Milk biomarkers in relation to inherent and external factors based on metabolomics

Milk is one of the most important food sources for human nutrition. The chemical composition of milk varies depending on many factors and may influence its nutritional value and safety. Metabolomics has recently become one of the major approaches applied to understand the chemical profiles and dynamic changes in milk products. The increase in the application of metabolomics in milk studies is, in large part, due to the development of high-throughput and high-sensitivity instruments and efficient chemometrics. This review focuses on the changing metabolites in milk, which are relevant to both the inherent factors, such as cow breed, dairy animal species and lactation stage, and external factors, including diet, season, geographic origin, disease, processing and storage stability. Additionally, the applications of candidate biomarkers for milk authentication are reviewed. The candidate biomarkers identified in previous studies are listed in this review. Also, some metabolites, which could be used as robust biomarkers, such as choline, citrate, valine, hippuric acid, 2-butanone, lactate and some fatty acids, for milk quality, traceability and safety studies are summarized. Challenges for this scientific field are also addressed.