Salting-Out Effect Behavior of Protein/λ-Carrageenan Composite Gels Enhanced by Enzymatic Pretreatment: Focusing on Microstructure, Interactions and the Potential for Dysphagia Food

This study aimed to investigate the effects of synergistic K⁺ immersion-induced salting-out on the rheological properties, microstructure, molecular interactions, and swallowing adaptability of soy protein isolate (SPI)/λ-carrageenan composite gels under different enzymatic pretreatment times (0, 10, 20, 30, 60, and 120 min) using Flavourzyme. The results showed that enzymatic hydrolysis increased the degree of hydrolysis of SPI from 1.11% to 11.46%, gradually degraded the 7S subunit, and reached the highest surface hydrophobicity at 30 min of moderate hydrolysis. After KCl immersion treatment, the K-λ/SPI gels exhibited lower water holding capacity and higher whiteness compared to those before immersion. Among them, the K-λ/SPH30 group demonstrated the best rheological properties. Moderate enzymatic hydrolysis synergistically promoted the formation of a dense network in K-λ/SPI gels. This process enhanced the stability of the composite gel through hydrophobic interactions, electrostatic interactions, and hydrogen bonds while simultaneously increasing the proportion of β-structures (reaching a maximum of 62.05%). The expanded binding sites from moderate enzymatic hydrolysis, combined with the dense network and enhanced interactions, collectively strengthened the salting-out effect. This ultimately enabled K-λ/SPH30 to achieve the highest crystallinity (93.57%), the highest K⁺ content (4.80%), and the optimal swallowing performance (IDDSI level 5). This study not only deepens the understanding of the synergistic mechanism between protein hydrolysates and salting-out but also provides an innovative solution for designing foods for dysphagia diets.