Researches On The Mechanism And Effects Of Rice Aging On Cooking And Eating Quality Of Rice

China is the world's highest producer of rice,and rice is the stable food of nearly 65%of the total population in China.Storage of rice grains is a crucial stage in the processing of paddy rice for consumption.The duration of the storage period is an average of 14 to 16 month in China.Rice will be aging during storage,which results in a decrease of eating quality.The mechanism of rice aging is not fully understood.The theory of rice protein oxidation was used to study the mechanism of rice aging in the dissertation,in order to explore the mechanism of rice aging,effect of rice protein oxidation on cooking and eating quality of rice was studied.The new harvest indica rice,japonica rice,and glutinous rice was used as raw materials to study effect of storage on cooking properties and storability of three kind of rice under artificial accelerated aging conditions.The results indicated that as storage time increased,peak viscosity in the rapid visco analyser profile first increased,and then decreased,setback and gelatinization temperature increased;water absorbing capacity of glutinous rice increased,water absorbing capacity of indica rice and japonica rice first increased,and then decreased;heating water absorbing capacity,elongation ratio,expansion ratio,light transmittance ratio of rice,and water absorbing capacity of three kind of rice increased.The extent of increase of elongation ratio and expansion ratio of glutinous rice was the most largest(55.88%,84.14%),the extent of increase of elongation ratio and expansion ratio of indica rice was the most smallest(26.18%,24.19%).In contrast,iodine blue number and pH of extraction liquid of three kind of rice decreased.The results implied cooking and eating quality of indica rice,japonica rice,and glutinous rice decreased as storage time increased.Comparison of change extent of indicators which characterized cooking and eating quality of three kind of rice indicated that storability of glutinous rice was worst,storability of indica rice was best.Rice protein-rice starch interaction of indica rice and japonica rice during storage was characterized by peak viscosity and setback in rapid visco analyser profile which different concentration of water,sodium chloride,sodium dodecyl sulfate,?-mercaptoethanol,and ascorbic acid used as test solvents.The results indicated that as storage time increased,peak viscosity firstly increased,and then decreased;value of setback gradually increased,which implied that rice aging lead to an increase of retrogradation extent of cooked rice.Cooking properties of rice was mainly ascribed to formation of rice protein disulphide of rice protein during storage,variation of hydrophobic interaction and electrostatic interaction during storage also related to the cooking properties of rice.Moreover,retrogradation extent of rice could be manipulated by adjustment of hydrophobic interaction of rice protein.Structure and functional properties of rice protein which prepared by indica rice and japonica rice during storage was also studied.The results indicated that as storage time increased,rice protein carbonyl content,disulphide content,surface hydrophobicity,and oil holding capacity of indica rice and japonica rice steadily increased;total sulphydryl groups,free sulphydryl groups,extraction yield,solubility,water holding capacity,emulsifying ability,and emulsifying stability gradually decreased;foaming ability and foaming stability of rice protein firstly increased,and then decreased.The results indicated that storage lead to exposure of hydrophobic groups and formation of oxdative aggregates which contain disulfide and non-disulfide covalent bond.Peroxyl radical,13-hydroperoxy-9Z,11E-octadecadienoic acid(HPODE)and malondialdehyde was selected as representatives of lipid peroxidation-derived free radicals,lipid hydroperoxides and reactive aldehydes,respectively,and effects of peroxyl radical,HPODE and malondialdehyde oxidation on structure and functional properties of rice protein were studied.The results indicated that incubation of rice protein with increasing concentration of oxidant lead to an increase of protein carbonyl content(MDA>ROO·>HPODE(412.41%>69.41%>21.23%),surface hydrophobicity,and oil holding capacity;loss of solubility,free sulphydryl groups,total sulphydryl groups,water holding capacity,foaming ability,foaming stability,emulsifying ability,emulsifying stability,and intrinsic fluorescence intensity.Blue shift of wavelength of the maximum emission was accompanied by formation of oxidation aggregates which involved in disulphide bonds and non-disulphide covalent.