after the breaking of the glycosidic bond and contact with atmospheric oxygen. This ROOd radical give a BB0 signal (Raffi et al., 1985). The intensity of free radicals is dependent on starch water content, irradiation dose, temperature and time of storage. The dispersion of ions and free radicals is higher when free water is in the liquid form and lower when free water is limited (dried products) or frozen (crystalline) (Raffi and Agnel, 1983). When water is frozen, free radicals tend to recombine forming the original substance, because diffusion is reduced limiting their access to other food components (Taub et al., 1978). Formation of a primary hydroxyalkyl (pHA) radical is a common event in
crystalline carbohydrates containing a primary alcohol group when they are irradiated at low temperature (Madden and Bernhard, 1980). Cooked rice is a major food in Asia. However, it is very perishable because of its high water activity and nutrients. Although irradiation is able to improve the shelf-life of cooked rice, its application has been limited because consumers are sensitive to changes of the sensory properties of cooked rice such as the flavor, texture, and color after irradiation (Lee et al., 2004). However, when cooked rice was irradiated in the frozen state, the browning reaction was effectively prevented during irradiation as well as during storage at room temperature in the present study (Table 1). Irradiation in the frozen state prevented a browning reaction not only in the aqueous model solution but also the cooked rice.
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References (a) 550 a.u.
(b) 1463 a.u.
(c) 1404 a.u.
(d) 1328 a.u.
(e) 815 a.u. 300
310
320 330 340 Magnetic Field (mT)
350
360
Fig. 4. ESR spectrum and signal intensity (a.u.) of gammairradiated cooked rice at various conditions: (a) 0 kGy; (b) 30 kGy at room temperature; (c) 30 kGy at 4 1C; (d) 30 kGy at 20 1C; (e) 30 kGy at 70 1C.
This result is supported by previous reports. Coleby et al. (1962) and Farkas (1987) reported that the threshold dose for an off-flavor, e.g., in frozen poultry and meat are at least two-fold higher when compared with the chilled ones.
5. Conclusion Based on the present results, gamma irradiation produces free radicals and radiolysis products of sugar and glycine. These products and free radicals may condense to produce colored products during the postirradiation period. However, when the food is irradiated in the frozen state, the production of free radicals and radiolysis products is inhibited and it prevents the browning reaction.
Ackowledgements This research was supported by Korea Science and Engineering Foundation (KOSEF) and Ministry of Science & Technology (MOST), Korean government, through its National Nuclear Technology Program.
Bertolini, A.C., Mestres, C., Colonna, P., Raffi, J., 2001. Free radical formation in UV- and gamma-irradiated cassava starch. Carbohydr. Polym. 44, 269–271. Brewer, S., 2004. Irradiation effects on meat color—a review. Meat Sci. 68, 1–17. Coleby, B., Ingram, M., Shepherd, H.J., Thronley, M.J., Wilson, G.M., 1962. Treatment of meats with ionizing radiation. VII. Effect of low temperature during radiation. J. Sci. Food Agric. 