Article pubs.acs.org/JAFC
Changes in Physicochemical, Structural, and Sensory Properties of Irradiated Brown Japonica Rice during Storage Yinji Chen,*,†,‡ Weixin Jiang,† Zhongqing Jiang,‡ Xia Chen,† Jun Cao,† Wen Dong,§ and Bingye Dai§ †
Deptartment of Food Science and Engineering/Collaborative Innovation Center for Modern Grain Circulation and Safety, Nanjing University of Finance and Economics, P.O. Box 16, Wenyuan Lu 3#, 210023 Nanjing, People’s Republic of China ‡ Department of Food and Environmental Sciences, University of Helsinki, P.O. Box 66, Agnes Sjörbergin katu 2, 00014 Helsinki, Finland § China Rural Technology Development Center, Sanlihe Lu54#, 100045 Beijing, People’s Republic of China ABSTRACT: Brown japonica rice was treated with 60Co γ irradiation at doses of 0, 0.2, 0.5, 1.0, and 2.0 kGy immediately after harvesting. The effects of irradiation on physicochemical, structural, and sensory properties during long-term storage (18 months) were investigated. The study revealed that the pasting properties, including peak, through, breakdown, final, and setback viscosities, decrease considerably in a dose-dependent manner and vary differently during 18 months of storage. Irradiation reduced the free fatty acid (FFA) content in comparison with unirradiated brown rice with long-term storage (from 12 to 18 months). Scanning electron microscope (SEM) observation showed that the mean range and shape of starch granules did not vary significantly. However, dark spots developed among starch granules and the narrow cracks became wider with increasing irradiation dose and storage time. During sensory evaluation, extremely low scores for odor and overall acceptability were obtained for medium-dose irradiated rice (1.0 and 2.0 kGy); however, no significant difference was found in acceptability between low-dose irradiated rice (0.2 and 0.5 kGy) and the control rice (0 kGy). Overall, low-dose (0.5 kGy or below) irradiation seems to be a promising alternative treatment to increase brown rice shelf life, without affecting the physicochemical and structural characteristics and sensory acceptability. KEYWORDS: brown japonica rice, storage, irradiation
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INTRODUCTION Rice (Oryza sativa L.) is one of the most important sources for staple foods which sustain about half of the world’s population. However, great loss and waste occurs along farm to fork production,1 which has been estimated to be 10% for cereal.2 Most of the losses occur during long-term storage, especially with inappropriate methods. During storage, rice is vulnerable to bacterial pathogens, toxins, bacteria, mildew bacterium, insects, etc.; as a result, shelf life, cooking quality, and edible value are affected. Living cells in bacteria or insects are inactivated on exposure to lethal factors that substantially change their cellular structure or physiological functions.3 γ irradiation can be an effective physical method to control the impact of harmful damage.2 Irradiation can extend the shelf life and reduce the spoilage of grain. Many studies have been conducted on the effect of γ irradiation on wheat,4−6 rice,4,7−9 corn,10,11 potato,12−14 and some legumes12,15−17 in recent years. During the irradiation, microorganisms and insect gametes are prevented from reproducing.18,19 The required radiation doses to achieve effective inactivation usually increase as follows: insects < parasites < molds and yeasts < bacteria < viruses. Therefore, insects are the most sensitive.20 Low-dose ( 0.05) was observed among brown japonica rice samples with different irradiation doses (0.2, 0.5, 1.0, and 2.0 kGy). With 9 months of storage, the FFA contents of all irradiated brown japonica rice samples increased significantly (P < 0.05) in comparison with the values after short-term storage (0, 3, and 6 months). During storage from 12 to 18 months, the FFA content of irradiated brown rice remained constant within a range of 22.7−25.0 (P > 0.05). However, the FFA content of unirradiated brown rice increased steadily and reached a peak of 29.9 mg of KOH/100 g at 18 months of storage. Changes in lipids due to ionizing radiation could be brought about in two ways: (1) by catalyzing their reaction with molecular oxygen, i.e., autoxidation, or (2) by the action of high-energy radiation (direct or indirect) on lipid molecules.53,54 In the presence of oxygen, both oxidative and radiolytic effects would occur and could be superimposed. The general mechanism of the radiolysis of lipids was thought to involve primary ionization, followed by migration of the positive charge toward the carboxyl group or double bonds. Cleavage occurred preferentially at the positions