The Effect Of Milling Degree On Cooking And Nutrient Quailty Traits Of Rice (Oryza Stativa L.)

Rice is the second largest food crop in world and also one of the major grain crops in China. It is an important source for people to obtain energy, protein, mineral elements and other nutrients. Increasing the content of beneficial nutrients in rice is of great significance to promote the health of people in our country. In this study, four varieties of early season indica rice, Zhenong7, Zhenong Futie, Zhenong Fuxin and Zhenong Fuxi, were selected to study the distribution of iron (Fe), zinc (Zn), protein and amino acids in different parts of rice and the effect of degree of milling (DOM) on the contents of these nutrients in rice. The influence of DOM on cooking quality and the correlation between cooking quality and nutrient quality traits of rice were also analyzed. The main results are as follows:1. Brown rice was divided into bran layer, outer endosperm layer, middle endosperm layer and inner endosperm layer from outside to inside, and their weight proportion of the whole grain were9%,6%,10%and75%respectively. The contents of nutrients in different parts of rice had significant differences for the quality traits studied. Analysis of their distribution showed that Fe was mainly distributed in the outer fraction of rice, but the distribution of Zn, protein and various amino acids were more uniform. Under different planting environmental conditions, the relative content of Fe in bran layer and outer endosperm layer were relatively stable while the relative content of Zn and protein were more stable in inner endosperm layer.2. The changes of rice cooking quality along with DOM were:(1) amylose content didn’t show obvious change tendency (2) pasting temperature declined very slowly (3) gel consistency gradually increased, and (4) the rising speed was from fast to slow and was flattened after reaching a certain value. The effect of DOM on the six rice RVA profile characteristics were:(1) the peak viscosity, hot paste viscosity and breakdown value increased gradually with milling degree,(2) setback value and consistence value gradually decreased with milling degree, and (3) hot viscosity fluctuated within a certain range with no apparent trend.3. During the milling process, Fe and Zn contents in milled rice decreased gradually, and tended to be stable towards the DOM of25%where the contents of Fe and Zn decreased by85.01%and37.82%on average, respectively. Protein, lysine and threonine contents also decreased gradually by19.49%,18.40%and24.98%at the DOM of25%, respectively. These results suggested that DOM had the largest effect on Fe content, larger effect on Zn content, and smaller effect on protein, lysine and threonine contents of rice.4. The correlation analysis between rice appearance traits and milling time showed that at the initial milling stage, larger grain length and ratio of length to width took less time to reach a certain DOM. Correlation between physicochemical properties and cooking quality showed that amylose content of rice was highly significantly positively correlated with gelatinization temperature, significantly positively correlated with hot paste viscosity and highly significantly positively correlated with cool paste viscosity, setback value and consistence value; Protein content was highly significantly negatively correlated with peak viscosity and breakdown value. Relationships among rice cooking qualities were also studied. Gel consistency was significantly negatively correlated with cold paste viscosity, highly significantly negatively correlated with setback value and consistence value and highly significantly positively correlated with breakdown value. Peak viscosity was significantly or highly significantly positively correlated with breakdown value and significantly negatively correlated with setback value. Hot paste viscosity was significantly positively correlated with cold paste viscosity and significantly or highly significantly positively correlated with consistence value. Cold paste viscosity was highly significantly positively correlated with setback value and consistence value. Breakdown value was highly significantly negatively correlated with setback value and highly significantly negatively correlated with consistence value. Setback value was highly positively correlated with consistence value.