Kinetic study of the semi-continuous extraction/hydrolysis of the protein and polysaccharide fraction of the industrial solid residue from red macroalgae by subcritical water

The valorization of the underexploited solid residue after agar extraction from red marine algae was studied by subcritical water treatment. Experiments were carried out in two different semi-continuous fix-bed reactor configurations at 185 ºC at different subcritical water residence times. The use of a by-pass section allowed to heat the water previous contact to the biomass, avoiding the exposure of the sample to high temperatures during the heating procedure and reducing the formation of degradation products. Higher hydrolysis yields were obtained for the protein fraction (reaching 96.1%) than for the carbohydrate fraction (reaching 45.7%, 11.3%, 27.5% and 57.6% for galactans, glucans, arabinans and uronic acids, respectively). With the decrease of the residence time, by increasing the flow rate, higher initial hydrolysis rates were obtained due to enhancing diffusion of the hydrolysis products into the bulk solution. It was determined a similar dependence of the initial hydrolysis rates on the residence time for the carbohydrate oligomers and total protein fraction, but the release of free amino acids was less dependent on increasing flow rate due to higher diffusion coefficients for small molecules.Agencia Estatal de Investigación (Spain) [grant number PID2019–104950RB-I00 / AEI / 10.13039/501100011033], the Junta de Castilla y León (JCyL) and the European Regional Development Fund (ERDF) [grant numbers BU301P18 and BU050P20] E. Trigueros and P. Alonso-Riaño predoctoral contracts were funded by the JCyL and the European Social Fund (ESF) by ORDEN EDU/574/2018, de 28 de mayo and ORDEN EDU/556/2019, de 5 de junio, respectively. The C. I. K. Diop contract was funded by the European Commission, Horizon 2020 program through the Marie-Curie Individual Fellowship (H2020-MSCA-IF-2019), with regards to the ALGWAS-BIOR project (Grant agreement number 898804)