6, 483–490. Farkas, J., 1987. Decontamination, including parasite control, of dried, chilled and frozen foods by irradiation. Acta Aliment 16, 351–384. Ferna´ndez-Artigas, P., Guerra-Herna´ndez, E., Garcı´ a-Villanova, B., 1999. Browning indicators in model systems and baby cereals. J. Agric. Food Chem. 47 (7), 2872–2878. Friedman, M., 1996. Food browning and its prevention: an overview. J. Agric. Food Chem. 44 (3), 631–653. Furuta, M., Dohmaru, T., Katayama, T., Toratani, H., Takeda, A., 1992. Detection of irradiated frozen meat and poultry using carbon monoxide gas as a probe. J. Agric. Food Chem. 40, 1099–1100. Hodge, J.E., 1953. Dehydrated foods: chemistry of browning reactions in model systems. J. Agric. Food Chem. 1 (15), 928–943. Kim, J.H., Ahn, H.J., Yook, H.S., Kim, K.S., Rhee, M.S., Ryu, G.H., Byun, M.W., 2004. Color, flavor, and sensory characteristics of gamma-irradiated salted and fermented anchovy sauce. Radiat. Phys. Chem. 69 (2), 179–187. Korkmaz, M., Polat, M., 2000. Free radical kinetics of irradiated durum wheat. Radiat. Phys. Chem. 58, 169–179. Lee, Y.S., Oh, S.H., Lee, J.W., Kim, J.H., Rhee, C.O., Lee, H.K., Byun, M.W., 2004. Effects of gamma irradiation on quality of cooked rice. J. Korean Soc. Food Sci. Nutr. 33 (3), 582–586. Liggett, R.W., Feazel, C.E., Ellengerg, J.Y., 1959. Browning reaction initiated by gamma irradiation. J. Agric. Food Chem. 7 (4), 277–280. Madden, K.P., Bernhard, W.A., 1980. A 1,2 hydrogen shift and other thermally induced free radical reactions in Xirradiated methyl a-D-glucopyranose single crystals: an electron spin resonace-electron double resonance study. J. Phys. Chem. 86, 4033–4036. Martins, S.I.F.S., Jongen, W.M.F., van Boekel, M.A.J.S., 2001. A review of Maillard reaction in food and implications to kinetic modeling. Trends Food Sci. Technol. 11, 364–373. Miller, G.L., 1959. Use of dinitrosalicylic reagent for determination of reducing groups. Anal. Chem. 31, 426–428. Nicoli, M.C., Casadei, M.A., Guerzoni, M.E., Lerici, C.R., 1994. Nonenzymatic browning reactions in irradiated glucose–glycine aqueous model systems. Appl. Radiat. Isot. 45 (3), 389. Oh, S.H., Lee, Y.S., Lee, J.W., Kim, M.R., Yook, H.S., Byun, M.W., 2005. The effect of gamma-irradiation on the nonenzymatic browning reaction in the aqueous model solutions. Food Chem. 92 (2), 357–363. Oh, S.H., Lee, Y.S., Kim, J.H., Kim, J.H., Lee, J.W., Kim, M.R., Yook, H.S., Byun, M.W., 2006. Effect of pH on nonenzymatic browning reaction during gamma-irradiation
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processing using sugar and sugar–glycine solutions. Food Chem. 94 (3), 420–427. Olson, D.G., 1995. Irradiation processing. In: Marano, E.A. (Ed.), Food Irradiation: A Source Book. Iowa State University Press, Ames, pp. 3–7. Raffi, J., Agnel, J., 1983. Influence of the physical structure of irradiated starches on their electron spin resonance spectra kinetics. J. Phys. Chem. 87, 2369–2373. Raffi, J., Agnel, J.P., Boizot, C., Thiery, C., Vincent, P., 1985. Flucose oligomers as models to elucidate the starch radiolysis mechanism. Starch 37, 228–231. Roushdi, M., Harras, A., El-Meligi, A., Bassim, M., 1981. Effect of high dose of gamma rays on corn grains. I. Influence on the chemical composition of whole grains and
the technololgical process of starch and by-product isolation. Cereal Chem. 58 (2), 110–112. SPSS, 1999. SPSS for windows Rel. 10.05. SPSS Inc., Chicago, USA. Stewart, E.M., 2001. Food irradiation chemistry. In: Molins, R.A. (Ed.), Food Irradiation: Principles and Applications. Wiley Interscience, New York, pp. 37–76. Swallow, A.J., 1997. Chemical effects of irradiation. In: Elias, P.S., Cohen, A.J. (Eds.), Radiation Chemistry of Major Food Components. Elsevier Scientific, Amsterdam, pp. 5–20. Taub, I.A., Karielian, R.A., Halliday, J.W., 1978. Radiation Chemistry of High Protein Food Irradiated at Low Temperature. Food Preservation by Irradiation, vol. 1. International Atomic Energy Agency, Vinnea, pp. 371–384.