near carbonyl groups. As a result, the yield of hydrocarbon was increased. These effects subsequently brought about an increase in TBA content and FFA content. Rice Color. Values of L* (brightness) and b* (yellowness) recorded for brown japonica rice are shown in Figures 4 and 5. After irradiation, in comparison to unirradiated rice (0 kGy), the L* values of irradiated rice (0.2, 0.5, 1.0, and 2.0 kGy) decreased (P < 0.05) whereas the b* values of irradiated rice (0.2, 0.5, 1.0, and 2.0 kGy) increased (P < 0.05). These results are consistent with research reported in the literature.14,30,40 Brightness and yellowness changes might be explained by the breakdown of glycosidic and peptidic linkages promoted during irradiation. The free radicals, including hydroxyl radical, and hydrogen atoms derived from water radiolysis substantially attacked the glycosidic bonding in carbohydrates, causing depolymerization or degradation and the Maillard reaction. The cleavage of glycosidic bonds subsequently caused the rupture of starch granules and gave rise to browning. With prolonged storage time, the L* and b* values of brown rice varied differently. However, the lightness (L*) values of brown rice irradiated with different doses began to converge and remained constant after 12 months of storage (varying within the range 57.3−58.0, P > 0.05). However, the yellowness (b*) values of brown rice varied differently within 9 months of storage and then remained constant after 9 months. The yellowness values over 18 months were linearly dose dependent as follows: 21.62 for 0 kGy, 21.71 for 0.2 kGy, 22.08 for 0.5 kGy, 22.65 for 1.0 kGy, and 23.10 for 2.0 kGy. The changes in brightness and yellowness of brown rice after long-term storage might be explained by the fact that the cleavage of glycosidic bonds
differences in hardness between nonirradiated rice and lowdose irradiated rice (0.2 and 0.5 kGy) were not significant. Changes in cooked rice hardness might be due to γ-irradiationinduced structural degradation in starch granules,51 which would enable water to penetrate relatively easily.30,36,52 When rice starch was heated with steam or excess water, the crystalline structure was disrupted due to the breakage of hydrogen bonds, and molecules of water became linked by hydrogen bonding to the exposed hydroxyl group of amylose and amylopectin. This phenomenon causes an increase in granule swelling and solubility. The hardness of cooked rice in the current study increased in all samples with prolonged storage times. After 18 months of storage, nonirradiated brown rice had the highest increase in hardness (45.64%), whereas there were relatively small increases (35.75%, 39.42%, 37.89%, and 24.55%) for brown rice irradiated at 0.2, 0.5, 1.0, and 2.0 kGy, respectively. Thus, we consider that γ irradiation could improve the cooking quality induced by rice aging during longterm storage. Lipid Properties. The lipid content of brown japonica rice obtained in this study was approximately 2.1−3.4% and did not change significantly during 18 months of storage. During storage, the surface lipids of brown japonica rice had undergone hydrolysis to form FFAs. These FFAs were particularly susceptible to oxidation and could form intermediate peroxides, followed by the decomposition of intermediate peroxides into various oxidation products. TBA reacted with oxidation products and produced a color pigment.30 Initially, the TBA content obtained from irradiated brown japonica rice (2.0 kGy) was significantly higher (P < 0.05) than other samples (0, 0.2, 0.5, and 1.0 kGy) (Figure 2). During storage, the TBA content of brown japonica rice irradiated at doses of 0.2, 0.5, and 1.0 kGy had no significant difference (P > 0.05) from that of nonirradiated brown japonica rice, whereas the TBA increased significantly (P < 0.05) with irradiation doses up to 2.0 kGy. This might be due to the medium dose of 2.0 kGy having a marked function on lipids and accelerating lipid oxidation. A sharp increase of TBA content in all samples was observed after 3 months of storage, and this content then decreased significantly (P < 0.05) after 6 months of storage. At the end of storage (18 months), the TBA contents did not show significant differences in nonirradiated and irradiated brown japonica rice and were in the range of 0.014−0.018 μmol/g. During long-term storage, the TBA content of brown japonica rice had shown a dynamic change; this might be ascribed to changes in the duration of end products. These results were consistent with the research reported in the literature.30 The FFA content in brown japonica rice is shown in Figure 3. Initially, the FFA content in nonirradiated brown japonica rice was 10.7 mg of KOH/100 g. After irradiation, the FFA content increased slightly (13.6, 14.1, 12.8, and 11.5 mg of KOH/100 g F
