Heterologous expression of protein-glutaminase in Escherichia coli Nissle 1917 for improved flavor properties of soy protein and its implications on high-moisture extrudates

Beany flavor affects sensory experience of soy protein -based food. Deamidation is an effective approach to improve the flavor of soy protein, but excessive hydrolysis has a negative effect on its molecular structure and processing property. In this study, protein-glutaminase (PG; EC 3.5.1.44) was heterologous expressed in E. coli Nissle 1917 (EcN) and used to specifically modify soy protein isolate (SPI) for improved flavor properties of textured protein. A fluorescence -based enzyme activity screening method was developed and optimized for fermentation conditions. After screening for optimal fermentation conditions based on cell fluorescence intensity, 3.01 U/mL of PG activity was achieved. SPI was treated with PG (0-24 h), and the degree of deamidation (DD) rapidly increased to 47.4% within 0-2 h. There was no significant decrease in the molecular weight of deamidated SPI (DSPI) after specific deamidation of PG within 0-10 h. The contents of main beany flavor substances (e. g., hexanal, benzaldehyde, and 2-pentylfuran) in DSPI decreased significantly with increasing DD. Compared with soy protein, moderate deamidation did not affect the processing properties and fibrous structure of DSPI in preparing high-moisture extrudates, while the structure of the extrudate decreased after a deamidation time of more than 6 h. This finding developed a safe expression system for PG and provided a promising method to address beany flavor in soy-based meat analogues produced by high moisture extrusion.