Surfactant-induced coagulation of agarose from aqueous extract of Gracilaria dura seaweed as an energy-efficient alternative to the conventional freeze–thaw process

Surfactant-induced coagulation of agarose from alkali-treated Gracilaria dura seaweed extract (SE) is reported. The new approach, which was suitable for linear galactans with low sulphate content is an alternative to the traditional energy intensive process of “freeze–thaw” cycles employed for product isolation from the extract. Only nonionic surfactants were effective, and detailed studies were undertaken with octyl phenol ethoxylate (Triton X-100). The coagulated product was successively washed with water and water–isopropyl alcohol (IPA) to yield a fine powder of agarose in 13–15% yield (with respect to dry biomass). The product exhibited excellent properties [sulphate content: 0.2% w/w; degree of electro-endosmosis: 0.13; gel strength: 2200 g cm −2 (1% gel, w/v); and gelling temperature: 35 ± 1 °C] essential for demanding molecular biology applications, and the desired gel electrophoretic separation of DNA and RNA was demonstrated. It was further confirmed that there was no degradation of nucleic acids in the gel. The agarose-depleted extract, along with water used for washings, was subjected to reverse osmosis for recovering the surfactant in concentrated form for its subsequent reuse. Energy savings from the improved process were assessed.