Title: Biorefinery for yellowfin tuna (thunnus albacares) by-products for characterization and identification of value-added materials

Valorization of fish by-products is gaining interest because a huge amount of biomass is discarded as by-products from fish processing industries. Seafood companies in Sri Lanka use only 50% of the total catch for actual human consumption. This work aims to find out the appropriate extraction methods to maximize the recovery of high-value-added compounds from juvenile yellowfin tuna by-products and extract those compounds using a novel biorefinery approach. The chemical composition of yellowfin tuna by-products was examined. Oil, collagen, and protein isolates were recovered using the pH shift method combined with the acid-soluble collagen extraction from the head, dorsal and caudal samples of juvenile yellowfin tuna (n=7, mean weight = 0.875 ± 0. 107 kg). In terms of protein content, the highest protein content was obtained in the caudal sample (23.80 ± 0.14%) followed by dorsal (22.75 ± 0.64%) and head (18.75 ± 0.21%). Omega-3 PUFA was higher in the dorsal (40.02%) than in the caudal (44.56%) and head (32.78%). The alkaline pH shift method resulted in high yield (Dorsal:10.9%, caudal (Post H₂O₂): 10.85%, caudal (Pre H₂O₂): 10.77%, head: 8.81% on a wet weight basis) and high purity (<90%) protein isolates from heads, caudal and dorsal samples. Collagen extracted with 0.6 M hydrochloric acid could increase the collagen yield from the head (21.16% on a dry weight basis) and caudal sample (39.44% -Pre H2O₂, 23.17% -post H2O₂; on a dry weight basis) than the dorsal sample (9.51% on a dry weight basis) which was extracted with 0.5M acetic acid. The oil was extracted through the pH shift method at room temperature and the conventional heat base method. Yellowfin tuna oil was rich in omega-3 polyunsaturated fatty acids, particularly DHA (26.84%). Freeze-thawing and repeated centrifugation at 4⁰C during the pH shift method recovered a higher oil yield (1.52%) than the conventional heat base method (0.75%). A higher yield could be expected from adult yellowfin tuna. The outcomes of this study depicted the maximum recovery of high-added value compounds with high purity of proteins from the raw materials through this biorefinery approach.