Hydrothermal fractionation of Gelidium sesquipedale alga residue by subcritical water extraction

Abstract

Introduction Gelidium sesquipedale residue after industrial agar extraction is a very valuable by-product because of its carbohydrate, protein and bioactive compounds content. Subcritical water (SW) extraction was carried out by using different solvent flow rates (4, 6 and 8ml/min) to study the effect of residence time (RT = 6.0, 4.2 and 3.0 minutes, respectively) on sugars hydrolysis and fractionation at 185°C. Results and discussion Macroalga residue after industrial agar extraction presents an important fraction of carbohydrate composed by glucans (23.4%), galactans (10.9%) and arabinans (2.9%). Accumulated hydrolysis curves for arabinans, galactans and glucans by using SW are shown in Figure 1. No monomers were observed under work conditions in subcritical water extracts so that carbohydrate fraction was recovered as sugar oligomers. Sugars extraction curves show how the lower the residence time, the higher extraction yields for all components. Moreover, a preferential hydrolysis was observed for galactans, followed by arabinans and glucans that showed the lowest yield. As has been seen, increasing and faster extraction has been found at increasing flow rate due to promotion of mass transfer of the solutes from by-product matrix1, that is translated into a greater compounds extraction. Similar results were found in literature about residence time effect on bioactive compounds extraction2,3. Conclusions SWE constitutes an interesting technique for sugars recovery, promoting a better use of the alga residue generated in the food industry without using organic solvents in compare to conventional treatments. Low residence times led to higher sugars recovery by SW treatment.