Membrane fractionation of hydrolysates of the water-soluble protein from tuna fish meal obtained by subcritical water and enzymatic treatments. Comparison of physical and chemical properties.

Two different hydrolysates of the water-soluble protein (WSP) fraction from tuna fish meal were obtained by subcritical water-CO₂ (subW-CO₂) and Alcalase^® treatments. Hydrolysates showed different chemical composition regarding their free amino acid (FAA) profile and molecular weight distribution of the peptides generated. Consequently, different strategies were proposed by using a stirred dead-end filtration system equipped with flat sheet membranes. Two nanofiltration (NF) consecutive steps were proposed for fractionation of subW-CO₂ hydrolysates with polyamide membranes (800-600 Da and 300-150 Da) yielding a first retentate where 79% of the protein fraction was retained, while 92% of FAA were retained in the second step with a purity index of 29%. Alcalase^® hydrolysates were fractionated by ultrafiltration (UF, 10 kDa) followed by NF (1-1.1 kDa) with polyethersulfone membranes obtaining a first retentate with more than 65% of protein fraction retention with a purity index of 50%. A relatively wide molecular weight distribution was observed in all streams, with high antioxidant and iron-chelating capacities in the retentate fractions.