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Table 3. Panelists did not report differences (P > 0.05) in appearance, color, odor, taste, and overall acceptability among samples after irradiation with 0, 0.2, and 0.5 kGy at the end of storage (18 months). However, very low scores for odor were obtained after γ irradiation, especially with medium-dose irradiated (1.0 and 2.0 kGy) brown japonica rice (P < 0.01). The disagreeable odor of brown japonica rice irradiated at 1.0 and 2.0 kGy might be ascribed to the increase in lipid degradation products (determined from an increase in TBA content) during 18 months of storage. The unsaturated fatty acids in brown japonica rice are easily oxidized to peroxides and change to carbonyl compounds,53,54 which caused disagreeable odor and flavor of brown japonica rice stored long-term. Poor scores of appearance, color, and taste from cooked rice were obtained for irradiation doses up to 1.0 kGy (P < 0.05). For overall acceptability, scores given by panelists varied from 4.6 to 6.7. Irradiated rice at 0.5 and 0.2 kGy had significantly high scores ranging from 6.6 to 6.7 in comparison scores with irradiation doses of 1.0 and 2.0 kGy (P < 0.01). The unpleasant odor and color might be the main reason for the low scores. These results meant that, at the end of storage (stored for 18 months), panelists preferred irradiated rice with doses as follows: 0.5 kGy > 0.2 kGy > 0 kGy > 1.0 kGy > 2.0 kGy. Therefore, rice irradiated with relatively low doses was the most acceptable. In conclusion, physicochemical, structural, and sensory properties of irradiated brown rice during long-term storage were characterized in the current work. A low or medium dose of γ irradiation (from 0.2 kGy to 2.0 kGy) increased the main pasting values and improved the storage performance of brown japonica rice. Irradiation decreased the FFA content of brown rice with long-term storage (from 12 to 18 months). Narrow cracks and dark spots developed among starch granules, especially in irradiated rice, which became wider and more dense with prolonged storage time. Extremely low scores for odor and overall acceptability were obtained from panelists on γ-irradiated brown rice especially with medium doses (1.0 or 2.0 kGy). On the basis of these observations, we considered that a dose of 0.5 kGy or below could be suitable for γ irradiation on brown japonica rice before long-term storage. These results could serve as a basis for developing an efficient method for grain storage.
carried out by free radicals became extremely weak after longterm storage. As the consumer favors rice with a low chroma value (i.e., yellowness) and a high lightness value, low-dose irradiation with short storage time is preferred for commercial storage. Correlation among Properties of Irradiated Brown Japonica Rice. Pearson’s correlation coefficients for the relationship among various functional properties of irradiated brown japonica rice during storage are shown in Table 2. Storage time had a positive correlation with hardness (r = 0.721, P < 0.01) and FFA content (r = 0.805, P < 0.01), whereas it had a negative correlation with SB (r = −0.542, P < 0.01) and PT (r = −0.691, P < 0.01). The irradiation dose had a positive correlation with PT (r = 0.504, P < 0.01), b* value (r = 0.624, P < 0.01), and TBA content (r = 0.407, P < 0.05) and a negative correlation with L* value and all pasting parameters except PT (r = 0.504, P < 0.01; these results were in agreement with the findings reported by Liu.37 PV had a positive correlation with TR (r = 0.972, P < 0.01), BD (r = 0.849, P < 0.01), FV (r = 0.941, P < 0.01), and SB (r = 0.441, P < 0.01). Similar phenomena have been revealed in pasting properties on irradiation.17,52 Kernel Morphological Structure. The SEM photographs of starch granules showed an uneven surface with angular and polygonal shapes in all rice samples (Figure 6). After irradiation, dark spots developed among starch granules, especially in irradiated rice (1.0 or 2.0 kGy). These dark spots were probably the small spherical holes with diameters of about 1 μm. As the storage time was prolonged, these dark spots became rather dense and the narrow cracks became wider. However, the mean granule range and the shapes of starches did not vary significantly. The reason different levels of cracks and spots formed in irradiated rice samples and how they formed during storage were not clear and required further investigation. The isolated starches of irradiated rice grains (0.35 kGy) also showed similar morphology with respect to polygonal