1.1. An Overview on Milk Composition
Milk is a natural beverage which meets the nutritional needs of infants, since it is one of the most complete and highly nutritious foods. Nutritionally, milk is defined as “the most nearly perfect food” due to its chemical composition. The principal constituents are water, lipids, carbohydrates, and proteins (caseins and whey proteins). Furthermore, other minor constituents are present in milk, such as vitamins, minerals, hormones, enzymes, and miscellaneous compounds. As well, there are numerous immune components like growth factors, cytokines, nucleotides, antimicrobial compounds, and specific immune cells. Thanks to these features, milk provides important nutritive elements, immunological protection, and biologically active substances. The principal milk constituents vary widely among species: lipids, less than 1–55%; proteins, 1–20%; lactose, 0–10%. Concentrations of the minor constituents are also variable. Within any species, milk composition changes among individual animals, between breeds, according to the stage of lactation, feed and health of the animal, the environment, and the climatic situation, among many other factors [
1].
The composition of milk reflects mainly the nutritional and physiological requirements of the newborn, and even the profile of constituents therein changes markedly during lactation. The main changes are evident during the first days post-partum, especially regarding the immunoglobulin fraction. The concentration of milk components remains relatively constant during mid-lactation and changes noticeably in late lactation, due to the involution of the mammary gland tissue and the greater influx of blood constituents [
2].
Among the milk constituents, fats vary widely between different species both in their concentration and chemical composition, from less than 1% in donkey milk, to more than 50% in aquatic mammals [
3]. Its composition depends on the energy requirements and on the nutritional, genetic, and lactation characteristics of the species. Milk lipids are also important as a source of essential fatty acids, such as linoleic and linolenic acid, and fat-soluble vitamins A, D, E, and K.
Considering this milk fraction, although 97–98% of lipids are triacylglycerols, small amounts of di- and monoacylglycerols, free cholesterol and cholesterol esters, free fatty acids, and phospholipids are also present. Phospholipids, which represent less than 1% of total lipid, are present mainly in the milk fat globule membrane and other membranous material in the milk [
4].
Regarding carbohydrate content in milk, lactose concentration varies from 0.7% to about 7.0% among the different mammal species [
4].
Lactose is synthesized in the epithelial mammary cells from two molecules of glucose: one molecule of glucose is converted into UDP-galactose, which is condensed with a second molecule of glucose by the complex of lactose synthetase. The latter is a dimer composed by the UDP-galactosyl transferase and the whey protein α-lactalbumin (α-La), which is a regulatory subunit able to make UDP-galactosyl transferase specific for glucose. Therefore, there is a positive correlation between the concentrations of lactose and α-La in milk, (the milk of the Californian sea lion, which contains no lactose, also lacks α-La). α-La probably has a regulatory role in the lactose synthesis, as well as in the control of the osmotic pressure of milk [
5].
Milk protein content ranges from 1% to 24% by milk weight depending on the mammalian species. There are two major categories of milk proteins that are defined by their chemical composition and physical properties: caseins and whey proteins. The caseins, which accounts for about the 80% of milk proteins in ruminants, are responsible for the transport of calcium and phosphate, and for the formation of a clot in the stomach for efficient digestion.
They are heat stable phosphoproteins synthesized in the mammary glands and found in the milk of all mammals. Caseins are divided in α-caseins, which in turn comprise αs
1-, αs
2-, β-, and κ-caseins. They differ in primary structure and type and degree of post-translational modifications. These proteins are able to bind and transport relatively large amount of calcium, thanks to the presence of phosphates: this makes caseins very important for the development of teeth and bones in newborns [
6].
The casein proteins are nutritionally important also because of their high content in lysine, an essential amino acid in humans: αs1- and αs2-casein have 14 and 24 lysine residues, respectively.
The whey proteins are those proteins which do not precipitate from solution when the pH of milk is adjusted to 4.6. These proteins are referred to also as serum proteins or non-casein nitrogen, and are a heterogeneous group of heat labile globular proteins constituted mainly by α-lactalbumin (α-La), β-lactoglobulin (β-LG), serum albumin and immunoglobulins, and to a lesser extent by lactoferrin and lysozyme. In the whey protein fraction, there are also enzymes, hormones, nutrient transporters, growth factors, disease resistance factors, and others. However, contrary to caseins, they do not contain phosphorus.
The ratio of casein/whey proteins is different among species—in human milk, the ratio is about 40/60, in mare’s milk it is around 50/50, while in cow, goat, sheep, and buffalo’s milk it is about 80/20. These differences reflect the nutritional and physiological requirements of the newborn of these species [
7].