Radiation Physics and Chemistry 76 (2007) 886–892 www.elsevier.com/locate/radphyschem
The effect of irradiation temperature on the non-enzymatic browning reaction in cooked rice Ju-Woon Leea, Sang-Hee Oha, Jae-Hun Kima, Eui-Hong Byuna, Mee Ree Kimb, Min Baekc, Myung-Woo Byuna, a
Radiation Application Research Division, Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, PO Box 1266, Jeongeup, Jeonbuk 580-185, Republic of Korea b Department of Food and Nutrition, Chungnam National University, Gung-Dong 220, Yuseong, Daejeon 305-764, Republic of Korea c Atomic Energy Policy Division, Ministry of Science and Technology, Government Complex-Gwacheon, Kyunggi 427-715, Republic of Korea Received 22 December 2005; accepted 22 July 2006
Abstract The effect of irradiation temperature on the non-enzymatic browning reaction in a sugar–glycine solution and cooked rice generated by gamma irradiation was evaluated in the present study. When the sugar–glycine solution and cooked rice were irradiated at room temperature, the browning reaction was dramatically increased during the post-irradiation period. In the case of irradiation at below the freezing point, the browning by irradiation was retarded during not only irradiation but also a post-irradiation period. The changes of the sugar profile, such as a sugar loss or reducing power of the irradiated sugar–glycine solution and the electron spin resonance signal intensity of the irradiated cooked rice were also decreased with lower irradiation temperature. The present results may suggest that the production of free radicals and a radiolysis product is inhibited during gamma irradiation in the frozen state and it may prevent the browning reaction generated by gamma irradiation from occurring. r 2006 Elsevier Ltd. All rights reserved. Keywords: Non-enzymatic browning reaction; Gamma irradiation; Cooked rice
1. Introduction The non-enzymatic browning reaction is a complex chemical reaction that produces brown pigments with a condensation of various derivatives during a food processing or storage (Hodge, 1953). Brown pigment formation is desirable during some types of food processing (baking, cocoa and coffee roasting, or cooking of meat), while it is undesirable for other Corresponding author. Tel.: +82 63 570 3200; fax: +82 63 570 3202. E-mail address: [email protected] (M.-W. Byun).
technologies (milk drying, thermal treatments for the stabilization of milk, fruit juices, and tomatoes) (Ferna´ndez-Artigas et al., 1999; Martins et al., 2001). Despite these investigations, the non-enzymatic browning reaction is notoriously difficult to control or prevent. Addition of sulfhydryl compounds, acetylation of the amino group, and anitioxidant treatment have been investigated to inhibit or prevent the non-enzymatic browning reaction, however, they have not been applied to food due to their toxicity or weak activity (Friedman, 1996). Irradiation processing of food is now recognized as another method of preserving food and ensuring its
0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2006.07.004
ARTICLE IN PRESS J.-W. Lee et al. / Radiation Physics and Chemistry 76 (2007) 886–892
wholesomeness by sterilization, and it has diverse applications worldwide (Olson, 1995). Some researchers have observed that the color intensity of irradiated foods was increased as the irradiation dose was increased (Roushdi et al., 1981; Kim et al., 2004; Lee et al., 2004). The mechanism of the color change in irradiated meat is clarified through many researches (Brewer, 2004; Taub et al., 1978). However, there have been no scientific publications concerning the mechanism of the nonenzymatic browning in irradiated starchy foods as yet. Nicoli et al. (1994) suggested that irradiation leads to non-enzymatic browning reactions similar to those induced in heat-treated food. They hypothesized that these observations were due to the formation of colored compounds by the Maillard reaction. Oh et al. (2005, 2006) reported on the non-enzymatic browning reaction in a gamma-irradiated aqueous model solution. They observed that the non-enzymatic browning reaction that resulted as a result of gamma irradiation was influenced by the conditions of a system such as the reactant type, pH or medium, and this was similar for the browning reaction during heat processing or storage. Gamma irradiation could be applied to chilled or frozen food at low temperatures. The product temperature during irradiation is important because the initial ionization, excitation events and the reactions of the active species are dependent on the temperature (Swallow, 1997). Especially, the free radical, which is a major factor for a chemical change, has a limited mobility in the frozen state. The present study was undertaken to evaluate the effect of temperature on the non-enzymatic browning reaction, which occurred as a result of gamma-irradiation in both an aqueous model solution (glucose/ sucrose–glycine solution) and food (cooked rice).
