Colour attributes (L*, a*, hue angle and chroma) of the steaks from the aged/frozen/thawed beef loins were not affected by different freezing methods (P > 0.05). Further, no interaction between ageing period and freezing method was found (P > 0.05). The freezing rate has a crucial impact on muscle structure, the extent of physical cell damage and chemical changes of proteins through ice crystal formation and thus likely affecting oxidation stability. However, the meat surface region is relatively less influenced by the freezing rate (mostly intracellular small ice crystal formation) compared to the centre portion of the meat. Therefore, it can be suggested that the freezing rate per se would not impact the colour stability of frozen/thawed beef loins as observed in the current study.
However, freezing influenced surface meat colour of beef steaks during display (P < 0.05; Fig. 4). Although there were significant freezing by display day interactions found in a*, hue angle, and chroma values (predominantly observed at day 7; Fig. 4), the numerical difference between the frozen/thawed treatments was very marginal (around 1–2 units), so it would not be practically important for further discussion. Conversely, steaks from the A4 – aged only (never frozen)- loins maintained the highest L* (lightness), a* (redness) and chroma (colour intensity) values and lowest hue angle values (less discolouration) compared to the other steaks from the frozen/thawed loins during retail display. It is well known that freezing/thawing results in surface darkening (due to less blooming ability) and reduced colour and colour stability of meat (due to increased myoglobin oxidation susceptibility). However, between the frozen/thawed loins, ageing-then-freezing (A4F2 and A3F2) improved meat colour and colour stability compared to the frozen only loins (F2) during display. The steaks from the aged/frozen/thawed loins had higher lightness, redness and colour intensity compared to the steaks from the frozen only loins (P < 0.05; Fig. 4) confirming the findings from previous studies on beef and lamb in that ageing-then-freezing can enhance meat colour and colour stability of frozen/thawed meat. The exact mechanism by which ageing-then-freezing improves colour stability has not been fully studied, but the aged/frozen meat might maintain higher endogenous myoglobin redox stability, while having relatively lower oxygen consuming mitochondrial enzyme activities through the ageing process compared to the aged only and/or frozen only meat.