Alkali treatment affects in vitro digestibility and bile acid binding activity of rice protein due to varying its ratio of arginine to lysine

To elucidate whether the alkali treatment, which is a common process for rice protein extraction, is responsible for the regulation of digestibility and bile acid binding of rice protein, the effects of different degree of alkali treatment (from 0.1% to 0.3% of NaOH), as well as different ratio of arginine (Arg) to lysine (Lys) by 3.0, 3.5, 4.0, 4.5 and 5.0, which was induced by the addition of Arg to rice protein, on the in vitro digestibility and the bile acid binding activity were investigated. The present study clearly demonstrates the alkali treatment plays a major role in controlling the digestibility and the bile acid binding of rice protein through the modification of amino acid composition. Results indicate that the digestibility and the bile acid binding activity of rice protein are closely varying with the ratio of Arg to Lys. This shows the inhibition of digestibility and stimulation of bile acid binding are enhanced by the increase Arg/Lys ratio through the addition of Arg to rice protein. The present study suggests the Arg/Lys ratio controlled by the concentration of Arg and the alkali treatment may be the main modulator responsible for the physiological function of rice protein involved in cholesterol metabolism.