Different freezing methods did not affect shear force values of the loins (Table 1; P > 0.05). Furthermore, no interaction between ageing and freezing method was observed (P > 0.05). The qualitative Western blot analysis of desmin also showed no substantial impact of different
freezing rates on the extent of desmin degradation (Fig. 3). 3). Desmin is a myofibrillar protein, and the extent of desmin degradation is highly related to meat tenderness development. The impact of freezing rates on meat tenderness has been debated by several researchers. The positive fast freezing impact on meat tenderness has been reported, where quick frozen samples showed less resistance to shearing or higher scores of sensory tenderness compared to slow frozen counterparts. In contrast, no substantial freezing rate effect on eating quality attributes of beef including tenderness and water-holding capacity has also been reported. The discrepancy between the studies could be attributed to the different freezing rate and/or possibly different freezing methods adopted in each study. The studies, where significant freezing rate impacts on meat tenderness were found, mostly applied to freezing temperature at −40 °C or even lower temperaturesreported that the solubility of myofibrillar proteins and meat tenderness increased with faster freezing rates, which could have resulted from the formation of smaller sized intracellular ice crystals within the fibres thus leading to less denaturation compared to the slower freezing rate. On the other hand, other studies found no substantial freezing rate impact on meat tenderness, when the targeted freezing temperature ranged between −18 and −28 °C.
Ageing-then-freezing significantly improved shear force values of the loins compared to both the aged only (A4) and frozen only (F2) loins (Table 1). It is of interest to note that the aged/frozen/thawed loins (both A3F2 and A4F2) had even lower shear force values than A4 (P < 0.001). The lower shear force values of the aged/frozen/thawed loins compared to the aged only (never frozen) loin did not seem to be attributed to the further myofibrillar protein degradation by protease enzymes, as no significant differences were shown between the aged
only and the aged/frozen/thawed beef loins based on the qualitative Western blot analysis of desmin (Fig. 3). Numerous studies have found a tenderising effect of freezing/thawing, 2009), which is likely associated with the loss of structural integrity induced by ice crystal formation. Furthermore, ageing-then-freezing might allow the formation of intracellular crystals between degraded myofibrillar proteins, which could subsequently induce greater muscle fibre fragmentation and/or structural weakening during thawing resulting in an increase in meat tenderness.