Freezing Promotes Postmortem Proteolysis in Beef by Accelerating the Activation of Endogenous Proteolytic Systems

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This study investigated the effect of freezing and subsequent aging on beef quality, particularly focusing on the extent of postmortem proteolysis and tenderization. The longissimus lumborum muscle was collected from 8 steers 24 h postmortem, sliced into 8 2.5-cm-thick steaks, and randomly allocated into 4 groups. Treatment groups consisted of 1) aging at 4°C for 24 h; 2) aging for 168 h; 3) freezing at −20°C for 24 h followed by thawing/aging for 24 h; and 4) freezing for 24 h followed by thawing/aging for 168 h. In general, freezing decreased the color intensity of the steaks, whereas aging increased it (P < 0.05). Freezing also increased water loss, evidenced by greater drip loss and purge loss (P < 0.05). On the other hand, both freezing and aging improved beef proteolysis and tenderness (P < 0.05). This was associated with enhanced protease activity, indicated by greater calpain-1 autolysis and cathepsin B activity (P < 0.05). Additionally, freezing may have accelerated the activation of caspase-3, but our sampling timing did not permit verifying this possibility. This increase in the activity of proteases is likely caused by ice crystals disrupting cellular organelles, releasing factors that trigger their activation. In support of this, frozen steaks displayed an elevated level of free calcium and mitochondrial dysfunction (P < 0.05). Collectively, these findings suggest that freezing enhances postmortem proteolysis and tenderness in beef, likely by compromising key cellular organelles and subsequently accentuating the activity of several endogenous protease systems during aging.