Causation Analysis Of The Off-flavor In Aged Japonica Rice And The Improvement Effect Of Controlled Soaking Treatment In Rice Cooker

Rice cooked with aged japonica rice usually suffers from the problems such as weakening of aroma and producing of off-flavor.The reasons for the off-flavor of cooked rice during the storage of japonica rice were analyzed,and the characteristic flavor compounds causing the offflavor of aged japonica rice were identified.Then the effect of soaking treatment in the water absorption stage of electric rice cooker on improving the flavor quality of aged japonica rice was investigated,and the effective soaking parameters were determined.The mechanism of improving the flavor quality of aged japonica rice was revealed from the change of endogenous enzyme activity and the control of lipid hydrolysis and oxidation degree.The object was to provide guidance for designing the cooking curve of rice cooker to cook aged japonica rice,and at the same time,provide a reference for the cooking of aged japonica rice in the canteen and the takeaway industry.The changes of volatile flavor compounds of aged japonica rice during one-year storage at room temperature were analyzed.By combining the odor activity value,gas chromatographyolfactometry-mass spectrometry,the flavor recombination and omission analysis,the characteristic compounds causing the off-flavor of aged japonica rice were identified,which include hexanal,(E)-2-heptenal,2-octenal,2-nonenal,nonanal,2-pentylfuran,(E,E)-2,4-decadienal,and 4-vinylguaiacol.Based on the changes of volatile flavor compounds,endogenous enzyme activity and the lipid hydrolysis and oxidation degree in the japonica rice during storage,it was found that the accumulation of aldehydes in japonica rice during storage and the further degradation of lipid oxidation precursors during cooking was the main reason for the deterioration of the odor quality of aged japonica rice.The effect of soaking temperature and time in water absorption stage of rice cooker on the odor quality of aged japonica rice was investigated by sensory evaluation,electronic nose and gas chromatography-mass spectrometry analysis.The results showed that the overall flavor profile of aged japonica rice was closer to fresh rice after soaking at the medium temperature for short time(50℃/10 min).Furthermore,the processing parameters significantly inhibited the formation of characteristic off-flavor compounds such as hexanal and nonanal,which originated from lipid oxidation in cooked rice.The odor quality of aged japonica rice was effectively improved.In order to clarify the reasons for the adjustment of the soaking temperature and time on the odor quality of aged japonica rice,the endogenous enzyme activity,lipid hydrolysis and oxidation products,microstructure,and water distribution in aged japonica rice grain after soaking were analyzed.It was found that compared with soaking at room temperature,the lipase activity in aged rice grain decreased by 86%,and the accumulation of total free fatty acids and free unsaturated fatty acids reduced by 21.6% and 28.2%,respectively after soaking at the medium temperature for short time(50℃/10 min).The activity of endogenous lipase in rice grains was effectively inhibited,the lipid hydrolysis was effectively slowed down,and subsequent auto-oxidative degradation reaction of accumulated lipid oxidation primary products was inhibited through controlling the water absorption degree of the rice grains by the short-term soaking at medium temperature.The effect and reason of vacuum-assisted soaking treatment in rice cooker on the odor quality of aged japonica rice were analyzed by using sensory evaluation,electronic nose,and gas chromatography-mass spectrometry.Compared with soaking at normal pressure,the contents of hexanal,nonanal,and 2-nonenal in aged japonica rice cooked by vacuum assistedsoaking for 10 min were significantly decreased by 20.1%,12.4%,and 9.8%,respectively,which further improved the odor quality of aged japonica rice.It was found that the vacuumassisted soaking for 10 min in the water absorption stage of the electric rice cookers not only removed some of the off-flavor substances that generated during storage in aged rice,but also reduced the dissolved oxygen content in the soaking solution,inhibited the lipase activity and delayed the lipid oxidation process of the rice grain,and further decreased the content of the characteristic off-flavor compounds in cooked rice.