Production and characterization of protein-rich extracts from the red macroalga Gelidium corneum and its industrial agar extraction residues

The red macroalga Gelidium corneum is a species commercially exploited for agar extraction, though with enormous potential for other industrial applications. This work aimed to produce protein-rich extracts from the crude alga and from residues of the industrial agar extraction process by applying different extraction and recovery procedures, and to characterize the obtained protein extracts. The sequential implementation of two water extractions followed by two alkaline extractions resulted in the recovery of 12.0 +/- 1.2 % and 15.4 +/- 0.9 % of the total protein contents of the crude alga and industrial residues, respectively. Two sequential water extractions assisted by Celluclast (R) and Alcalase (R) enzymes followed by two alkaline extractions recovered 52.1 +/- 1.7 % and 36.5 +/- 1.3 % of the protein contents of the crude alga and industrial residues, respectively. Extracts produced by these processes were characterized focusing on proximal composition, protein accessibility, nutrition and antioxidant potential, showing value as aquaculture feed ingredients. As all protein products revealed high carbohydrate contents and, aiming at enriching protein content, different protein precipitation methods were assessed, namely through ammonium sulfate precipitation and pH-shift protocols. The pH-shift method in sequential mode proved to be simple and fast with low reagent consumption. The purification of extracts by this precipitation mode allowed precipitating up to 72 % of the extracted protein. The results obtained suggest that protein concentrates extracted from Gelidium corneum appear to be promising sustainable sources for fish feed production, owing to their essential amino acid content, protein bioaccessibility and antioxidant properties.