Effect Of Slaughter Age On The Quality Characteristics Of Yak Beef And Its Mechanisms

China is rich in yak resources,with about 16 million yaks,accounting for more than 95% of the world.It is generally believed that yak beef has high nutritional values and could be served as important supplements to China’s beef market and has great development potential.However,it was generally believed that the processing quality of yak beef was relatively poor and ranged considerably.Among many factors affecting beef quality characteristics,slaughter age and marbling were generally considered the most important criterion for beef quality classification,and the richness of marbling was significantly affected by age.As such,slaughter age has more influence on yak beef quality.According to the limited standardization of yak breeding and slaughtering in China,one of the most undesirable consequences was that the slaughtering age was generally older(average 6 years old)and widely ranged,over 80% of which were distributed from 3 to 7 years old.Therefore,it is necessary to systematically carry out research on the influence mechanism of slaughtering age on yak meat quality characteristics,so as to determine the appropriate slaughtering age under the existing slaughtering and processing conditions,explore the influence mechanism of slaughtering age on quality,and provide scientific basis for slaughtering process optimization and yak meat deep processing.In order to ensure the consistency of samples as much as possible,the experiment took Datong yak(Bos grunniens)with genetic stability,which was uniformly bred and bred in Qinghai yak breeding and extension service center,as the research object.According to the experimental design,the effects of slaughtering age(2,4,6 and 12 years old)on the oxidation characteristics,tenderness,water retention of longissimus doracis muscle and the gel and emulsifying properties of myofibrillar protein were studied during acid excretion(0-4 ℃)at 72 hours after slaughter.The main research contents and conclusions are as follows:1.Effects of slaughter age on protein and fat oxidation of yak beefThe differences in protein and fat oxidation modification degree between slaughter ages were studied,and the content of related endogenous oxidants,antioxidants,and antioxidant enzyme activity was analyzed accordingly.Based on these indices,the potential mechanism of age affecting the oxidation degree of protein and fat in yak beef was explored.In the range of age and postmortem aging time,the results showed that the slaughtering age would not directly lead to the difference in protein oxidative modification,but would directly cause the difference in fat oxidation.With the extension of postmortem time(the aging temperature at 4℃),the oxidation levels of protein,fat,and myoglobin in yak meat of all age groups increased significantly,while the content of endogenous antioxidants and antioxidant enzyme activity decreased.Compared with the older group(6 and 12 years old),yak beef of the younger(2 and 4 years old)can delay the oxidation of fat,myoglobin,and protein caused by postmortem aging,which may be due to the lower initial myoglobin(e.g.2-year-old showing 30.7% lower myoglobin than that of 12-year-old),H2O2(42.4% lower),IMF(65.6% lower),a higher proportion of polyunsaturated fatty acids(366% higher)and greater antioxidant capacity(or antioxidant enzyme activity)in the younger yak meat,such as glutathione reductase activity(50.6% higher)and so on.2.Effect of slaughter age on meat tenderness of yakThe changes of shear force with age at 1,24,and 72 h postmortem were studied.The age-related differences in yak carcass p H and temperature,sarcomere length,intermuscular fat(IMF)content,collagen content,myofibril fragmentation index(MFI),and muscle fiber structure were analyzed,and the potential mechanism of age’s effect on tenderness was explored.In the range of age and postmortem aging time,the results showed that the yak meat of all age groups was affected by the cold shrinkage effect caused by postmortem aging conditions.The elderly(6 and 12 years old)group was relatively conducive to postmortem aging probably because of their large carcass weight and high intermuscular fat content,which decreased the cold shrinkage effect,resulting in better protein hydrolysis degree and tenderer beef at 72 h after slaughter.As yak age grew,yak carcass weight increased;muscle fiber became thicker,IMF content increased while the muscle fiber density decreased;the content of collagen increased without signification.3.Effect of slaughter age on WHC of yak meatThe changes in drip loss and cooking loss with age were studied.The differences in sarcomere length,phospholipase A2(PLA2)content,water content,water distribution,DSC,protein structure,and muscle fiber structure in yak meat of different ages were analyzed,and the age-related WHC mechanism was explored.In the range of age and postmortem aging time,the results showed that compared with the younger group,the meat of the older group,compared with younger one,was less affected by the cold shrinkage effect caused by acid excretion conditions,the sarcomere shortened more slightly and the formation of drip channel was relatively late,leading to better WHC at 72 h postmortem,among which the 6-year-old showed greater WHC.With the increase of age,the moisture content and PLA2 content in yak meat decreased;the degree of protein hydrolysis increased and the hydrophobic interaction decreased.4.Effects of slaughter age on the characteristics of myofibrillar protein(MP)and MP heat-induced gel propertiesThe changes in gel centrifugal loss and textural properties with age were studied.The variation of oxidation degree,particle size distribution,surface hydrophobicity,solubility,dynamic rheological properties,the protein structure of MP,and gel microstructure of different ages were analyzed.The potential mechanism of age affecting MP gel characteristics was expounded.In the range of age and postmortem aging time,the results showed that the WHC and textural properties of MP gel in the younger age group were better than those in the elder one,among which MP from 4-year-old yak possessed better WHC and textural properties at 24 h postmortem.With yak age grew,the oxidation degree and surface hydrophobicity of MP increased;the hydrolysis degree of MP increased,the average particle size decreased and the solubility increased;The hydrophobic interaction of MP increased,the proportion of free water increased,and the centrifugal loss increased significantly;MP dynamic rheology storage modulus(G’),gel hardness and elasticity decreased;the compactness of the gel structure decreased with the increased gaps.5.The effect of slaughter age on the MP emulsifying characteristicsThe differences in MP emulsifying activity index(EAI)and stability index(ESI)between the ages were studied.The variation of physicochemical properties of MP and the particle size distribution,protein structure,and rheological properties of MP emulsion with age were analyzed,and the potential mechanism of EAI and ESI changes with age was explored.In the range of age and postmortem aging time,the emulsifying EAI,ESI,and apparent viscosity of MP were better when MP was extracted at 24 h postmortem,and there was no age difference.Within 72 h postmortem,the influence of age on the emulsifying properties of MP was relatively limited,which showed that the ESI of the younger group was lower at 1 h postmortem(e.g.6.8% lower than that of the 6-year-old group)while the EAI(8.39% higher)and apparent viscosity were higher at 72 h postmortem.In conclusion,slaughter age has a significant impact on the quality characteristics of yaks beef.Yak beef in the elderly group(6 and 12 years old)was more prone to protein oxidation caused by postmortem aging.Under the current aging conditions,the yak meat of the elderly group was less affected by the cold shrinkage effect,with sound postmortem aging effect,and displayed better tenderness and WHC,among which the 6-year-old group was the best.The key to improving the beef tenderness and WHC of younger yaks lies in the prevention and control of post-mortem cold shrinkage.At 24 h postmortem,gel properties of MP from the younger age group were better,among which the 4-year-old group was the best.Meanwhile,the MP gained fair emulsifying characteristics and there was no age difference at 24 h postmortem.