Effect of deamidation by protein-glutaminase on physicochemical and functional properties of skim milk

Protein-glutaminase (PG) is an enzyme that catalyzes the deamidation of glutamine residues in proteins, and protein deamidation is a promising method of improving solubility and other functional properties of foods. We investigated the effect of PG on skim milk proteins, finding that their deamidation degree increased dose-dependently. The solubility and relative viscosity of skim milk solution were remarkably improved as the deamidation degree increased, while its turbidity decreased until the milk became translucent; emulsifying capacity also increased, and the mean droplet diameter decreased to <4 μm. Particle size distribution analysis and transmission electron microscopy showed that smaller submicelle particles were produced in highly deamidated skim milk. These observations suggest that PG deamidation induces the dissociation of casein micelle, resulting in an increase of the oil/water surface area. This enzymatic deamidation helped to improve the functionality of skim milk by increasing the electrostatic repulsion of carboxyl groups in caseins.