887
temperature for 20 min. The rice soaked in water was cooked by a rice cooker (SR-0611, Cuckoo homesys. co., Ltd., Yangsan, Kyungnam-Do, Korea). The cooked rice was stored at room temperature (2573 1C) after irradiation. 2.3. Gamma irradiation The aqueous solutions and cooked rice were prepared for four treatment batches (25, 4, 20, and 70 1C). Each batch was adjusted to each temperature before irradiation in a refrigerator or a deep-freezer. The samples were irradiated in a cobalt-60 irradiator (IR-7P, MDS Nordion Intl., Ottawa, Ont., Canada) at a chilled or a frozen state by using ice or dry ice. The source strength was about 100 kCi with a dose rate of 10 kGy/h at 1570.5 1C. Dosimetry was performed by using 5-mm-dia alanine dosimeters (Bruker Instruments, Rheinstetten, Germany), and the free radical signal was measured by using a Bruker EMS 104 EPR Analyzer. The absorbed doses in this study were 0, 10, 20, and 30 kGy and the actual doses were typically within 2% of the target doses. 2.4. Determination of the color changes The browning of the irradiated sugar–glycine solutions was determined via an absorbance at 420 nm as a index of the brown polymers formed in the more advanced stages of a non-enzymatic browning reaction in a spectrophotometer (UV-1601 PC, Shimadzu Co., Tokyo, Japan) by using an untreated solution as a reference (Hodge, 1953). The color of the cooked rice was measured at the surface using a Color Difference Meter (Spectrophotometer CM-3500d, Minolta Co, Ltd., Osaka, Japan).
2. Materials and methods
2.5. Determination of the reducing level of the sugar
2.1. Preparation and storage of the aqueous solutions
Reducing sugars were determined by the 3,5-dinitrosalicylic acid (DNSA) method (Miller, 1959). One milliliter of the sample was transferred into 15 ml glass tubes and 2 ml of the modified DNSA reagent (0.5 g dinitrosalicylic acid, 8 g sodium hydrate, and 150 g rochell salt in distilled water up to 500 ml) was added. The solution was mixed with a vortex mixer for 5 s and it was boiled for 10 min, and then cooled in ice. The reducing sugar level of the sample was analyzed by the spectrophotometer in reference to the glucose standard.
The sugar–glycine solutions were prepared as follows: each sugar (glucose or sucrose) and glycine was dissolved in deionized water at a concentration of 0.4 M. The sugar–glycine solutions were prepared as equimolar (0.2 M) mixtures. The suagr–glycine solution was stored at room temperature (2573 1C) after irradiation. 2.2. Preparation and storage of the cooked rice
2.6. Electron spin resonance (ESR) analysis Milled, non-waxy and medium grain rice (Oryza sativa L. cv Dongjinbyeo, harvested in Dangjin, Chungnam-Do, Korea) was purchased at a local supermarket in Daejeon, Korea in 2005. Raw grains (300 g) were soaked in 450 ml of deionized water at room
The cooked rice was dried in a freeze dry system (SFDSF12, Samwon freezing co., Seoul, Korea) to remove moisture and short-life ESR signals, and ground into powder (ca. 20 mesh) and about 0.2 g was placed in
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a ESR quartz tube. The ESR spectrum was measured at a microwave frequency of 9.18 GHz, a magnetic field of 328.2070.5 mT, a microwave power of 0.4 mW, modulation of 100 kHz, a time constant of 0.03 s, and a sweep time of 30 s by using an ESR spectrometer (JESTE300, Jeol Co., Tokyo, Japan). At this time, the spectra of the samples were scanned to record the signal intensity (peak-to-peak height). 2.7. Statistical analysis For the statistical analysis of the samples Windows SPSS 10.0 (SPSS, 1999) was used. The general linear model procedure was processed and Student–Newman– Keul’s multiple range test was used to compare the differences in the mean values. Mean values and pooled standard errors of the mean (SEM) were reported, and the significance was defined at Po0:05.
3. Results Fig. 1 shows the brown color development, measured at 420 nm, of the sugar–glycine solutions after gamma irradiation at various temperatures. After gamma irradiation, the browning of the irradiated solution was continued to develop during storage at room temperature in both the glucose–glycine solution and the sucrose–glycine solution. However, the brown color development differed with the temperature condition during irradiation. In the case of irradiation at room temperature, the browning was dramatically increased during the post-irradiation period. However, the browning developed more slowly at low irradiation temperatures. When the sugar–glycine solutions were irradiated
below the freezing point in a container within dry-ice, the browning was controlled not only during irradiation but also during the post-irradiation period. The irradiation temperature also influenced the sugar loss (Fig. 2) and the reducing power production (Fig. 3). When the sugar–glycine solution was irradiated in a frozen state, the sugar loss is much less than in the unfrozen state, in both the glucose–glycine and the sucrose–glycine solution. The reducing power of the sucrose–glycine solution was produced by irradiation. However, the reducing power production was decreased at low temperatures. In the present study, there were no further changes of the sugar profile (contents and reducing power) during the post-irradiation periods (data is not shown). The effect of irradiation temperature on the browning reaction was also investigated in rice. Table 1 shows the hunter color values of the cooked rice irradiated at various temperatures. The yellowness (b-value) of the cooked rice was significantly (Po0:05) increased with the irradiation dose after irradiation at room temperature. When the cooked rice was irradiated in the frozen state, the browning was effectively controlled during irradiation as well as storage at room temperature. Fig. 4 shows the ESR spectrum and the signal intensity of the irradiated cooked rice at various conditions. As the irradiation temperature was lowered, the ESR signal intensity was decreased.
