Preparation and radical scavenging activity of papain-catalyzed casein plasteins

Abstract

The aim of this study was to prepare papain-catalyzed casein plasteins and to demonstrate the influence of the plastein reaction on the radical scavenging activity. Casein was initially hydrolyzed with papain under selected conditions to give a degree of hydrolysis of 9.6%. The casein hydrolysates were then subjected to papain-catalyzed plastein reaction to prepare casein plasteins. The formation of plasteins in the prepared casein plasteins was confirmed by size exclusion chromatography. The optimal conditions for the plastein reaction, determined by response surface methodology using the decrease of free amino groups in the casein plasteins as the response variable, were as follows: papain addition level was fixed at 500 U·g−1 proteins, reaction temperature 30 °C, concentration of casein hydrolysates 50% and reaction time 5.6 h. Six different casein plasteins were prepared and the radical scavenging activity on two radicals, 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS), was evaluated by the Trolox equivalent antioxidant capacity (TEAC). Analysis showed that all casein plasteins had improved radical scavenging activities. Compared to casein hydrolysates, the scavenging activity of the casein plasteins on DPPH radical had a seven- or ninefold increase, as TEAC increased from 10.45 to 73.41 or 90.31 mmol·g−1 proteins, while a onefold increase was detected on ABTS+ (TEAC increased from 8.33 μmol·g−1 to 15.09 or 16.45 μmol·g−1 proteins). The results of this study clearly showed that the papain-catalyzed plastein reaction could be applied to prepare casein antioxidant peptides with higher activity, and that the plastein reaction may have a new application in dairy science.