Optimization of Deamidation of Casein by Protein-Glutaminase and Its Effect on Structural and Functional Properties

Casein is a commonly used protein in the food industry, with its related products such as beverages and desserts. However, the further application of casein is limited by its solubility and stability. This study aimed to improve the functional of casein through protein-glutaminase (PG) deamination. The deamination of casein using PG was optimized through central composite design experiments, and its impact on the structure, solubility, and stability of casein was investigated. The results demonstrate that the optimal conditions for PG deamidation were determined at pH 6.0, E/S 15 U/g, and a temperature of 45 °C. The deamidation process alters the secondary structure of casein, resulting in a decrease in α-helix structure and an increase in β-sheet structure. The modification of casein improved emulsifying activity at pH 8.0 and 10.0, respectively, while significantly enhancing the solubility from 5.0 to 6.0. Furthermore, the deamidation of casein caused an increase in zeta potential and a decrease in particle size, resulting in improved stability of the protein solution due to reduced particle aggregation. The 3% deamidated casein-based beverage with carrageenan exhibited reduced precipitation rates compared to the control after sterilization at 121 °C for 15 min. In summary, PG deamidation offers a promising method for the modification and enhancement of the functional properties, including solubility, stability, and emulsifying activity of casein, thereby expanding its use of casein in the food industry.