Effect of ethanol concentrations on rice bran protease activity and ester synthesis during enzymatic synthesis of oleic acid ethyl ester in a fed-batch system using crude rice bran (Oryza sativa) lipase

Abstract

Crude rice bran lipase was used for the enzymatic synthesis of Oleic Acid Ethyl Ester (OAEE) in a fed-batch system. It was found that rice bran contains protease (4.19 U/g) and lipase (324.03 U/g). The results show that protease inhibits lipase at a low concentration of ethanol (5 μmol/ml), in which lipase esterification activity was only 45.02 U/g. However, protease activity decreased about 11 times at a higher concentration of ethanol (5–50 μmol/ml). It resulted in an increase in the esterification activity of lipase (324.03 U/g). This finding could be used for explaining the effects of ethanol on the enzymatic synthesis of OAEE using crude rice bran lipase in a fed-batch system. When the ethanol concentration in the reaction system was low (≤ 4 μmol/ml), the final OAEE conversion was only 30%. It was caused both by the inhibition of lipase by rice bran protease and by the effect of low ethanol concentration on the equilibrium position of the lipase-catalysed esterification reaction. However, the final OAEE conversion reached 76–92% when higher ethanol concentrations (12.5–50 μmol/ml) were added at initial reaction. It is suggested that the addition of higher ethanol concentrations (12.5–50 μmol/ml) into the reaction system may reduce protease activity and also increase ethanol concentration in the reaction system to be converted into OAEE.