shape and size of starch granules but with few round starches.50 However, with high-dose irradiation (maximum dose range from 10 to 50 kGy), microscopic observation of the starches indicated surface cracking of the starch samples with irradiation treatment, and the extent of surface cracking was dose-dependent and increased with increasing irradiation dose in rice,9 corn,10 potatoes,12,14 and beans.12,16 These changes were due to the cleavage of large starch molecules by free radicals generated from the treatment of γ irradiation;55 the breakage or cleavage of long chains in amylopectin decreased the apparent amylose content, which might aid humans in digesting rice starch.9,56,57 However, rice starch granules are the smallest granules known in cereal grains,58 with sizes ranging from 4 to 8 μm in the current study; the granules were loosely packed and look like clusters. Wu et al.51 observed the shape of the starch granule deformed by medium-dose γ irradiation (1 kGy) and considered that γ irradiation was capable of degrading starch through the cleavage of large molecules of starch into smaller fragments. However, in the current study, we did not observe any significant changes in starch granule shape except for the appearance of narrow cracks and dark spots among starch granules, even with medium-dose irradiation (1 and 2 kGy; Figure 6). Panelist Acceptance of Cooked Rice. Sensory evaluation was one of the most important methods for shelf life determination in food products.59 The results of sensory qualities of cooked rice stored for 18 months were presented in
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AUTHOR INFORMATION
Funding
This research was supported by a project from the State Administration of Grain in China (201313010), a project from Jiangsu Education Department (13KJB550010), and a project from the Priority Academic Program Development of Jiangsu higher education institutions (PAPD). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank the JAAS for providing rice samples. REFERENCES
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Changes in Physicochemical, Structural, and Sensory Properties of Irradiated Brown Japonica Rice during Storage Yinji Chen,*,†,‡ Weixin Jiang,† Zhongqing Jiang,‡ Xia Chen,† Jun Cao,† Wen Dong,§ and Bingye Dai§ †
Deptartment of Food Science and Engineering/Collaborative Innovation Center for Modern Grain Circulation and Safety, Nanjing University of Finance and Economics, P.O. Box 16, Wenyuan Lu 3#, 210023 Nanjing, People’s Republic of China ‡ Department of Food and Environmental Sciences, University of Helsinki, P.O. Box 66, Agnes Sjörbergin katu 2, 00014 Helsinki, Finland § China Rural Technology Development Center, Sanlihe Lu54#, 100045 Beijing, People’s Republic of China ABSTRACT: Brown japonica rice was treated with 60Co γ irradiation at doses of 0, 0.2, 0.5, 1.0, and 2.0 kGy immediately after harvesting. The effects of irradiation on physicochemical, structural, and sensory properties during long-term storage (18 months) were investigated. The study revealed that the pasting properties, including peak, through, breakdown, final, and setback viscosities, decrease considerably in a dose-dependent manner and vary differently during 18 months of storage. Irradiation reduced the free fatty acid (FFA) content in comparison with unirradiated brown rice with long-term storage (from 12 to 18 months). Scanning electron microscope (SEM) observation showed that the mean range and shape of starch granules did not vary significantly. However, dark spots developed among starch granules and the narrow cracks became wider with increasing irradiation dose and storage time. During sensory evaluation, extremely low scores for odor and overall acceptability were obtained for medium-dose irradiated rice (1.0 and 2.0 kGy); however, no significant difference was found in acceptability between low-dose irradiated rice (0.2 and 0.5 kGy) and the control rice (0 kGy). Overall, low-dose (0.5 kGy or below) irradiation seems to be a promising alternative treatment to increase brown rice shelf life, without affecting the physicochemical and structural characteristics and sensory acceptability. KEYWORDS: brown japonica rice, storage, irradiation
