Shyozo Wada

Studies on Cereals

Five carbonyl compounds were found in the vapour of cooked rice, of which acetaldehyde, and n-caproaldehyde were identified from their retention times in gas chromatography and by their infrared spectra. Propionaldehyde or acetone, methylethylketone, and n-valeraldehyde were tentatively identified. The mechanisms of the formation of these carbonyl compounds were discussed from the view-point of Strecker degradation and lipid oxidation.When stored rice (40°C, two months) was cooked, the stale flavor (komai-shu) was clearly detected by sensory test. Direct gas chromatographic analysis of head space vapors over cooked rice showed three main peaks which corresponded to propionaldehyde or acetone, n-valeraldehyde and n-caproaldehyde. On the other hand, the content of linoleic and linolenic acids of the rice decreased during storage at 40°C. This means that the unsaturated fatty acids autoxidized during storage and gave rise to carbonyl compounds responsible for the stale flavor of cooked rice.