4. Discussion The non-enzymatic browning reaction generated by gamma irradiation has been hypothesized to be the result of the breakdown of the glycosidic and peptidic linkages being promoted during irradiation and then the
0.30
1.0
0.8 Absorbance at 420 nm
Absorbance at 420 nm
0.25 0.20 0.15 0.10
0.4
0.2
0.05 0.00
0.0 0
(A)
0.6
20 40 60 80 100 Post-irradiation period (hr)
0 (B)
20 40 60 80 100 Post-irradiation period (hr)
Fig. 1. Effects of the irradiation conditions on a brown color development of a sugar–glycine solution after gamma-irradiation at 30 kGy: (A) glucose–glycine; (B) sucrose–glycine. Room temperature (J), 4 1C (’), 20 1C (,), 70 1C (&).
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16
7
14
6
Loss of sucrose (%)
Loss of glucose (%)
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12 10 8 6
889
5 4 3
4
2
2
1 0
0 0 (A)
10 20 Dose (kGy)
30
0 (B)
10 20 Dose (kGy)
30
Fig. 2. Effects of the irradiation conditions on a sugar loss (%) of a sugar–glycine solution during gamma-irradiation: (A) glucose–glycine; (B) sucrose–glycine. Room temperature (J), 4 1C (’), 20 1C (,), 70 1C (&).
0.4
Reducing power (%)
0.3
0.2
0.1
0.0 0
10
20
30
Dose (kGy) Fig. 3. Effects of the irradiation conditions on a reducing power (%) production of a sucrose–glycine solution during gamma-irradiation: (A) glucose–glycine; (B) sucrose–glycine. Room temperature (J), 4 1C(’), 20 1C(,), 70 1C (&).
breakdown products, such as the carbonyl and amino compounds, react forming colored compounds (Liggett et al., 1959). In the present study, the browning of a sugar–glycine solution was developed not only during gamma-irradiation but also during the post-irradiation period (Fig. 1). This observation may suggest that the breakdown products which were produced by the water radicals or due to direct effects of the ionizing radiation
during irradiation were condensed to colored copolymers during the post-irradiation period as previous hypothesis (Liggett et al., 1959). However, a little browning was observed when the sugar–solution was irradiated in the frozen state not only during irradiation but also during the post-irradiation periods (Fig. 1). The water state is important to the chemical changes induced by ionizing radiation because the changes are caused mostly by the products of water radiolysis. When pure water is irradiated, a number of highly reactive entities such as aqueous electrons (e aq), hydroxyl radical (dOH), hydroperoxyl radical (dH2O) and superoxide anion radical (dO 2 ) are formed. In the frozen state, the reactive intermediates of water radiolysis are trapped, and hence, not free to interact with each other or with other components (Furuta et al., 1992). In fact, little changes of the sugar profiles, such as the content and reducing power, in a solution irradiated at the frozen state were observed (Figs. 2 and 3). In a non-enzymatic browning reaction, the reducing power production is important because it has a reactive site (Hodge, 1953). Irradiation induced scission of a glycosidic linkage leads to the generation of the free radical at the C1 position on the glucose molecule in starch or disaccharide in the presence of water and, consequently, these radiolytic end-products have a reducing power (Stewart, 2001). These results may suggest that when the solution or food was irradiated in the frozen state, the reactive intermediates of water radiolysis are trapped, therefore the radiolysis of a component such as sugar and amino acid is inhibited and finally the browning reaction is prevented. This hypothesis was supported by the ESR spectrum of the irradiated cooked rice at various temperatures (Fig. 4). The ESR results in the present work on cooked rice are in agreement with those reported for starch (Korkmaz and Polat, 2000; Bertolini et al., 2001). The ESR pattern of irradiated starch can
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Table 1 Effects of the irradiation conditions on the hunter color values of gamma-irradiated cooked rice during a storage at room temperaturea Storage time (days)