The inorganic elements are defined as macroelements or microelements, depending on whether they are present in the animal organism in smaller or greater quantities. The most important mineral salts in milk are calcium, sodium, potassium, magnesium, phosphorus (inorganic), chlorides, and citrates. Mineral elements are indispensable for the life of animals, as they participate in numerous physiological functions.
In milk, the inorganic elements are in solution or complexed into casein micelles.
In
Table 1 and
Table 2 are reported respectively, the quantities of the main macroelements and microelements in the milk of various species.
Vitamins are important bioregulators and are classified as fat-soluble and water-soluble. They are essential compounds for nutrition as they are involved in several functions: some are hormone precursors (i.e., vitamin D), others act as antioxidants by sequestering free radicals (i.e., vitamins C and E), and others constitute the prosthetic group of enzymes (i.e., vitamin B complex). Vitamin nutritional requirement varies from a few mg, to a few μg per day, depending on the considered vitamin. In milk, vitamin content depends on the maternal vitamin status and, especially for the water-soluble vitamins, from the maternal diet. In
Table 3, the content of some fat-soluble and water-soluble vitamins in the milk of various species is summarized.
1.2. Cow’s Milk Protein Allergy
It is well-known that the prevalence of adverse reactions to food is higher in children (6–8% in the early years of life) than in adults (2.4%) [
11]. Prospective studies, performed in various countries, showed that about 2.5% of children in the first year of life have allergic reactions to cow’s milk, which is therefore considered the main cause of allergies in children [
12].
Cow’s Milk Protein Allergy (CMPA) is clinically an abnormal immunological reaction to cow milk proteins. It may be due to the interaction between one or more milk proteins and one or more immune mechanisms, and resulting in IgE-mediated reactions. If the reactions do not involve the immune system, they are defined as cow milk protein intolerance. IgE-mediated forms are often associated with a higher risk of multiple food allergies and atopic conditions, such as asthma in later periods of life. The causes of allergy to cow’s milk are manifold: first, the protein content of cow’s milk is higher than that of human milk (3.6% versus 0.9–1%); second, cow’s milk proteins are represented by casein for the 80% and by the whey proteins for the 18% (in human milk, 40% caseins and 60% whey proteins); third, β-lactoglobulin is absent in human milk, and for this reason, this protein is considered as a potential allergen (allergen Bos d 5).
Cow’s milk caseins (αs
1-, αs
2-, β- and κ-casein) constitute the allergen Bos d 8, however, there is a greater sensitiveness to the α-caseins (100%) and the κ-casein (91.7%). Among whey proteins, β-LG is the most abundant whey protein in cow’s milk and it affects allergic patients in a percentage from 13% to 76%. Also, bovine immunoglobulins (allergen Bos d 7), and in particular γ-globulin, are responsible for CMPA clinical symptoms [
13]. However, the allergic reaction to cow’s milk proteins occurs when these proteins are poorly digested by the infant and hence their structure remains substantially unchanged, since the polypeptide chain is not fractionated into amino acids because of insufficient protease enzymatic attack, or a lack of a specific intestine enzyme. Therefore, the undigested protein behaves as an antigen. This is not enough to determine allergic reactions, since the amount of absorbed antigen is limited by anatomical barriers (mucus, epithelium) and/or eliminated by immunological barriers, by a combination on the epithelial surface of antigen with secretory immune globulin (IgA). However, if the anatomic barriers are weakened, because of inflammatory or IgA deficiency, the penetration of large amounts of antigens occurs and the awareness and allergy in atopic subjects could increase. CMPA occurs with gastrointestinal disorders for 50–60% of cases, skin diseases for 50–70%, respiratory affections for 20–30% respirators, and anaphylaxis for 5–9% of cases [
14].
Although milk allergy is considered transient, there are still some children who do not outgrow their allergy by the age of 10, but also by their adulthood [
15]. These latter patients have more severe reactions and a different immunological pattern than those with transient allergy [
16].
Once CMPA has been established, the therapy, as in all food allergies, should consist of the absolute exclusion of milk or allergenic proteins of the cow’s milk (formulated milk and derivatives), whereas in the case of breastfed children, on the complete elimination of milk protein from the mother’s diet [
17].
When breast milk is insufficient or unavailable, it is indispensable to choose an alternative milk formula. In the case of infants affected by CMPA, the elimination of allergen administration usually leads to the disappearance of the symptoms. However, some patients may also show the allergic reaction if a substitute for cow’s milk is used, thus indicating multiple food allergies [
18].
Multiple food allergies are difficult to treat—elimination from the diet of some nutrients, if not properly balanced by the assumption of alternative nutrients, can lead to malnutrition and therefore can stop growing [
18].