*4.4. Protein Oxidation*

Protein oxidation during retail display has been shown to lead to changes in protein aggregation and degradation, with implication for meat tenderisation. Carbonyl content substantially increased after 10 days of retail display in both TrigasMAP and HioxMAP (Table 3). Interestingly, TrigasMAP reduced the extent of formation of carbonyl groups compared to HioxMAP. This is similar to that observed in lipid oxidation, and agrees with previous studies [19] which reported increases in carbonyl content with increases in oxygen concentration. This would suggest that reducing the oxygen concentration in the packaging system to 50% reduced the extent of post-mortem oxidative processes.

Morzel et al. [43], using an ·OH radical generating system from pig *Longissimus*, showed oxidation induced formation of disulfide bridge and protein polymerisation led to a reduction in proteolysis susceptibility of myofibril proteins. Free thiol groups (sulfhydryl) are susceptible to oxidation; therefore, the quantification is a useful measure to determine the extent of protein oxidation in muscle foods. The present study showed the free thiol content of both *Semimembranosus* and *Vastus lateralis* did not differ between breed and finishing feed treatments. However, significant differences were observed between VSP and HioxMAP and TrigasMAP treatments. Bao and Ertbjerg [34] reported no difference in free thiol content between 80% O2 and 60% O2 in HioxMAP packaged beef. The underlying mechanisms behind differences in free thiol content between Composite and Merino on SPD is not understood. However, differences in muscle fibre type, lipid content and composition, and antioxidant capacities between breeds are likely to be involved [44].

#### **5. Conclusions**

By examining the colour, water holding capacity, texture, and oxidative stability of sheep meat from different breeds, finishing feed, and retail packaging methods, this study demonstrated the complexity in how different sheep breeds and muscles respond to variations in finishing feeds and packaging methods. Packaging of sheep meat in low, moderate, or high oxygen environments affected the colour to a greater extent than breed and finishing feeds. However, supplementation of the finishing feed with either camelina forage or camelina meal significantly reduced the lipid oxidation of sheep meat. Understanding how and to which extent supply chain factors affect the quality of sheep meat enables sheep producers and processors to prioritise intervention strategies to ensure optimal quality.

**Author Contributions:** Conceptualization, E.N.P. and M.H.; methodology, E.N.P. and M.H.; validation, M.H., S.W. and C.K.; formal analysis, M.H.; investigation, S.W. and C.K.; resources, E.N.P. and R.D.W.; data curation, M.H.; writing—original draft preparation, M.H., C.K. and S.W.; writingreview and editing, M.H., R.D.W. and E.N.P.; supervision, M.H. and R.D.W.; project administration, E.N.P. and M.H.; funding acquisition, E.N.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** The funding for the animal feeding experiment was provided by the Victorian Department of Jobs, Precincts and Regions (Agriculture Victoria Research), grant number CMI 105393.

**Institutional Review Board Statement:** The study was conducted according to the Australian Code of Practice for the Care and Use of Animals for Scientific Purposes, and approved by the Agricultural Research and Extension Animal Ethics Committee of the Department of Jobs, Precincts and Regions (AEC Approval No: 2016-17).

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on reasonable request.

**Acknowledgments:** The authors gratefully acknowledge the technical contributions of staff from Hamilton and Bundoora during animal feeding, slaughter, and primal cuts collection. The camelina forage and camelina meal were purchased from Yarrock Pty Ltd., Kaniva, VIC 3419, Australia. Technical and operational support from the Faculty of Veterinary and Agricultural Sciences, The University of Melbourne is also acknowledged for the collection of muscle cuts, biochemical analyses, and meat quality measurements.

**Conflicts of Interest:** The authors declare no conflict of interest. The funder had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**


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