Freezing is one of the most widely practiced, effective and efficient methods for food preservation. Freezing has been extensively used in the meat industry, as it can maintain a reasonably acceptable level of meat quality for months and even years. The freezing process has substantial impacts on the quality attributes of the frozen/thawed meat products. In particular, the rate of freezing plays a crucial role in determining the size and location (or distribution) of ice crystals within the frozen meat tissue, which subsequently influences the quality of the frozen/thawed meat. It has been generally agreed that slow freezing rates cause the formation of extracellular/large ice crystals within muscle resulting in considerable damage in muscle proteins and cell membranes, consequently diminishing meat quality characteristics (particularly showing unacceptably high moisture loss as purge and/or drip). Conversely, fast freezing results in the formation of numerous fine ice crystals, which are uniformly distributed mostly at intracellular levels within muscle. Thus, the extent of quality deterioration caused by freezing is relatively less for the fast frozen meat compared to the slow frozen counterpart.
Although the influence of freezing rate on ice crystal formation per se is well established, research outcomes of its direct impact on meat quality have not been consistent. While some of the studies showed improved frozen/thawed meat quality attributes (particularly minimized purge and/or drip loss) due to fast freezing rates, others did not. This inconsistency could be attributed to several critical aspects, such as 1) the subjective definitions of “fast” and “slow” freezing rates, as not all the studies actually measured the freezing rate or the freezing velocity the time that elapsed from −1.5 °C (beginning of freezing) to −7 °C, where 80% of water in meat tissue is being frozen, 2) the different freezing methods used (e.g. cryogenic, air blast, freezing tunnel, nitrogen chamber and/or high-pressure-assisted freezing), and 3) post mortem ageing time prior to freezing, as substantial changes/improvement in meat tenderness and water-holding capacity occur during ageing through myofibrillar protein degradation.
Although numerous studies determined the effects of ageing, freezing, or thawing on meat quality attributes, the combined impacts of ageing and freezing on meat quality characteristics have not been extensively studied. In fact, several recent studies reported that the difference in meat quality, such as tenderness, colour and water-holding capacity, between chilled (never frozen) and frozen/thawed meat can be greatly reduced by sufficiently ageing the meat prior to freezing. Therefore, it can be hypothesised that by applying fast freezing (through cryogenic immersion freezing) to aged beef loins, the positive effect of ageing-then-freezing on meat quality attributes will be enhanced by minimising the freezing-related cell/muscle tissue damage. The objective of the current study was to determine the effects of ageing period and freezing rate on meat quality attributes of beef loins.