Ultrasound-Assisted Extraction of Native Agar from Gracilaria corticata: Optimization by Response Surface Methodology and Evaluation of Its Physicochemical, Rheological, and Cytotoxic Properties

Gracilaria corticata, an abundant red seaweed rich in agar, is a valuable resource for sustainable hydrocolloid production. High-yield native crude agar with preserved structural and rheological properties was extracted using ultrasound pre-treatment followed by hot-water extraction (95 °C, 2 h) without alkali pre-treatment. Response surface methodology with central composite design predicted 32.87 min and 61.58% amplitude as optimum conditions, giving a predicted yield of 28.66% (w/w). Acoustic cavitation-induced cell wall disruption significantly enhanced agar yield (validated: 29.31 ± 0.70%) versus conventional hot-water extraction (14.01 ± 0.43%). Commercial agar (C-AP) had higher gel strength. Ultrasound-assisted (US-AP) and conventionally extracted agar (CE-AP) had higher sulfate and lower 3,6-anhydrogalactose contents (p < 0.05). C-AG showed higher gel strength. FTIR confirmed agar functional groups in all samples, indicating preserved functional groups. Syneresis remained below 2%. Microstructure showed a more porous network in CE-AG and US-AG than in C-AG. Rheology demonstrated thermo-reversible gel behaviour, with gelation onset at 38–46 °C. Extracted agars were cytocompatible with BJ fibroblasts after 48 h, supporting soft-gel applications without chemical modification. Low gel-strength seaweeds such as G. corticata can be utilized for high-yield agar extraction via ultrasonication.