Study On The Nutrition,taste Quality,volatile Flavor Substance And Storage Characteristics Of The Moderate Milled Black Rice

Black rice is a precious rice variety and is obtaining increasing attentions of researchers due to its abundant anthocyanins.But its dense bran layer,poor cooking and eating qualities greatly limit the utilization of black rice.Milling is a typical method of rice processing,in which moderate milling treatment is considered as a processing method with the processing degree between brown rice and polished rice,generally ranging among 0-10%.Moderate milling can balance the nutrition and the cooking as well as tasting qualities,resulting in improving its cooking and tasting attributes while retaining the nutrition.Therefore,black rice was taken as the research object in the present work and six different treated samples were obtained by moderate milling,including 0%?MD₀?,2%?MD₂?,4%?MD₄?,6%?MD₆?,8%?MD₈?and 10%(MD₁₀),to explore the effect of different milling degrees on the nutritional quality,gelatinization characteristics,tasting quality,sensory property,storage characteristics and the volatile flavor compounds of the cooked black rice.The primary results are shown as follows:?1?With the increase of milling degree,the nutrients and bioactive components of black rice decreased,while its cooking quality and gelatinization characteristics were improved.The contents of the starch increased significantly?p < 0.05?with milling levels increased,but the contents of protein,fat,crude fiber,mineral,free phenol and flavone obviously declined,as well as the abilities of DPPH and ABTS+ free radical scavenging?p < 0.05?.The results indicated that the nutritional and bioactive components of black rice were mainly concentrated in the outermost bran layer,which would be partially lost when grinding.Further analysis found that,at 2% milling level?MD₂?,a remarkable reduction of 1.90% in the moisture?p < 0.05?was observed,as well as an evident elevation of the water absorption?9.12%?after 20 min immersion and an obvious shortening of the cooking time to 42.67min?p < 0.05?.However,there were no significant changes when milling degrees continuously increased from 2%?MD₂?to 10%(MD₁₀).The water absorption,volume expansion,solid loss,Peak Viscosity?PV?,Final Viscosity?FV?,Holding Strength?HS?and Breakdown?BD?increased significantly?p < 0.05?with the increase of milling levels,but Setback?SB?decreased gradually.?2?A total of 85 compounds in black rice samples with different milling degrees were detected by GC-MS,with MD₀-MD₁₀containing 55,74,80,78,70 and 64 components,respectively.Hexanal was the dominant volatile flavor substance of black rice.And the lower milling treatment can promote the release of volatile compounds.Principal component analysis?PCA?and Linear discriminant analysis?LDA?showed that the overall odor of MD₄,MD₆ and MD₈ were similar,which was difficultly distinguished using electronic nose?E-nose?.Sensory evaluation observed that MD₂ and MD₄ possessed an evidently higher score?p <0.05?,but there was no significant changes when milling degree was increased continuously.Partial least-squares regression?PLS?was employed to establish the relationship between sensory evaluation and flavor components,finding that butyl ethyl ether?hexyl formate,acetic acid,3-methyltetrahydropyran,2-methylbutyraldehyde,2-methylenehexanal,phenylacetaldehyde were positively correlated with the aroma of the cooked black rice.Moderate milling significantly reduced the hardness and increased the viscosity?p <0.05?,but no obvious differences in hardness were determined among different milling degrees?p > 0.05?.?3?During storage,the fatty acid value and PV value of black rice with different milling degrees increased first and then decreased with the storage periods prolonged,while HS value gradually rising.The initial peroxide value of all tested samples decreased with the increase of milling degree,and the fatty acid value,peroxide value and water content of MD₀ were observed the least variations during storage process.BD values of MD₀-MD₆ tended to firstly increase and then decline during the storage of six months,with only FV value of MD₆ declining after three months.With the extension of storage time,the free phenol content of MD₀-MD₈ increased first and then decreased,while MD₁₀decreased gradually.MD₂-MD₆ reached the highest contents of the free phenol after two months,with 2.27?1.85 and 1.52 mg/g GAE,respectively.Meanwhile,the levels of the free phenol in MD₂-MD₆ respectively decreased lower than that of the unstored sample after three,five,four and three months.MD₈and MD₁₀respectively presented their higher decrease,with 1.04 and 0.06 mg/g GAE,in the first and fourth month storage period.The obtained results indicated that the oxidization and rancidity occurred during the storage of black rice,and nutrients and bioactive components were continuously lost.The longer the storage time and the greater the milling degree,the more serious these unfavorable influences.This might be attributed to the distribution of nutritional components of black rice and the intrinsic instability of their properties.