L-value 0
11
a-value 0
11
b-value 0
11
Dose (kGy)
Irradiation conditionsb
SEMd
RTc
4 1C
20 1C
70 1C
0 10 20 30
50.37a 48.36a 47.85b 47.31b
49.09a 48.76ab 48.83ab
50.25a 50.67a 50.06a
50.26a 50.83a 50.94a
1.202 0.941 0.805
0 10 20 30
46.11a 47.58a 47.27a 46.42a
47.44a 47.11a 47.67a
50.17b 50.26b 50.43b
50.44b 50.23b 50.30b
1.081 1.172 0.762
0 10 20 30
1.03a 0.92a 0.90b 0.90a
0.97a 0.93ab 0.92a
1.03a 1.01ab 1.02a
1.03a 1.03ab 1.01a
0.082 0.060 0.061
0 10 20 30
1.04a 0.80c 0.79b 0.75b
0.89bc 0.89ab 0.88ab
1.04ab 1.02a 1.05a
1.00a 1.04a 1.10a
0.062 0.073 0.081
0 10 20 30
2.00a 1.19b 0.98b 0.90c
1.27b 0.99b 0.98c
1.52ab 1.39b 1.27bc
1.98a 1.99a 1.74ab
0.305 0.221 0.232
0 10 20 30
1.75a 0.74c 0.07c 0.35c
0.67c 0.31c 0.12c
1.05bc 1.07b 1.00b
1.39ab 1.02b 1.03b
0.251 0.251 0.182
a
Values with different letters (a–c) within a row and control (0 kGy irradiated sample) differ significantly (Po0:05). Temperature during gamma-irradiation. c Room temperature. d Standard error of the mean (n ¼ 50). b
present two main signals, AA0 and BB0 : the AA0 signal has been tentatively assigned to an R OR radical at C1 of anhydroglucose which turns into an ROOd radical after the breaking of the glycosidic bond and contact with atmospheric oxygen. This ROOd radical give a BB0 signal (Raffi et al., 1985). The intensity of free radicals is dependent on starch water content, irradiation dose, temperature and time of storage. The dispersion of ions and free radicals is higher when free water is in the liquid form and lower when free water is limited (dried products) or frozen (crystalline) (Raffi and Agnel, 1983). When water is frozen, free radicals tend to recombine forming the original substance, because diffusion is reduced limiting their access to other food components (Taub et al., 1978). Formation of a primary hydroxyalkyl (pHA) radical is a common event in
crystalline carbohydrates containing a primary alcohol group when they are irradiated at low temperature (Madden and Bernhard, 1980). Cooked rice is a major food in Asia. However, it is very perishable because of its high water activity and nutrients. Although irradiation is able to improve the shelf-life of cooked rice, its application has been limited because consumers are sensitive to changes of the sensory properties of cooked rice such as the flavor, texture, and color after irradiation (Lee et al., 2004). However, when cooked rice was irradiated in the frozen state, the browning reaction was effectively prevented during irradiation as well as during storage at room temperature in the present study (Table 1). Irradiation in the frozen state prevented a browning reaction not only in the aqueous model solution but also the cooked rice.
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References (a) 550 a.u.
(b) 1463 a.u.
(c) 1404 a.u.
(d) 1328 a.u.
(e) 815 a.u. 300
310
320 330 340 Magnetic Field (mT)
350
360
Fig. 4. ESR spectrum and signal intensity (a.u.) of gammairradiated cooked rice at various conditions: (a) 0 kGy; (b) 30 kGy at room temperature; (c) 30 kGy at 4 1C; (d) 30 kGy at 20 1C; (e) 30 kGy at 70 1C.
This result is supported by previous reports. Coleby et al. (1962) and Farkas (1987) reported that the threshold dose for an off-flavor, e.g., in frozen poultry and meat are at least two-fold higher when compared with the chilled ones.
5. Conclusion Based on the present results, gamma irradiation produces free radicals and radiolysis products of sugar and glycine. These products and free radicals may condense to produce colored products during the postirradiation period. However, when the food is irradiated in the frozen state, the production of free radicals and radiolysis products is inhibited and it prevents the browning reaction.
Ackowledgements This research was supported by Korea Science and Engineering Foundation (KOSEF) and Ministry of Science & Technology (MOST), Korean government, through its National Nuclear Technology Program.
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