Kiwifruit protease extract injection reduces toughness of pork loin muscle induced by freeze-thaw abuse

Kiwifruit juice injection was used to tenderize freeze-thaw abused porcine meat, and the enzyme activity in relation to meat tenderness was determined. Whole pork loins (Longissimus lumborum) were subjected to 5 freeze-thaw cycles (-29 °C ↔ 4 °C). Non-frozen control and freeze-thaw treated loins were either non-injected or injected with 10% water or diluted kiwifruit juice (meat weight basis). After 48 h of incubation, samples were analyzed for pH, proteolysis, cooking loss, and Warner-Bratzler shear force. Shear force of meat increased 2-fold (P < 0.05) after 1 freeze-thaw cycle but decreased when loins were subjected to additional freeze-thaw treatments. The kiwifruit juice injection lowered the pH of meat from 5.6 to 5.2 and increased the cooking loss from 21% to 30% (P < 0.05). However, as indicated by the shear force reduction, tenderness of meat improved more than 2-fold (P < 0.05) after kiwifruit juice injection, in agreement with the SDS-PAGE patterns that showed substantial degradation of myosin in samples treated with kiwifruit juice. The actinidin assay confirmed the existence of significant proteolytic activity in kiwifruit juice extract and in injected loin samples, i.e., 4.65 units/mL and 1.27 units/g, respectively.