Thermal inactivation of rice bran lipase by extrusion-cooking for production of edible oil

Abstract

For the development of the effective method to prevent the deterioration of rice bran after milling, series of the experiments for characterization of lipase action in low-miosture soilid state, and for analysis of relation of water activity to thermal inactivation of rice bran lipase $\underline{in}$ $\underline{situ}$, have been conducted. And to produce edible oil from rice bran, optimization of the operation variables in extrusion cooking of rice bran also have been carried out through the evaluation of the extraction and refining characteristics of the extrusion/stabilized rice bran. Two species of rice bran lipase was isolated by using hydrophobic chromatography on otyl-Sepharose CL 4B column. The isolated major lipase showed an isoelectric point of 8.53 and a specific activity of 5.83U/mg of enzyme protein. This enzyme showed its maximal reaction rate at pH 8.0 and 37$^\circ$C and preferentially hydrolized the shorter chain length of fatty acid ester of triglyceride. In low-moisture solid state, the lipolysis was not required the mediation of the aqueous phase as a vehicle, however, the reaction rate was controlled by the rate of diffusion from the enzyme to lipid substrate. It was confirmed by the results as followings; The measurable lipolysis action occured at a water activity as low as 0.21. Lipolysis rate, however, increased by increasing water activity and the maximum extent of lipolysis tends toward a different final value at each different water activity. The rate of lipolysis was directly propotional to enzyme concentration only at the water activity above 0.75. In thermal inactivation of lipase in bran, the rate constant on destrution of lipase in the bran of different moisture contents was increased as increase the moisture content. The D-value at 110$^\circ$C was increased in the order of 33, 72, and 154 sec for the bran hydrated at 23.3, 12.7 and 5.88\%, respectively. The free energy changes was between 23 and 26 kcal/mole, and the entrophy ...