Identification of Properties and Structure of ι-Carrageenan Prepared with Different Alkali Treatment Times

This study systematically investigated the influence of alkaline treatment duration (2–6 h) on the structural characteristics and functional properties of ι-carrageenan derived from Eucheuma denticulatum. The results indicated that a 3 h alkaline treatment achieved the highest yield (46.6%) and significantly enhanced gel strength by facilitating the formation of 3,6-anhydrogalactose (3,6-AG). In contrast, prolonged treatment reduced both molecular weight and yield. Moderate alkaline treatment (3–4 h) improved crystallinity and thermal stability while maintaining water-holding capacity above 80% after freeze-thaw cycles. FT-IR and XRD analyses confirmed that alkaline treatment preserved the fundamental structure of ι-carrageenan while altering molecular packing order. These findings provide valuable guidance for industrial production, indicating that a 3–4 h alkaline treatment not only optimizes yield but also enhances functional performance. Overall, the study elucidated how alkaline treatment time modulates ι-carrageenan quality through 3,6-AG formation, molecular chain degradation, and crystalline reorganization, thereby offering theoretical guidance for optimizing food-grade carrageenan production.