Green extraction and esterification of marine polysaccharide (ulvan) from green macroalgae Ulva sp. using citric acid for hydrogel preparation

A green strategy of using citric acid as both an extractant and a cross-linking agent to expel and esterify ulvan – a marine polysaccharide from foliose Ulva sp. and filamentous Ulva sp. for hydrogel preparation was developed. Ulva sp. biomass of two different morphologies, foliose and filamentous Ulva sp., was collected from the Spiddal coast of Ireland to extract ulvan using a novel citric acid method which compared with conventional hydrochloric acid or water-based extraction methods. Extraction process parameters, viz. Ulva sp. load, extraction time, extraction temperature, and citric acid concentration, were optimised using the Taguchi experimental design. The optimised process parameters were 3 wt%, 4 h, 90 °C, and 1 wt%, respectively, for foliose and filamentous Ulva sp. At the optimised conditions, a maximum of 0.41 (±0.02) g/g and 0.39 (±0.04) g/g of ulvan yield with a protein content of 0.010 (±0.003) g/g and 0.007 (±0.002) g/g (dry weight basis) were obtained, respectively. This ulvan yield by citric acid-based extraction was superior in comparison with conventional hydrochloric acid (0.29 g/g) and water (<0.1 g/g) based extraction. Extracted ulvan was characterised and compared with commercial ulvan using proton - Nuclear Magnetic Resonance (NMR), Fourier transform infrared (FTIR) spectroscopy, and differential scanning spectroscopy (DSC) analysis. Extracted ulvan was cross-linked with 2 wt% of citric acid to form hydrogel, and the cross-linking mechanism was found to be an esterification reaction using FTIR spectroscopy. Hybridization of ulvan with varying proportions of sodium carboxymethyl cellulose (Na-CMC) was carried out to uncover the broad applications of the hydrogel in health care, agriculture, and environmental fields.