A proteomic insight into oxidative modifications of myofibrillar and sarcoplasmic proteins in venison after PEF treatment and ageing

For optimal and desired nutritional qualities, modern meat industries use various processing and ageing procedures. As a consequence of meat processing and ageing, it has been increasingly recognized that potential oxidation events may negatively impact on meat quality. Protein oxidation (PROX) is a chemical process that is well described in various pathophysiological diseases in humans, but has largely been ignored in the quality and preservation of food materials. Not until the early 2000s, the impact of oxidative effects in meat was increasingly recognized as a limiting and even harmful process that influences both meat quality, industries and the marginal inclination of consumers. A series of experiments have successfully investigated the extent of both protein and lipid oxidation in various meats that include, but are not limited to, beef, pork, lamb, chicken and fish. Although, venison is a frequently consumed type of meat, well known for its high export rates in New Zealand and also subject to processing and ageing, no published investigations have outlined oxidative events in venison in the context of processing and ageing. Hallmarks of meat protein oxidation include myofibrillar cross-links and aggregation products due to radical-induced oxidation of the protein backbone or individual amino acid residues. Reactive radicals can originate from external environments or intracellular oxidising systems. This research investigated to what extent external factors commonly applied in the preservation of meat influenced oxidative modifications in venison. The project examined oxidation due to a modern meat processing technology, named pulsed electric fields (PEF). PEF is a technique that introduces pores into membrane structures of myocytes. Due to proteolytic and non-thermal properties of PEF, the technique is associated with enhanced meat tenderness while minimising oxidation. The study focused predominantly on the degree and type of oxidation that occurred in venison as a consequence of a high PEF treatment and subsequent ageing regimes. Common ageing procedures include a 21 day dry and vacuum-packaged meat ageing. Initial PEF treatments and ageing were carried out in a complementary investigation by PhD student Tanyaradzwa (TA) Mungure, who previously examined physiochemical properties and LIPOX in venison, but no information was obtained about PROX. Samples provided by TA consisted of a temperature control, a no PEF and a high PEF control. Here, it was investigated how 21 days dry or vacuum-packaged (VP) ageing affects oxidation in venison following each of the treatment groups. The experiment initially extracted sarcoplasmic and myofibrillar proteins from pre-treated and aged sample groups. In the present study, actin and myosin were found to be the most abundant myofibrillar proteins in venison. Myoglobin, a sarcoplasmic protein, associated with meat colour was also examined based on oxidative modifications. Samples of protein extraction were visualised on SDS-PAGE and using a tryptic in-gel digest were isolated and analysed by LC-MS. The fundamental aim was to provide an insight into the oxidative modifications on actin, myosin and myoglobin in each venison sample group. The objective was to gather information to understand how PEF and ageing may link to oxidation mechanisms in venison. Peptides were analysed using a red deer sequence database of which only 43 % of proteins were annotated. Using quantitative and proteomic analyses, the number of oxidative modifications between venison sample groups were investigated and compared. Dry-ageing resulted in a 23 % increase of oxidation events compared to VP-aged meat. The time period of 21-d ageing revealed a 17 % increase in oxidations compared to 0-d. The effect of PEF resulted in lower oxidation quantities compared to no PEF stimulation. The results indicated that oxidation was not directly affected by the low level of thermal input or by PEF itself. Myosin was the most susceptible protein to oxidation, whereas oxidation on myoglobin remained low in both 0-d and 21-d samples. Analyses of amino acid oxidation revealed methionine oxidation as the most prevalent and little difference was found with oxidised residues between treatment and ageing groups. Overall, the research described in this thesis provided an insight into the oxidations occurring in venison. Oxidative mechanisms in venison had not previously been studied in the context of PEF and meat ageing, and were discussed in this study based on oxidative mechanisms investigated in other meat materials