Effects Of Far-infrared Treatment On Volatile Substances During Storage Of Rice With Different Milling Degrees And Development Of Rice Cake Based On Lightly Milled Rice

Rice as a staple food for two thirds of the world’s population,has a high annual yield.However,over-processed of commercial polished rice consumes energy and reduces the rice yield.Long-term consumption of those over-polished rice will also results in a lack of nutrition.Though brown rice is rich in nutrients,it is difficult for consumers to accept because of its long cooking time and rough taste.Lightly milled rice will have a broad market because it can balance nutrition and taste.The aging process of rice during storage will produce unpleasant smell,which directly affects the commodity value of rice.In this study,the odor changes of milled rice with different milling degrees during storage were studied;and far infrared(FIR)was introduced to the pretreatment of the milled rice.The odor changes of those milled rice treated with FIR were also monitored to investigate the effect of FIR;Then the volatile flavor components of rice cake with different milling degrees were analyzed,and a rice cake was prepared with lightly milled rice.These studies can guide for the milled rice storage and provide a reference for the utilization of lightly milled rice.The details are as follows:1.Two varieties of indica rice were polished with different milling degrees(grind reduction rates were 0%,3%,6%,and 10%,respectively).The effects of milling degree on the water content,crude fat,MDA,free fatty acid,peroxidase activities and odor of rice samples during storage were investigated.(1)The water content of rice samples fluctuated slightly with the milling degree and storage time.The crude fat,MDA and peroxidase activities in the initial rice samples decreased with the increase of milling degree;the free fatty acid content was the highest in lightly milled rice,followed by brown rice and polished rice.(2)The volatile substances of rice samples during early storage mainly contained aldehydes and alcohols.The milling degree had significant effect on the composition of initial volatile substances in rice(p<0.05).Compared with polished rice,storage time significantly affected the types and contents of volatile compounds in brown and lightly milled rice.During accelerated storage,nonanol and thymol as the main contributors were related to the flavor of brown rice after short-term storage(30-60d).Butylated hydroxytoluene(camphor flavor)and benzyl alcohol(bitter flavor)as the main contributors were related to the flavor of 3%lightly milled rice after long-term storage(150d).Thymol,nonanal and nonanol as the main contributors were related to the flavor of 6%lightly milled rice after early storage.2-ethylhexanol was a unique contributor in polished rice.It could be used as a marker to distinguish it from other samples.Combined with the content and odor changes of main contributors during storage,it was found that the flavor deterioration of all rice samples was small in the first three months of accelerated storage at 37℃(corresponding to 4.5-6 months at room temperature).(3)In this experiment,two kinds of indica rice samples showed the same trend in the flavor substances changes during storage.2.FIR was applied to inactivate the peroxidase in rice samples(FIR-rice).The results showed that FIR affected the contents of water,crude fat,MDA,free fatty acids and volatile substances in rice samples.(1)Compared with untreated rice,the water,crude fat and free fatty acid contents in FIR-rice decreased,but the content of MDA increased.FIR inhibited the hydrolysis rate of triglycerides and improved the storage stability of lightly milled rice.However,it also promoted the oxidation of unsaturated fatty acids.The types and contents of aldehydes and ketones in the fresh FIR-rice increased,and pyrazines with baking flavor appeared.(2)During storage the variation trend of the crude fat,MDA and free fatty acid in FIR-rice was similar to that of untreated rice.As emerging compounds,the content of 3-ethyl-2,5-dimethylpyrazine and 2,5-dimethyl-pyrazine(the flavor of cocoa and roast nuts)in FIR-rice decreased with increase of milling degree.(3)The storage time had significant effect on the composition of volatile substances in FIR-rice with different milling degrees(P<0.05).The aroma volatile 3-ethyl-2,5-dimethyl-pyrazine could be used as a characteristic marker to distinguish polished rice samples with 10%milling degree from the others.The contents of 3-ethyl-2,5-dimethyl-pyrazine in FIR brown rice and lightly milled rice were decreased with the storage time.The content of 2ethylhexanol in FIR lightly milled rice and polished rice decreased with the extension of storage time.It could be used as a characteristic marker distinguished from brown rice.Our results showed that the best flavor storage period of FIR brown rice was 90d,while that of lightly milled rice and polished rice was 120d.3.The rice cake was prepared from rice flour with different milling degrees.The effects of different milling degrees on specific volume,chromatic aberration,texture,flavor and sensory evaluation results of rice cakes were investigated.(1)Considering the texture and sensory evaluation,lightly milled rice with 6%milling degree was suitable for rice cake.(2)The volatile flavor substances especially for esters in rice cakes were more abundant than those in raw rice.The volatile compounds in rice cake were mainly alcohols,aldehydes and esters.The increase of esters and alcohols give the rice cake fruity and floral aroma.