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INTRODUCTION Rice (Oryza sativa L.) is one of the most important sources for staple foods which sustain about half of the world’s population. However, great loss and waste occurs along farm to fork production,1 which has been estimated to be 10% for cereal.2 Most of the losses occur during long-term storage, especially with inappropriate methods. During storage, rice is vulnerable to bacterial pathogens, toxins, bacteria, mildew bacterium, insects, etc.; as a result, shelf life, cooking quality, and edible value are affected. Living cells in bacteria or insects are inactivated on exposure to lethal factors that substantially change their cellular structure or physiological functions.3 γ irradiation can be an effective physical method to control the impact of harmful damage.2 Irradiation can extend the shelf life and reduce the spoilage of grain. Many studies have been conducted on the effect of γ irradiation on wheat,4−6 rice,4,7−9 corn,10,11 potato,12−14 and some legumes12,15−17 in recent years. During the irradiation, microorganisms and insect gametes are prevented from reproducing.18,19 The required radiation doses to achieve effective inactivation usually increase as follows: insects < parasites < molds and yeasts < bacteria < viruses. Therefore, insects are the most sensitive.20 Low-dose ( 0.05) was observed among brown japonica rice samples with different irradiation doses (0.2, 0.5, 1.0, and 2.0 kGy). With 9 months of storage, the FFA contents of all irradiated brown japonica rice samples increased significantly (P < 0.05) in comparison with the values after short-term storage (0, 3, and 6 months). During storage from 12 to 18 months, the FFA content of irradiated brown rice remained constant within a range of 22.7−25.0 (P > 0.05). However, the FFA content of unirradiated brown rice increased steadily and reached a peak of 29.9 mg of KOH/100 g at 18 months of storage. Changes in lipids due to ionizing radiation could be brought about in two ways: (1) by catalyzing their reaction with molecular oxygen, i.e., autoxidation, or (2) by the action of high-energy radiation (direct or indirect) on lipid molecules.53,54 In the presence of oxygen, both oxidative and radiolytic effects would occur and could be superimposed. The general mechanism of the radiolysis of lipids was thought to involve primary ionization, followed by migration of the positive charge toward the carboxyl group or double bonds. Cleavage occurred preferentially at the positions near carbonyl groups. As a result, the yield of hydrocarbon was increased. These effects subsequently brought about an increase in TBA content and FFA content. Rice Color. Values of L* (brightness) and b* (yellowness) recorded for brown japonica rice are shown in Figures 4 and 5. After irradiation, in comparison to unirradiated rice (0 kGy), the L* values of irradiated rice (0.2, 0.5, 1.0, and 2.0 kGy) decreased (P < 0.05) whereas the b* values of irradiated rice (0.2, 0.5, 1.0, and 2.0 kGy) increased (P < 0.05). These results are consistent with research reported in the literature.14,30,40 Brightness and yellowness changes might be explained by the breakdown of glycosidic and peptidic linkages promoted during irradiation. The free radicals, including hydroxyl radical, and hydrogen atoms derived from water radiolysis substantially attacked the glycosidic bonding in carbohydrates, causing depolymerization or degradation and the Maillard reaction. The cleavage of glycosidic bonds subsequently caused the rupture of starch granules and gave rise to browning. With prolonged storage time, the L* and b* values of brown rice varied differently. However, the lightness (L*) values of brown rice irradiated with different doses began to converge and remained constant after 12 months of storage (varying within the range 57.3−58.0, P > 0.05). However, the yellowness (b*) values of brown rice varied differently within 9 months of storage and then remained constant after 9 months. The yellowness values over 18 months were linearly dose dependent as follows: 21.62 for 0 kGy, 21.71 for 0.2 kGy, 22.08 for 0.5 kGy, 22.65 for 1.0 kGy, and 23.10 for 2.0 kGy. The changes in brightness and yellowness of brown rice after long-term storage might be explained by the fact that the cleavage of glycosidic bonds
differences in hardness between nonirradiated rice and lowdose irradiated rice (0.2 and 0.5 kGy) were not significant. Changes in cooked rice hardness might be due to γ-irradiationinduced structural degradation in starch granules,51 which would enable water to penetrate relatively easily.30,36,52 When rice starch was heated with steam or excess water, the crystalline structure was disrupted due to the breakage of hydrogen bonds, and molecules of water became linked by hydrogen bonding to the exposed hydroxyl group of amylose and amylopectin. This phenomenon causes an increase in granule swelling and solubility. The hardness of cooked rice in the current study increased in all samples with prolonged storage times. After 18 months of storage, nonirradiated brown rice had the highest increase in hardness (45.64%), whereas