Ultrasound modified flaxseed protein-rice starch composite system for texture-modified soft gels: structural, functional, and swallowing properties

Meeting the growing demand for safe, nutritious, and swallowable foods for the elderly with dysphagia is hindered by persistent difficulties in achieving appropriate texture and stability. This study investigated the effects of ultrasonic pretreatment on the structural, functional, and swallowing properties of a flaxseed protein-rice starch (FP-RS) composite system used as potential texture-modified soft gels suitable for dysphagia patients. Ultrasonic treatment (0-80 min) modulated the gel network in a non-monotonic manner. Moderate ultrasonication (40 min) promoted protein unfolding and starch fragmentation, enhancing intermolecular hydrogen bonding and hydrophobic interactions, and facilitating the formation of a homogeneous, tightly cross-linked three-dimensional network. This optimized structure significantly improved the water-holding capacity from 83.66% to 94.98%, increased hardness to 0.86 N and cohesiveness to 0.26, and enhanced thermal stability by raising the initial degradation temperature from 293.4 °C to 298.3 °C while reducing the weight loss from 78.7% to 76.2%. Atomic force microscopy revealed a minimum surface roughness (Rq) of 1.40 nm at 40 min, compared with 15.1 nm for the untreated control, confirming the highest nanoscale structural uniformity. Notably, gels subjected to 40-60 min of ultrasonication achieved International Dysphagia Diet Standardisation Initiative (IDDSI) Level 5 classification, demonstrating appropriate moldability, cohesiveness, and low adhesiveness. However, prolonged ultrasonication induced protein over-aggregation and starch chain scission, leading to network degradation and functional deterioration. This study provides mechanistic insights into ultrasound-induced structural modifications of FP-RS composite gels and highlights their potential as texture-modified soft gel foods for dysphagia management.