Abstract: In order to establish a rapid sample pretreatment method for spectrometrically (based on Biuret reaction) determining peptides in rice hydrolysate prepared with alcalase, trichloroacetic acid (TCA) and ethanol were separately used as precipitants to remove high molecular weight proteins from the hydrolysate and three conditions including precipitant dosage and concentration, temperature and standing time were examined. The optimal conditions for TCA precipitation were adding a 3-fold volume of 150 g/L TCA solution to the hydrolysate and then standing at 35 ℃ for 10 min, and under these conditions, the peptide content in the hydrolysate was determined to be (54.2 ± 0.6)%. In the ethanol precipitation, a-fold volume of 75% ethanol solution was added to the hydrolysate and thoroughly mixed prior to standing at 25 ℃ for 10 min, and the resultant peptide content in the hydrolysate was (50.6 ± 0.1)%. Biuret reaction at 30 ℃ for 30 min proved optimal for stable color development. In summary, the TCA precipitation method has the advantages of wider linear range as well as higher sensitivity and repeatability compared with the ethanol precipitation method.

Key words: trichloroacetic acid, ethanol, rice peptide, Biuret reaction