The effect of electrical stimulation on beef tenderness, protease activity and myofibrillar protein fragmentation

From six one-year-old White red bulls, one randomly assigned carcass half was subjected to high voltage electrical stimulation (HVES) at 1 h post mortem (pm). Longissimus dorsi (LD) steaks from control (C) and stimulated sides (ES) were removed at 24 h pm, stored at 4-degrees-C and sampled at 1, 6, 8 and 12 days pm. HVES caused an immediate fall of pH with about 0.5 units (P < 0.01), and an increase in temperature of about 1-degrees-C (P < 0.05). HVES lowered LD shear force values (P < 0.05), both at 1 and 8 days pm. Sarcomere length was not affected by stimulation, but cooking losses were increased in ES. Semi-quantitative SDS-PAGE, using BSA as internal standard showed the rate of degradation of several myofibrillar proteins to be increased by HVES. Levels of 43-kDa peptide increased upon ageing and were higher in ES (P < 0.001). The level of 30-kDa peptide increases strongly in the early ageing period and faster in ES. It is striking that 1 h pm ES contain more 30 kDa (P < 0.05), suggesting activation of calpain 1. The increase upon ageing in levels of 34- and 35-kDa peptides, often considered as the tropomyosins, suggests that these compounds are myofibrillar degradation products. Troponin-T (TNT) was degraded very fast in the early ageing period. The degradation rate was increased by HVES (P < 0.005) and related to rate of tenderization (r = 0.90). Irrespective of HVES, titin was degraded to a considerable extent (35%) during the first 6 days of ageing, whereas nebulin and filamin were far less degraded. HVES induced a small increase in both calpain 1 activity and calpastatin capacity at 1 h pm, but the remaining calpain 1 and calpastatin levels at 24 h pm were decreased significantly (P < 0.05). Ageing decreased calpain 2 activities by about 15% between 1 and 24 h pm, and no effect of HVES could be detected. Total and free cathepsin B and cathepsin L levels were not affected by ageing or HVES. Endogenous total proteolytical activity (measured from release of peptide bound tryptophan of TCA soluble peptides) increased upon ageing, suggesting an increase in myofibrillar substrate availability. We conclude that HVES caused significant tenderization, enhanced degradation of TNT and increased rate of formation of 30- and 43-kDa peptides. Tenderness is improved by HVES within 24 h, and rate of ageing later on is not affected by HVES. Part of the data suggest that the early post mortem tenderization could be due to calpastatin-regulated calpain 1 activity. The data do not suggest a role for cathepsins B and L in tenderization. To investigate the relative importance of various proteases in meat conditioning or ageing, in situ measurement of protease activity is necessary, rather than measuring total enzyme concentrations. Indeed, the classical calpain and cathepsin assays do not take into account in situ enzyme regulation and changing myofibrillar substrate availability

Meat Toughening During Aging ?

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