Changes in physico-vhemical characteristics of stored brown rice

Abstract

For the investigation on the effect of lipid on the changes in off-flavor and texture of stored brown rice, and for the determination of a simple method as a quality indicator of stored brown rice, various experiments for the analysis of physicochemical properties of stored brown rice have been conducted. Storage temperature caused no difference in fatty acid composition of neutral lipid (NL), glycolipid (GL) and phospholipid (PL) fractions, but caused remarkable differences in fatty acid composition of free fatty acid (FFA) fractions. Amount of the increase in oleic acid of FFA fraction during storage is roughly accounted for by the amount of decrease in corresponding fatty acid of NL fraction. However, the decreased amount of linoleic acid in the NL fraction was greater than its increased amount in the FFA fraction. Furthermore, the amount of n-hexanal was found to be linearly proportional to the amount of oxidized linoleic acid (r=0.99), which indicates that part of the linoleic acid from the NL fraction was converted to oxidized compounds, especially hexanal, by the action of lipoxygenase present in the brown rice. n-Hexanal in headspace over the cooked brown rice stored at the various conditions was determined by a modified direct vapor injection gas chromatographic method. The retention time of n-hexanal was 3.5 min and n-hexanal could be rapidly separated from other compounds at the operational conditions of gas chromatography. n-Hexanal contents of cooked brown rice also showed a standard deviation of less than 10\% on the average. Therefore, direct analysis of n-hexanal in the headspace of cooked brown rice is a useful indicator of the lipid oxidation in the brown rice. The amylogram peak viscosity and total setback of undefatted brown rice flour pastes increased substantially during the 12 month storage period at 35$^\circ$C, but their breakdown did not show noticeable changes during the same storage period. Ether defatted brown rice flour (EDBR), whi...