Study On Causes And Control Method Of Cooked Rice Quality Deterioration In The Heat Preservation Stage Of Electric Rice Cooker

In the process of heat preservation,rice cooked by electric rice cooker(ERC)generally suffers from the deterioration of quality,such as hardening of surface,undesirable flavor and yellowing of color.In this paper,the effect of water distribution and water migration on the texture uniformity of cooked rice during heat preservation were analyzed.Meanwhile the variety and content of volatile flavor substances in cooked rice during the heat preservation process were explored,and the relationship between the change of 5-hydroxymethyl furfural content and the yellowing of cooked rice during the heat preservation process was clarified.This research could provide a rational guideline for the modulation of the heat preservation process of commercial electric rice cooker.The sensory quality,odor characteristics and cooked rice color were used as indexes to compare the eating quality of cooked rice after heat preservation in S mode or Z mode by ERC.It was found that the quality of cooked rice unfortunately deteriorates during the heat preservation process,especially after 12 hours of heat preservation,such as yellowing of color,hardening of texture and disappearance of cooked rice aroma.The difference of cooked rice before and after heat preservation in S mode by ERC is mainly reflected in the odor and the hardness value of texture characteristics.This is related to the condensate return caused by the lower temperature of the lid during the heat preservation process in this mode.The difference of cooked rice before and after heat preservation in Z mode by ERC is mainly reflected in the color difference b*value.This is mainly due to the water loss caused by the overheating of the lid during the heat preservation process in this mode.The relationship between water reflux and the uniformity deterioration of cooked rice texture during the heat preservation process in S mode was further explored by means of physical property analyzer and low field nuclear magnetic resonance.Texture analysis showed that cooked rice in the middle-upper layer of ERC was drier and harder due to the decrease of water content,while cooked rice in the outer-upper layer of ERC swelled obviously due to the increase of water content.Further,by comparing the relaxation signals of the each sampling sites,it was found that the binding ability of the structural water and the matrix of the two sampling sites was weakened.At the same time,the proportion of structured water in the outer-upper layer decreased,and the proportion of free water increased,and the binding ability between free water and matrix decreased,which was conducive to promoting the chemical reaction that affected the quality of cooked rice in the heat preservation process.The change of the variety and content of volatile flavor substances in cooked rice during S mode heat preservation by ERC were investigated by gas chromatography-mass spectrometry to identify key substances that were important for the odor of cooked rice.The results showed that the total amount of flavor substances in cooked rice decreased gradually with the prolongation of heat preservation time.Partial least-squares regression was used to analyze the correlation between the content of volatile flavor substances and the sensory evaluation results.It was found that the odor during the heat preservation progress was mainly attributed to the reduction of the aldehydes which were characteristic flavor compounds of cooked rice,and the formation of undesirable flavor compounds such as 2-methyltetrahydrothiophene-3-one.5-Hydroxymethylfurfural(5-HMF)is an important product of Maillard reaction.The reason for the yellowing of cooked rice in the Z mode heat preservation process was investigated by analyzing the production of 5-HMF.In order to eliminate the influence of cooked rice matrix on the signal intensity of 5-HMF detection,a pretreatment method for the simple and rapid separation and enrichment of 5-HMF in cooked rice was established,named QuEChERS combined with dispersed liquid-liquid microextraction.The limit of quantification was 0.3μg/L,and the recovery rate was 82.87%~104.86%.The results showed that the 5-HMF content increased significantly in Z mode heat preservation by ERC.The correlation between the 5-HMF content and the b~*value in cooked rice was analyzed.It was found that the holding time had significant positive correlation with the 5-HMF level and the b~*value of color.Based on the correction mode of commercial prototype,the lid temperature was adjusted,and the effect of the condensate reflux and water dispersion on the quality of cooked rice after heat preservation was further verified.The results showed that the difference in hardness between the middle-upper layer and the outer-upper layer of cooked rice was reduced after 24hours of heat preservation,and there was no significant difference in relaxation signals between the sampling sites,which maintained the homogeneity of texture.In addition,there was no undesirable flavor generated and obvious yellowing phenomenon after heat preservation.