Structural and functional analysis of soy protein-skipjack tuna protein gels prepared by two-screw extrusion and their effects on metabolites and gut microbiota in vitro

This study examined the impact of varying skipjack tuna fish protein (FP) concentrations on the structural and functional properties of soy protein (SP)-based gels produced via two-screw extrusion. Scanning electron microscopy showed that gels with ≤10 % FP had compact structures, while ≥20 % FP caused voids, cracks, and phase separation. Fluorescence spectroscopy indicated disrupted protein conformation with increasing FP. X-ray diffraction revealed enhanced semi-crystalline structures, while textural analysis showed reduced hardness and chewiness but consistent springiness with higher FP. Gels with FP improved gelation behavior and thermal stability, with SP@20 %FP demonstrating optimal thermal resistance. Rheological analysis showed higher FP improved gel strength but weakened structural integrity beyond 20 %. Short-chain fatty acids analysis indicated increased acetate and propionate levels with FP, enhancing fermentation potential. Microbial diversity decreased with increasing FP, favoring fermentative bacteria. Moderate FP addition optimized gel properties, but excessive FP weakened structure and altered microbial dynamics, emphasizing balanced FP usage. • Moderate fish protein improved gel structure, thermal stability, and gelation behavior. • High fish protein caused voids, phase separation, and reduced gel textural properties. • Fish protein addition increased acetate and propionate, enhancing fermentation potential. • Higher fish protein reduced microbial diversity, favoring fermentative bacterial growth. • Soy protein with 20 % fish protein achieved optimal structural and microbial stability.