# A Comprehensive Review of Milk Components: Recent Developments on Extraction and Analysis Methods
## 5 Micro-Constituents  
Milk micro-constituents include substances such as vitamins, minerals, biogenic amines, organic acids, nucleotides, polyphenols and animal metabolites. Their number corresponds to several hundred molecules [107].  
Vitamins are a heterogeneous group of organic substances essential for the growth and maintenance of the organism. Milk is a source of fat-soluble vitamins (A, D, E and K). Vitamin A is the precursor of \(\beta\)-carotene responsible for the yellow color of cow's milk. The water-soluble vitamins B1, B2, B6, B12, pantothenic acid, niacin, biotin, folic acid and vitamin C are also present [108].  
As regards minerals, they constitute only a small part of milk. Calcium, potassium and sodium are the most abundant cations, while phosphate, citrate and chloride, are the most abundant anions [109]. In milk, they are associated with proteins. In particular, calcium, magnesium, phosphate and citrate form colloidal particles with casein [110].  
Biogenic amines (BAs) can be found in milk alongside with some fermented foods such as cheese, sausage, fermented vegetable, wine and fish and they are produced by bacterial decarboxylation of the corresponding amino acids [111]. If present in traces, BAs are essential for life as they involve protein production and DNA replication [112]. However, when occurring in high concentrations, they are toxic and could cause diarrhea and tachycardia [113]. Also, nucleotides, i.e., the precursors of nucleic acids, are involved in improving growth performance [114]. The nucleotides present in milk are ribonucleotides and usually have concentrations in the order of \(\mu\)mol/L [115].  
Polyphenols and MetabolitesPolyphenols are a heterogeneous class of aromatic compounds that have two or more alcoholic groups in their structure. They are usually found in fruit and vegetables but, in many cases, they occur also in milk and dairy products. Polyphenols are exclusively synthesized by plants [116], serving in pollination function, or as a response to environmental stress [117]. Polyphenols in milk can originate from various sources, namely animals' diet  
Figure 4: Example of peptide mass spectra obtained by MALDI (+)-TOF MS analysis of \(\alpha\)-casein, \(\beta\)-casein, cow milk and goat milk using dihydroxybenzoic acid as matrix. Ions at \(m/z\) 830, 1195 and 1759 used as markers for the identification of cow milk are marked. Reproduced under the terms of the Creative Commons CC BY license [106].  
or fortification. If animals consume feed containing polyphenol-rich plants (e.g., grasses, clover, or other vegetation with high polyphenol content), traces of these compounds may be found in their milk. On the other side, some milk products are fortified with polyphenols during processing to enhance their health benefits. For example, dairy manufacturers may add plant-derived polyphenols (e.g., from tea, cocoa, or berries) to create functional or enriched dairy products [118, 119, 120, 121]. The exact type of polyphenols found in milk depend greatly on the animal's origin and any added ingredients. While raw cow's milk contains negligible polyphenols, fortified or flavored milk can serve as a vehicle for various polyphenic compounds, enhancing its nutritional profile.