In this work valorization of the underexploited industrial solid residue generated after agar extraction from Gelidium sesquipedale
was studied by using subcritical water in a semicontinuous fix-bed reactor. First, a complete characterization of this by-product
was carried out, determining up to 21%(w/w) of protein content (for a nitrogen factor of 4.9) with high content of essential amino
acids, 37% (w/w) of carbohydrate fraction and high amount of ash, 22% (w/w). The effect of temperature, in the range from 129
to 200 °C, and flow rate, in the range from 2 to 6 mL min−1, on protein and carbon fraction extraction/hydrolysiswas studied.At
constant flow rate of 2 mL min−1, a maximum in the protein extraction was achieved at 185 °C. Higher temperatures led to
degradation of protein or its hydrolysis products. Free amino acids release followed the same trend as the protein fraction. The
most temperature sensitive amino acids, as determined by gas chromatography (EZ:faast Phenomenex), were serine and aspartic
and glutamic acids. As a consequence, the selectivity towards non-polar amino acids increased by working at high severity
factors. A Pearson correlation between antioxidant capacity of the collected extracts with the bioactive compounds determined in
the extracts (total polyphenolic compounds –TPC-, peptides and free amino acids) was established, being stronger for TPC. The
ash content in the solid residue after treatment steadily increased with temperature due to non-solubilization being possible its
application in agriculture as fertilizers.
