Changes of myofibrillar protein oxidation and in vitro digestion in yak meat during air drying

Air-dried yak meat can be produced by a traditional procedure, where the raw yak meat is divided into strips, then naturally air-drying (about 40 days) under a low temperature condition. This ancient processing is still popular in China at present, particularly on the high altitudes areas (generally over 3 000 m), probably due to the influence of traditional eating habits and inheritance of culture. High-protein yak meat can be used to enhance the resistance of cold climates in these areas. However, the protein digestibility of air-dried yak meat has not been evaluated during traditional processing. Consequently, the objective of this study was to evaluate the changes in digestibility of myofibrillar protein during the processing of traditional air-dried yak meat, and further to investigate the underlying effect of protein oxidation on protein digestion. Taking the healthy 3 to 4-year-old male yaks as subject, the semimembranosus was selected to remove the fascia and fat on the surface, and then cut into a cross-section of 2×2 cm2 strip in Gannan prefecture of Gansu province. The meat was hung on the wire, and kept the distance of 1 to 2 cm between two adjacent pieces of meat in a dark ventilated naturally place in Xiahe County, Gannan Prefecture, Gansu Province. The temperature was set as ?10 to ?15 ℃, and the relative humidity was 50% to 75%. During air drying, changes were monitored in lipid oxidation, protein oxidation, protein particle distribution, and in vitro digestibility, in order to explore the effects of lipid and protein oxidation on protein digestibility. Samples were collected to analyze during natural air-drying for 40 days. A model was introduced to simulate the digestion of proteins in the stomach and duodenum, specifically to investigate the nutritional change of the air-dried yak meat in the processing. The results showed that the lipid Primary Oxidation Value (POV), Thiobarbituric Acid Reaction Substrate (TBARS) and protein carbonyl compounds increased dramatically with the extension of processing time (P<0.05), and there was a significant correlation between them (P<0.01). In addition, the content of disulfide bonds and dimeric tyrosine increased significantly (P<0.05), which is the main form of protein cross-linking. The digestibility of protein decreased by 19.58% during the whole process (P<0.05). The hydrolysis rate of protein by protease showed a significant correlation with the carbonyl group, disulfide bond, dimeric tyrosine, and protein particle size (P<0.01). Lipid oxidize products can promote protein oxidation, while the carbonylation and cross-linking of proteins can cause a decrease in protein hydrolysis rate by proteases. This study has a positive role to understand the traditional processing of air-dried yak meat, and further to improve processing technology and quality of yak meat.