A high yield of sodium alginate with high mannuronic content and low viscosity was extracted by using a greener and cleaner organic acid pre-treatment in the presence of the eco-friendly microwave and ultrasound techniques for a sustainable extraction route.
Abstract
In this study, alginate was extracted from Padina gymnospora (PG) by applying the sequential combination of microwaves followed by ultrasounds (at low power and amplitude settings) using an organic acid pre-treatment. The optimum alginate yield was investigated by varying the type of acid (oxalic and formic) and seaweed particle size range (0.295–0.495 and 0.146–0.246 mm). The extracted sodium alginate was then characterised in terms of FTIR, 1H-NMR, M/G ratio, viscosity and colour. The synergistic effect of the microwave-ultrasound hybrid system for short timeframes (15 min each) successfully improved the alginate yield without compromising its structural composition, while allowing for a milder and greener organic acid to be used for the acid pre-treatment. The highest alginate yield (~64 %) was obtained using formic acid and the smaller particle size range (0.146–0.246 mm). The latter thus highlights the importance of particle size reduction and optimisation. FTIR, NMR and viscosity analyses revealed a low guluronic acid content of alginate with a high M/G ratio (7.7±1.0–9.3±0.1) and low viscosity (2.92±0.09–3.05±0.06 cP), which was found to be inherent to the species used. The proposed novel method therefore opens prospects for the industry since it is both environmentally and economically sustainable.