Milk protein comprises caseins (CNs) and whey proteins, each of which has different genetic variants. Several studies have reported the frequencies of these genetic variants and the effects of variants on milk physicochemical properties and functionality. For example, the C variant and the BC haplotype of αS1-casein (αS1-CN), β-casein (β-CN) B and A1 variants, and κ-casein (κ-CN) B variant, are favourable for rennet coagulation, as well as the B variant of β-lactoglobulin (β-lg). κ-CN is reported to be the only protein influencing acid gel formation, with the AA variant contributing to a firmer acid curd. For heat stability, κ-CN B variant improves the heat resistance of milk at natural pH, and the order of heat stability between phenotypes is BB > AB > AA. The A2 variant of β-CN is more efficient in emulsion formation, but the emulsion stability is lower than the A1 and B variants. Foaming properties of milk with β-lg variant B are better than A, but the differences between β-CN A1 and A2 variants are controversial. Genetic variants of milk proteins also influence milk yield, composition, quality and processability; thus, study of such relationships offers guidance for the selection of targeted genetic variants.
Human milk is recommended as the optimal source of nutrition for babies up to six months of age, as it provides individualized, specific and abundant nutritional compounds, as well as immunological protection. Currently, there is wide interest in studying human milk because of this key role, as well as identification of biologically active compounds. In addition, human milk banks play an important role in supplying donor human milk to infants, especially premature infants, in hospitals, which involves processing and storage of such milk. This review summarises recent studies on macronutrients in human milk, including their structure, levels, influencing factors, and contributions to the health benefits of infants. Physico-chemical properties of human milk are also reviewed, including pH, colour, stability of casein micelles, size and zeta-potential of fat globules, heat stability, freeze-thaw effect, dehydration and phase separation, and the impact of preservation processes on these. The levels of human milk macronutrients and factors that influence these, such as lactation and gestational age, are ongoing research topics. Studies on physicochemical properties of some human milk components, such as caseins, are comparatively few, as research has mainly focused on bioactive peptides, milk fat globule membrane and human milk oligosaccharides. Processing and preservation are applied to human milk and the effects of these methods on the retention of human milk nutrients for use in human milk banks and hospitals must be considered. Some physico-chemical properties, such as milk pH, colour, and physical stability, may be possible indicators of milk quality.
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