there were relatively small increases (35.75%, 39.42%, 37.89%, and 24.55%) for brown rice irradiated at 0.2, 0.5, 1.0, and 2.0 kGy, respectively. Thus, we consider that γ irradiation could improve the cooking quality induced by rice aging during longterm storage. Lipid Properties. The lipid content of brown japonica rice obtained in this study was approximately 2.1−3.4% and did not change significantly during 18 months of storage. During storage, the surface lipids of brown japonica rice had undergone hydrolysis to form FFAs. These FFAs were particularly susceptible to oxidation and could form intermediate peroxides, followed by the decomposition of intermediate peroxides into various oxidation products. TBA reacted with oxidation products and produced a color pigment.30 Initially, the TBA content obtained from irradiated brown japonica rice (2.0 kGy) was significantly higher (P < 0.05) than other samples (0, 0.2, 0.5, and 1.0 kGy) (Figure 2). During storage, the TBA content of brown japonica rice irradiated at doses of 0.2, 0.5, and 1.0 kGy had no significant difference (P > 0.05) from that of nonirradiated brown japonica rice, whereas the TBA increased significantly (P < 0.05) with irradiation doses up to 2.0 kGy. This might be due to the medium dose of 2.0 kGy having a marked function on lipids and accelerating lipid oxidation. A sharp increase of TBA content in all samples was observed after 3 months of storage, and this content then decreased significantly (P < 0.05) after 6 months of storage. At the end of storage (18 months), the TBA contents did not show significant differences in nonirradiated and irradiated brown japonica rice and were in the range of 0.014−0.018 μmol/g. During long-term storage, the TBA content of brown japonica rice had shown a dynamic change; this might be ascribed to changes in the duration of end products. These results were consistent with the research reported in the literature.30 The FFA content in brown japonica rice is shown in Figure 3. Initially, the FFA content in nonirradiated brown japonica rice was 10.7 mg of KOH/100 g. After irradiation, the FFA content increased slightly (13.6, 14.1, 12.8, and 11.5 mg of KOH/100 g F
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Table 3. Panelists did not report differences (P > 0.05) in appearance, color, odor, taste, and overall acceptability among samples after irradiation with 0, 0.2, and 0.5 kGy at the end of storage (18 months). However, very low scores for odor were obtained after γ irradiation, especially with medium-dose irradiated (1.0 and 2.0 kGy) brown japonica rice (P < 0.01). The disagreeable odor of brown japonica rice irradiated at 1.0 and 2.0 kGy might be ascribed to the increase in lipid degradation products (determined from an increase in TBA content) during 18 months of storage. The unsaturated fatty acids in brown japonica rice are easily oxidized to peroxides and change to carbonyl compounds,53,54 which caused disagreeable odor and flavor of brown japonica rice stored long-term. Poor scores of appearance, color, and taste from cooked rice were obtained for irradiation doses up to 1.0 kGy (P < 0.05). For overall acceptability, scores given by panelists varied from 4.6 to 6.7. Irradiated rice at 0.5 and 0.2 kGy had significantly high scores ranging from 6.6 to 6.7 in comparison scores with irradiation doses of 1.0 and 2.0 kGy (P < 0.01). The unpleasant odor and color might be the main reason for the low scores. These results meant that, at the end of storage (stored for 18 months), panelists preferred irradiated rice with doses as follows: 0.5 kGy > 0.2 kGy > 0 kGy > 1.0 kGy > 2.0 kGy. Therefore, rice irradiated with relatively low doses was the most acceptable. In conclusion, physicochemical, structural, and sensory properties of irradiated brown rice during long-term storage were characterized in the current work. A low or medium dose of γ irradiation (from 0.2 kGy to 2.0 kGy) increased the main pasting values and improved the storage performance of brown japonica rice. Irradiation decreased the FFA content of brown rice with long-term storage (from 12 to 18 months). Narrow cracks and dark spots developed among starch granules, especially in irradiated rice, which became wider and more dense with prolonged storage time. Extremely low scores for odor and overall acceptability were obtained from panelists on γ-irradiated brown rice especially with medium doses (1.0 or 2.0 kGy). On the basis of these observations, we considered that a dose of 0.5 kGy or below could be suitable for γ irradiation on brown japonica rice before long-term storage. These results could serve as a basis for developing an efficient method for grain storage.
