Proteolytic and structural mechanisms in tuna tenderization by papain, bromelain and ficin.

Thermal processing could reduce tenderness of cooked tuna. Papain, bromelain and ficin were employed to solve this issue, and their tenderizing mechanisms were investigated. Protein profiles showed that macromolecular proteins in tuna were degraded into smaller fractions by proteases, which led to increases in the myofibrillar fragmentation index (MFI) values. All proteases reduced shear force and hardness but increased cooking loss of tuna (except ficin), accompanied by the formation of unevenly distributed pores in tuna microstructures. Compared with control, bromelain decreased shear force and hardness by 86.95 % and 58.25 %, showing the best tenderizing effect. Moreover, reduction in content of disulfide bonds, the primary molecular force in tuna network, also contributed to the improvement of tenderness. Additionally, the ordered structures (α-helix and β-sheet) were transformed to disordered structures (β-turn and random coil) by proteases. These findings showed that proteases could significantly enhance the tenderness of tuna, especially bromelain.