Physical and chemical modification of SPI as a potential means to enhance small peptide contents and antioxidant activity found in hydrolysates

The hydrolytic behaviour of the thermal and chemical modified soy protein isolate (SPI) and the antioxidant activity of the resultant hydrolysates have been evaluated in the present study. Thermal treatment, especially in combination with the addition of sodium sulfite, increased the susceptibility of protein hydrolysis due to the unfolding of proteins and the cleavage of disulfide bonds. Succinylation and acylation resulted in significant decrease in amino acids content than that of phosphorylation due to steric hindrance effect as a result of introduction of longer and bulky side chains. Thermal and chemical modification resulted in a significant increase in small peptide content found in hydrolysates, which in turn enhanced significantly the 2,2-Diphenyl-1-picryhydrazyl (DPPH) radical scavenging activity of the hydrolysates, indicating that modification of proteins before hydrolysis could be an efficient way to enhance the small peptide content with desired antioxidant activity. Industrial relevance: Enzymatic hydrolysis of SPI using selected protease has been intensively studied, however, reports on optimization of the substrate by thermal and chemical modifications before hydrolysis to obtain desired hydrolysates are still limited. In this work, SPI was thermally treated or chemical modified and then hydrolyzed by Alcalase. The results indicated that physical and chemical modification of SPI could change the levels of formaldehyde nitrogen and small peptide nitrogen, and consequently enhanced significantly the DPPH radical scavenging activity of the hydrolysates. This work was helpful to provide a feasible and economic way to enhance the contents of small peptides with desired antioxidant properties. (C) 2010 Elsevier Ltd. All rights reserved.