Microwave Puffing-Mediated Structural Modification of Soy Protein: Effects on Dynamic Changes in Key Properties During Soybean Paste Fermentation and Umami Improvement.

This study investigated the effects of microwave puffing-induced soy protein structural modification on the dynamic changes in physicochemical properties and umami formation during soybean paste fermentation. The results indicated that optimal microwave puffing (400 W, 90 s) disrupts protein non-covalent bonds, depolymerizes protein aggregates, exposes the internal hydrophobic groups to enhance surface hydrophobicity, forms a loose porous structure, and creates favorable conditions for enzymatic hydrolysis during soybean paste fermentation. Conversely, excessive treatment (400 W, >90 s) induced protein re-aggregation and inhibited proteolysis, as confirmed by SEM. Furthermore, microwave puffing (400 W, 90 s) promoted Aspergillus oryzae growth and protease activity, and significantly improved physicochemical properties of soybean paste, including lower pH and higher contents of total acid (5.8%), amino acid nitrogen (11.8%), small peptides (13.3%), and reducing sugars (14.95%) (p < 0.05), plus a brighter, redder color. LC-MS/MS revealed microwave puffing (400 W, 90 s) reshaped the peptide profile of soybean paste, increasing the relative abundance of the umami amino acids (Glu, Asp) and sweet amino acid (Ala) and decreasing proportions of the bitter amino acids in the peptides. Sensory and electronic tongue analyses confirmed enhanced umami and weaker bitterness and saltiness. These findings demonstrate that microwave puffing (400 W, 90 s) effectively improves the quality and flavor characteristics of soybean paste, providing technical support for high-quality fermented soybean products.