Protein hydrolysates from Bluefin Tuna (Thunnus thynnus) heads as influenced by the extent of enzymatic hydrolysis

Functional properties and antioxidant activities of protein hydrolysates from tuna (Thunnus thynnus) heads (THPHs), with different degrees of hydrolysis, obtained by treatment with Bacillus mojavensis A21 alkaline proteases and Alcalase, were investigated. Protein content of all freeze-dried THPHs ranged from 73.74 +/- 0.5 to 78.56 +/- 1.2%. The THPHs had excellent solubility, compared to untreated tuna head proteins and possessed interfacial properties, which were governed by their concentrations. Similarly, at a degree of hydrolysis (DH) of 12 and 15%, > 90% nitrogen solubility was observed at all experimental pH values tested. The emulsifying activity index (EAI) and emulsion stability index (ESI) of both hydrolysates at different DHs decreased (p < 0.05) with increasing DH. At low DH (5%), hydrolysates exhibited strong emulsifying properties. All THPHs produced by the A21 proteases generally showed higher antioxidative activity than that of the Alcalase protein hydrolysates. The highest DPPH radical-scavenging activity (78 +/- 2.1% at 3 mg/mL) was obtained with a DH of 15%. The IC50 value for the beta-carotene bleaching assay was 0.5 +/- 0.03 mg/mL. Alcalase (DH = 12%) and A21 (DH = 15%) protein hydrolysates contained glutamic acid/glutamine and arginine as the major amino acids, followed by lysine, aspartic acid/ asparagine, histidine, valine, phenylalanine, and leucine. In addition, the THPHs had a high percentage of essential amino acids, which made up 50.52 and 50.47%, of the protein hydrolysates obtained by the Alcalase and A21 proteases, respectively. Therefore, THPHs can be used as a promising source of functional peptides with antioxidant properties.