| Rice cooked with aged rice usually suffers from deterioration in quality,such as weak aroma,strong off-flavor,and hard texture.In this paper,the reasons for the formation of off-flavor during the aging process of indica rice were analyzed based on the perspective of lipid hydrolysis and oxidation.Furthermore,the effect of the change in lipase activity and the degree of lipid hydrolysis and oxidation in indica rice grains during the soaking stage by electric cooker on the odor improvement of aged indica rice were clarified.On the basis of this,the relationship between the changes in the internal structure and component distribution of rice grains caused by water absorption and the palatability of aged rice was explored.The aim was to provide guidance for the design and improvement of the curve to cook aged indica rice in the electric cooker.At the same time,it provided a reference for the cooking of aged indica rice in the canteen and takeaway industries.The eating quality of cooked rice with fresh and one-year-old aged indica rice was compared based on the sensory quality,flavor characteristics,texture,and cooking characteristics.It was found that the aged rice had a weaker aroma,obvious off-flavor,harder texture,and lower viscoelasticity.By combining odor activity value and gas chromatography-mass spectrometry-olfactometry,the odor-active compounds that had an important contribution to the aroma of aged indica rice were identified including(E)-2-nonenal,(E,E)-2,4-decadienal,1-octene-3-ol,and 2-pentylfuran.Besides,the odor-active compounds contributing to the off-flavor were hexanal,octanal,nonanal,decanal,and 1-heptanol.Cooking indexes such as iodine blue value and gelatinization degree,which tightly related to palatability,were significantly decreased in aged indica rice.In order to investigate the relationship between odor characteristics of indica rice and lipid hydrolysis and oxidation of indica grain,the accelerated aging of indica rice was carried out under the simulated storage environment of high temperature and high humidity.The results showed that the aroma of indica rice gradually weakened with the extension of storage time,and the off-flavor appeared and strengthened from the fourth week.After eight weeks of storage,the content of aldehydes in raw rice increased significantly,among which the content of hexanal,octanal,nonanal,and decanal increased to 3.9,2.2,8.4,and 8.0 times of the initial content,respectively.In addition,after eight weeks of storage,the off-flavor and content of key odor active compounds in the cooked rice were the closest to those of one-year-old indica rice,which well simulated the aging process of indica rice.The lipase activity in the rice grains remained at a high level during the aging process,resulting in the accumulation of free fatty acids,especially unsaturated free fatty acids in indica rice grains,which were used as precursor substances to further oxidize during storage and subsequent cooking stages to form off-flavor compounds.The effect of soaking parameters in the water absorption stage of the electric cooker on the odor quality of aged indica rice were investigated by means of sensory evaluation,electronic nose,and gas chromatography-mass spectrometry.By tracking the change of endogenous lipase activity,lipid hydrolysis,primary oxidation products,and secondary oxidation products in the rice grains,the reasons for controlling the off-flavor of aged indica rice by soaking treatment were revealed.It was found that soaking at 50? for 10 minutes had a significant effect on improving the odor of aged indica rice,and it was close to the overall flavor profile of fresh rice.Compared with unsoaked aged rice,the lipase activity of rice grains treated at 50? for 10min decreased by 78.2%,the peroxide value decreased to 3.04×10-2 meq/kg,and the malondialdehyde content increased by only 7.5%.Soaking at 50? for 10 minutes effectively delayed the further hydrolysis of lipids and decreased the accumulation of free fatty acids in rice grains.At the same time,the automatic oxidation of lipids during the soaking process was inhibited,which reduced the formation of undesirable odor-active compounds(saturated aldehydes)during the cooking process of aged indica rice.The improving effect of soaking parameter adjustment on the palatability of aged indica rice were investigated based on sensory evaluation and texture characteristics.The effect of soaking treatment on the presence and distribution of water,starch gelatinization,and crystal structure of aged indica rice grains were analyzed by low-field nuclear magnetic resonance,rapid viscosity analyzer,and X-ray diffraction.The results showed that the soaking treatment(50?/10 min)had a good control effect on the odor of aged indica rice.It could increase the proportion of structural water in aged rice grains and decrease the crystallinity of starch,which was beneficial to the gelatinization of starch and the formation of good taste in the subsequent cooking stage.The microstructure of rice grains after soaking showed that the porosity of the aged rice tissue was decreased and the distribution of protein-containing starch granules was relatively uniform,which was beneficial to promote the water absorption of starch granules and improved the viscosity of aged indica rice. |
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