Effect of Exogenous Phytate on Gelation and In Vitro Digestibility of CaCl₂-Induced Soy Protein Isolate

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Phytate interacts with proteins and metal ions to form complexes owing to its strong negative charge, which affects the gelation, structure, and digestibility of protein gels. In this study, different levels of exogenous phytate were added to the soy protein isolate (SPI) treated with phytase to investigate their effects on CaCl2-induced gelation and in vitro digestibility. The surface hydrophobicity and free sulfhydryl groups content of the phytate–protein binary complexes decreased from 1530.87 and 2.50 μM/g to 757.74 and 1.36 μM/g, respectively, with increasing phytate addition. Fourier transform infrared spectroscopy showed an increase and decrease of 4.88 and 3.81% in β-folding and random curling, respectively, in the gel added with 0.5 mg/mL phytate (PG5) compared to the gel with no added phytate (PG0). Lower gel hardness and higher water-holding capacity were observed for PG5, which were 66.87 g and 89.24%, respectively. Scanning electron microscopy indicated that an increase in phytate content resulted in a more uniform and ordered gel structure with smaller pore sizes. During in vitro digestion, PG5 exhibited a lower bioavailability than PG0. The presence of exogenous phytate resulted in CaCl2-induced soy isolate protein gels with a denser network structure and softer texture while increasing the water-holding properties of the gels and reducing protein digestibility.