Meat quality has been an area of major development in the past century: a result of increasing standards and expectations of producers, scientists and consumers. Meat safety aside, toughness is considered the meat characteristic of highest importance when it comes to consumer eating satisfaction. However, toughness is a difficult trait to manage as it does not result from inheritance of a single gene, it cannot be reliably predicted from animal growth rate or level of nutrition nor can it be completely overcome by post-slaughter processing. Four areas of importance to meat structure, and hence toughness, have been investigated in this thesis.
Firstly, the role played by an enzyme system present in meat has been investigated. Matrix metalloproteinases (MMP's) are zinc-dependent enzymes involved in tissue remodeling via the regulation of the extracellular matrix components such as collagen and proteoglycans. This study aimed to extract and characterise MMP-2 due to its reported role in beef cattle tissue remodeling and fat cell infiltration into the connective tissue matrix. Extraction of active MMP-2 was achieved, however, the quantity of enzyme present did not allow unequivocal identification by micro-sequencing.
Secondly, variation in muscle fascicular structure between different breeds of beef cattle, particularly Bos indicus and Bos taurus breeds, has been quantified and a novel image analysis technique verified. Both connective tissue content and distribution in muscle samples from both breed types was quantified using image analysis of stained tissue sections and hence the Random Line Allocation Method (RLAM) was devised and verified in this study. Whilst fascicular structure was macroscopically similar in the two breed groups, direct measurement using RLAM demonstrated statistically significant differences in perimysial seam width and frequency. Bos indicus animals tended to have fewer, thicker perimysial seams in the longissimus dorsi muscle. It was tempting to hypothesize that these structural differences might be associated with the higher average toughness generally observed in Bos indicus meat.
Thirdly, the effects of nutritional restriction on the fascicular structure of meat were investigated using the RLAM method. Periods of restricted nutrition followed by periods of repletion, induce weight loss followed by compensatory growth: an experimental circumstance that mirrors the environmental variation experienced in Northern Australia. This study undertook to measure structural changes in muscle in terms of fascicular structure and myofibrillar bundle dimensions, in animals undergoing weight loss and compensatory growth.
In terms of myofibrillar bundle size, atrophy was exhibited as a reduced average bundle diameter. Compensating animals started to recover muscle mass m the short term, and this was exhibited by an increase in perimysial seam thickness, as well as an increase in the average diameter of the muscle fibre bundles. In the longer term, this meant these animals during recovery developed increased average myofibrillar bundle diameter while maintaining connective tissue seam dimensions. Animals on slow growth diets and growth diets displayed no compensatory growth. Nonetheless muscles continued to grow and this was indicated by a general increase in the perimysial seam thickness as well as the myofibrillar bundle diameter. The lack of significant differences between slow growth and fast growth dietary treatments may indicate above adequate nutrition does not enhance fascicular structure.
A specific experiment addressed concerns about the robustness of the fascicular structure measurements in cold shortened muscle. This experiment aimed to characterise the differences in muscle structures, namely connective tissue and myofibrillar bundle size, in cold shortened sheep meat. The results indicate that muscle, which has cold shortened, determined by significantly shorter sarcomere lengths and smaller myofibrillar bundles, had no structural difference in connective tissue seams. This suggested that the methodology was robust and that differences observed between Bos indicus and Bos taurus animals were of genetic origin, rather than a processing artifact.
In summary, this study has established a new technique for the quantitative description of muscle fascicular structure, and has added to knowledge of cold shortening effects. This procedure is valuable for quantification of muscle structural variation due to genotypic differences and nutritional challenges and some of these were observed in the current study. This new descriptor will have value in the mechanistic description of population variation in meat toughness.