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 degrees C 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.

Automated summary

The valorization of underexploited solid residue from red marine algae after agar extraction was studied through subcritical water treatment. The use of by-pass section allowed for pre-heating the water and avoiding exposure to high temperatures, leading to higher hydrolysis yields. Increasing flow rate led to higher initial hydrolysis rates due to enhanced diffusion of hydrolysis products, with different effects on carbohydrate fractions and protein fractions.