Effect of enzymatic deamidation and heat deactivation on structure and functional properties of whey and pea protein isolates

Proteins, especially plant proteins, often require modification to improve their techno-functional properties. Enzymatic modifications are preferred over chemical processes due to specificity, safety, and clean label status. Deamidation to increase net negative charge and enhance solubility could be performed using glutaminases. Two protein glutaminases (PGs), PG-1 and PG-2, which target the deamidation of glutamine to form glutamic acid, were used to modify whey protein isolate (WPI) and pea protein isolate (PPI). The effect of enzyme inactivation at 75°C for 5 min and at 83°C for 20 min was investigated. Protein profiling, surface hydrophobicity and surface charge, particle size distribution, and protein solubility were determined. While PG-1 showed no residual protein hydrolysis, PG-2 showed evidence of proteolysis as evidenced by the presence of low molecular weight peptides. Deamidation resulted in a significant increase in the net negative charge at pH 7 for WPI compared to its control, but no significant effect was observed for PPI. Additionally, inactivation of PG at 83°C for 20 min contributed to an increase in surface hydrophobicity in both WPI and PPI, leading to extensive protein polymerization. Protein solubility improved at pH 7 upon deamidation in both WPI and PPI. Marked improvement in protein solubility at both pH 7 and 3.4 was noted in the samples that experienced proteolytic activity in addition to deamidation. Results highlighted that effects of deamidation on protein structural properties and on solubility are not only dependent on the type and purity of the enzyme, but also on the inactivation process and the type of protein. The findings can serve as the basis for optimizing the deamidation process for a specific protein and its targeted application, such as high protein beverages.