Physicochemical, structural, foaming, and rheological properties of soybean peptide aggregates obtained from enzymatic by-products

Soybean peptide aggregates (SPAs), enzymatic by-products derived from the hydrolysis of soybean protein isolate (SPI), are often considered "ready for disposal" owing to their limited utilization, thereby posing an additional burden on the food industry. However, these by-products possess substantial nutritional value and a rich protein content. This study investigated the changes in the structure, physicochemical, foaming, and rheological properties of SPAs with different degrees of hydrolysis. The results showed that the SPAs comprise peptides with molecular weights below 35 kDa. The increased particle size and decreased absolute zeta potential of the SPAs with degrees of hydrolysis demonstrate the enhanced formation of aggregates. Moreover, enzymatic hydrolysis led to a decrease in alpha-helix content and an increase in the beta-sheet content of SPI, facilitating the formation of SPAs aggregates through hydrophobic interactions. The SPAs exhibited higher intrinsic fluorescence and ultraviolet spectral intensities than SPI, suggesting that the soybean protein unfolds during hydrolysis to expose more polar sites, which could enhance structural flexibility and improve their foaming capacity. In addition, the increased interfacial adsorbed protein content, reduced surface tension, and higher adsorption rate indicated that SPAs with flexible structures could rapidly absorb at the interface, enhancing their foaming capacity. However, the decrease in absolute potential, oversaturation of intramolecular hydrogen bonds due to excessive hydrolysis, and insufficient relative molecular mass resulted in the formation of less dense interfacial membranes, leading to reduced foaming stability than SPI. Among all the samples of SPAs, the SPAs produced after 90 min of hydrolysis exhibited the highest foaming capacity (67.5 +/- 3.37%) and foaming stability (31.91 +/- 1.59%). This is attributed to its higher protein adsorption, lower surface tension, more flexible structure, and highest elastic modulus, facilitating rapid adsorption at the air-water interface and the formation of foams with sufficient rigidity. These findings highlight the potential of SPAs as promising foaming agents in the food industry.