## 3 An Outline of Metabolomics
### Metabolomic Analyses in Meat Quality Control  
In meat quality studies, metabolomics could enable drawing maps of the metabolic network in postmortem muscle aging and flavor development during cooking. Despite the impact on meat quality, the metabolic changes have not been easily predicted because  
Figure 3: Classification of metabolomics and analytical approaches [9; 42; 43; 44; 45; 46]. coordinated metabolism in live muscle is no longer maintained due to a lack of energy supply and arrest of de novo gene expression after animal death. Postmortem metabolic maps could result in the identification of the metabolic factors responsible for meat quality traits and thereby contribute to the exploration of biomarkers in quality monitoring, processing and authentication of meat products [51]. In the last decade, extensive research has been conducted to determine and evaluate the volatile organic compounds in meat products such as aldehydes, ketones, hydrocarbons, alcohols, as well as sulfur-containing compounds [51], and identified the most important compounds in sausages by the SPME-GC-MS technique [52; 53; 54]. Flavor substance formation is directly related to metabolic mechanisms and LC-MS/MS is widely used in food flavor metabolomic research. According to a recent study [41], metabolomics was employed to characterize key aroma substances and their water-soluble markers affecting chicken meat. Metabolomics analyses showed the presence of L-glutamine, a key metabolite of nonanal, hexanal, heptanal, octanal and 1-octen-3-ol affecting and improving chicken flavor through the Maillard reaction. Moreover, physiological and metabolic differences in meat quality between two local breeds of Tibetan sheep were investigated by analyzing differential metabolites through untargeted metabolomic analysis. White Tibetan sheep were clearly distinguished from Black Tibetan sheep as 49 differential metabolites were successfully identified, including carbohydrates, amino acids and derivatives, fatty acids and derivatives and other organic compounds [9]. Through metabolomic profiling, more information regarding quality of beef muscles can be reported using the exudate as an analytical sample and indicator due to the presence of sarcoplasmic proteins, amino acids, sugars, lipids and enzymes [55]. According to [PERSON] et al. [18], metabolomic analysis of meat exudate used aging periods for clustering of the metabolites by muscle type (beef loin muscles and tenderloin muscles) and PCA of exudate metabolites was employed. Another metabolomic study, according to [PERSON] et al. [56], revealed significant differences between the meat extract from Liancheng white duck breast meat and Cherry Valley duck meat. The differentiated metabolites were categorized into 28 classes. Of these, there were four main ones: carbohydrates, amino acids, fatty acids and eikosanoids. The results revealed that breed has a great impact on meat quality and metabolomic profiling of duck meat and quality metabolites (correlated with meat) may function as markers for the above breeds. Similarly, metabolomic profile was employed for the determination of the metabolites of volatile compounds in slow-growing Liancheng white duck and fast-growing Cherry Valley duck meat. Targeted metabolomics showed a lower carnitine content in Liancheng white duck meat. This might promote lipid deposition for production of additional octanal and nonanal. Conversely, the higher carnitine content in Cherry Valley duck meat may be attributed to fewer lipid oxidation products. In addition, the results showed that the sweet and meaty aroma in slow-growing Liancheng ducks was derived from higher sugar and amino acid contents [57]. In a recent study by [PERSON] et al. [43], the effect of age (70, 120 and 300 days old) on the nutritive profile of goose meat was investigated. In detail, using widely targeted metabolomics analysis, a total of 776 metabolites were detected in goose meat.