229. Award Talk: Beef Loin Colour and Ultimate pH: Mechanisms, and Influence of Climate-change Induced Bushfires.

Abstract Beef loins exhibiting dark color and high ultimate pH are associated with the unfavorable quality condition “dark-cutting”. Colorimeters produce CIE-L*a*b* values which are described as chromatic (a* and b*) and achromatic (lightness; L*); myoglobin, and light scattering within the muscle structure, are respectively main contributors. Observed under a reflection microscope, high pH muscle cells have a dark, swollen appearance, and when the surrounding pH is reduced, the cells can be seen undergoing transverse shrinkage, and specks of white (scattered) light appear. This can explain the 5-15% reduction in dark coloured beef loins subjected to delayed grading of beef carcases. Pre-slaughter stress in cattle, including heat stress, depletes glycogen reserves and is associated with dark-cutting. During bushfires, which are increasingly prevalent under climate change, animals can be exposed to extreme heat stress events, with detrimental consequences for animal health and productivity, and potentially product quality. MSA grading data from six beef processing plants in eastern Australia was collected, resulting in data on > 450,000 beef carcases slaughtered over the bushfire period in 2018-2020. Modelling of the incidence of dark color (>3) was conducted using grading data (ossification, fat depth, carcase weight, marbling score), fire and pasture variables. For analysis of the incidence of ‘dark color’ defects, the predictors were fitted for the binary outcome ‘compliant’ (0; meat color ≤ 3) or ‘dark’ meat color (1; meat color > 3) using a generalized linear mixed model (GLMM) with logit link. The final model for meat color at time of grading is presented in Table 1. Note that a similar model was developed for proportion of high ultimate pH loins and results were similar, with some variations, to meat colour. There was an interaction between ‘distance from closest bush-fire’ and ‘feed type’ (P < 0.001); grass-fed animals were darker than grain-fed and this difference increased with increased distance from closest bushfire. There was also an interaction between ‘log days of bushfire’ and ‘HGP’ (P < 0.001); no HGP treatment cattle had lighter color than HGP-treated, with HGP-treated having increasing color scores as ‘days of bushfire’ increased. Fig. 1 provides further detail on the effects of these two interactions. Overall, HGP treated animals were darker than animals with no HGP treatment under the same feed type and fire conditions. For grass-fed HGP treated animals, predicted color scores were consistently highest (∼ 2.7) across all the fire exposure variables modelled in Fig. 1, and higher than all other animals. Development of nutritional and genetic strategies for ameliorating heat and other stresses, particularly following a bushfire are recommended. Also, consideration of the role of light scattering in meat color can assist in understanding future production of color-acceptable beef, particularly for grass-fed cattle. For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text.