carried out by free radicals became extremely weak after longterm storage. As the consumer favors rice with a low chroma value (i.e., yellowness) and a high lightness value, low-dose irradiation with short storage time is preferred for commercial storage. Correlation among Properties of Irradiated Brown Japonica Rice. Pearson’s correlation coefficients for the relationship among various functional properties of irradiated brown japonica rice during storage are shown in Table 2. Storage time had a positive correlation with hardness (r = 0.721, P < 0.01) and FFA content (r = 0.805, P < 0.01), whereas it had a negative correlation with SB (r = −0.542, P < 0.01) and PT (r = −0.691, P < 0.01). The irradiation dose had a positive correlation with PT (r = 0.504, P < 0.01), b* value (r = 0.624, P < 0.01), and TBA content (r = 0.407, P < 0.05) and a negative correlation with L* value and all pasting parameters except PT (r = 0.504, P < 0.01; these results were in agreement with the findings reported by Liu.37 PV had a positive correlation with TR (r = 0.972, P < 0.01), BD (r = 0.849, P < 0.01), FV (r = 0.941, P < 0.01), and SB (r = 0.441, P < 0.01). Similar phenomena have been revealed in pasting properties on irradiation.17,52 Kernel Morphological Structure. The SEM photographs of starch granules showed an uneven surface with angular and polygonal shapes in all rice samples (Figure 6). After irradiation, dark spots developed among starch granules, especially in irradiated rice (1.0 or 2.0 kGy). These dark spots were probably the small spherical holes with diameters of about 1 μm. As the storage time was prolonged, these dark spots became rather dense and the narrow cracks became wider. However, the mean granule range and the shapes of starches did not vary significantly. The reason different levels of cracks and spots formed in irradiated rice samples and how they formed during storage were not clear and required further investigation. The isolated starches of irradiated rice grains (0.35 kGy) also showed similar morphology with respect to polygonal shape and size of starch granules but with few round starches.50 However, with high-dose irradiation (maximum dose range from 10 to 50 kGy), microscopic observation of the starches indicated surface cracking of the starch samples with irradiation treatment, and the extent of surface cracking was dose-dependent and increased with increasing irradiation dose in rice,9 corn,10 potatoes,12,14 and beans.12,16 These changes were due to the cleavage of large starch molecules by free radicals generated from the treatment of γ irradiation;55 the breakage or cleavage of long chains in amylopectin decreased the apparent amylose content, which might aid humans in digesting rice starch.9,56,57 However, rice starch granules are the smallest granules known in cereal grains,58 with sizes ranging from 4 to 8 μm in the current study; the granules were loosely packed and look like clusters. Wu et al.51 observed the shape of the starch granule deformed by medium-dose γ irradiation (1 kGy) and considered that γ irradiation was capable of degrading starch through the cleavage of large molecules of starch into smaller fragments. However, in the current study, we did not observe any significant changes in starch granule shape except for the appearance of narrow cracks and dark spots among starch granules, even with medium-dose irradiation (1 and 2 kGy; Figure 6). Panelist Acceptance of Cooked Rice. Sensory evaluation was one of the most important methods for shelf life determination in food products.59 The results of sensory qualities of cooked rice stored for 18 months were presented in
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AUTHOR INFORMATION
Funding
This research was supported by a project from the State Administration of Grain in China (201313010), a project from Jiangsu Education Department (13KJB550010), and a project from the Priority Academic Program Development of Jiangsu higher education institutions (PAPD). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank the JAAS for providing rice samples. REFERENCES
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