A Comparative Study of Soy Protein Isolate-κ-Carrageenan Emulsion Gels and Bigels for the Encapsulation, Protection, and Delivery of Curcumin

Protein-based emulsion gels and bigels serve as ideal delivery systems owing to their distinctive structural properties, high encapsulation efficiency, and adjustable digestive behavior. However, limited research has examined the differences between emulsion gels and bigels as polyphenol delivery systems. In this study, oil-in-water (O/W)-type emulsion gels formulated with soy protein isolate (SPI) and κ-carrageenan (κ-CG) were fabricated using a cold-set gelation method, and then the bigels were prepared through further oil gelation by the addition of glycerol monostearate (GMS). Both SPI-κ-CG emulsion gels and bigels were mainly stabilized by electrostatic and hydrophobic interactions, exhibiting high gel strength, varying from 940 g to 1304 g, and high water holding capacity (~84%). Both the SPI-κ-CG emulsion gels and bigels demonstrated high curcumin encapsulation efficiency, reaching 98~99%. Stability testing revealed that bigels prepared with 15% and 20% GMS exhibited the highest curcumin retention ratios, with a value of around 78% after storage for 21 days at 25 °C, suggesting that denser network structures more effectively prevent the degradation of the encapsulated compound. During the in vitro simulated gastric digestion, higher GMS content significantly delayed curcumin release by over 7%. Increasing GMS concentration from 0% to 20% elevated lipolysis by over 8% and concurrently improved the release of curcumin by more than 18% during the in vitro simulated intestinal digestion. This study provides comparative insights into polyphenol delivery performance between emulsion gels and bigels, offering valuable guidance for developing functional foods based on gel delivery systems.