Effect of Different Freeze–Thaw Cycles and Fucoidan on Structural and Functional Properties of Lotus Seed Starch Gels: Insights from Structural Characterization and In Vitro Gastrointestinal Digestion

The influence of freeze-thaw (FT) cycling and fucoidan incorporation on the structural and digestive characteristics of lotus seed starch (LS) gels was systematically examined. Fucoidan-lotus seed starch (F-LS) gels were exposed to 0, 1, 3, and 5 FT cycles. Repeated FT treatments were found to disrupt the gel matrix and decrease thermal stability, whereas the addition of 1-2% fucoidan effectively alleviated these degradations. Crystallinity was significantly reduced from 37.62% to 26.38% (<i>p</i> < 0.05), indicating suppressed retrogradation. Thermal gravimetric and low-field NMR analyses revealed reinforced matrix cohesion. <i>In vitro</i> digestion assays demonstrated that fucoidan significantly retarded starch hydrolysis and promoted resistant starch (RS) formation. After five FT cycles, the RS content of 2% F-LS gels reached 29.03%, a 30.24% increase compared to the control. These findings suggest that fucoidan could serve as a natural and effective cryoprotectant and digestibility modulator in starch-based functional foods, offering both technological and nutritional benefits.