The Chemical Composition of Milk

In the various texts available regarding milk steaming, there is often focus on the physical modifications that this practice entails and little is said about the chemical composition of milk and the modifications that the various components undergo, this often gives rise to the creation of urban legends that have little to do with reality.

Before discovering what chemical modifications occur in milk during steaming, it is necessary to know the main substances present within it.

Milk is a white liquid that is secreted by the mammary glands of mammals. From a chemical point of view it is an emulsion, that is a stable mixture of substances that normally do not mix with each other, but that are held together thanks to the presence of a substance called surfactant.

In milk the surfactants are the proteins that stabilize the carbohydrates and fats in water. The white color of the liquid is due to the refractive index of the fats dispersed in water.

The composition of milk can vary depending on the mammal from which it is produced and from treatments subsequent to milking, but water in all cases is always the main component. The milk used in coffee shops is cow’s milk generally consisting of: 87.7% water, 4.9% carbohydrates, 3.4% lipids (for whole milk), 3.3% proteins and 0.7% minerals, including calcium (1).

The Composition of Milk: Lipids

Before starting we must understand some terms in “chemistry speak”: it might seem complicated, but it is essential to have a minimal foundation to then understand the main changes that occur in milk during steaming.

Here is a legend that will help us better understand the concepts presented, to read and review when encountering the name of an unknown substance in the text.

Fatty acids: The molecules of fatty acids are structural components of fats, which in turn belong to the category of lipids. They are formed by long chains of carbon and hydrogen with a carboxylic group (symbol -COOH) that gives the characteristic of “acid”.

Saturated and unsaturated fatty acids: Fatty acids can be “saturated” when the carbon atoms of the chain are linked together only by single bonds and “unsaturated” when instead in the chain there is a double bond (monounsaturated fatty acids) or multiple double bonds (polyunsaturated fatty acids) connecting the carbon atoms together.

Glycerol: Known as we will see later for being a component of glycerides, it is an alcohol that appears viscous, odorless and sweetish. It should be remembered that in its structure there are three -OH groups (hydroxyl group).

Glycerides: In glycerides, the -OH group of glycerol has reacted with a fatty acid, to form precisely a glyceride. As we have said, glycerol possesses three hydroxyl groups, if only one of them reacts with a fatty acid we speak of monoglycerides, if two groups each react with a fatty acid we speak of diglycerides, if all three groups react with a different fatty acid we speak of triglycerides.

Phospholipids: Phospholipids are complex lipids in which one or more fatty acid molecules are linked to a phosphoric group and a nitrogenous base.

Sterols: these are polycyclic chemical compounds, that is formed by multiple carbon rings attached to each other and present an alcoholic function on the first ring and a branching on the last. They are the precursors of steroids and belong to the lipid family.

Lipids, also called fats, are the main energy source contained in milk and generally are found in the form of globules. The most abundant in milk are triglycerides (figure 1), that is molecules composed of fatty acids (saturated and unsaturated) and glycerol.

In milk over 400 fatty acids have been identified, but only 20 exceed 90% in the total composition. Abundant are the saturated fatty acids with 4 to 8 carbon atoms, monounsaturated and polyunsaturated with 18 carbon atoms. Others have been found in much lower concentrations but contribute significantly to the aroma of milk.

Approximately milk contains 65% saturated fats, 30% monounsaturated and 5% polyunsaturated (2).

It is known that saturated fatty acids are often associated with high cholesterol and cardiovascular problems; however those consisting of between 4 and 8 carbon atoms are metabolized differently from others with a higher carbon content and do not constitute in any way a danger to our health.

Instead oleic acid (figure 2) and linoleic, both unsaturated and present in milk, have a beneficial effect on the organism. The triglycerides of milk are in the form of globules, surrounded by proteins and by phospholipid membranes that stabilize them in the aqueous phase of milk (3).

In addition to triglycerides, the lipid component in milk is also constituted by glycerides, phospholipids, in particular by phosphatidylcholine (figure 3) and phosphatidylethanolamine, and sterols. These components, although present in lower concentration compared to triglycerides, play a fundamental role in milk as they contribute to the stabilization of the emulsion (4).

The Chemical Composition of Milk: Carbohydrates

Also in this case, before starting here is a small legend:

Monosaccharides, disaccharides and polysaccharides: They are all sugars, the simplest are monosaccharides. When two monosaccharides are joined together we can have disaccharides, when instead from two to ten monosaccharides are joined together we speak of oligosaccharides, when more than ten monosaccharides are joined together to form a substance we speak of polysaccharides.

Lactose: Milk sugar is a disaccharide formed in turn by two simpler sugars (monosaccharides) linked together, glucose and galactose.

In cow’s milk there is a carbohydrate component that can vary between 3 and 5% approximately, consisting predominantly of lactose and traces of mono and oligosaccharides. Lactose (figure 4) is a disaccharide, that is a carbohydrate consisting of 2 simple sugars (glucose and galactose).

Lactose dissolved in milk has been found in two forms called α and β that possess the same nutritional properties. The two forms can convert into each other and this equilibrium is strongly influenced by temperature. At room temperature (about 20°C) there is a predominance of the β form (63%), while at temperatures above 90°C the α form prevails (5).

The Composition of Milk: Proteins

Finally let’s talk about the fundamental proteins, a new legend can help us better understand the topic to discover.

Amino acids: Amino acids are the fundamental structural unit of proteins. For simplicity we can imagine them as small building blocks joined together by a glue (peptide bond) to form a long sequence called protein.

Proteins: They are macromolecules formed by long chains of amino acids, linked together by the peptide bond. The total protein content in milk is around 3.3%. Milk proteins contain all 9 essential amino acids, important for human nutrition.

60% of the amino acids that are used for protein formation come from the animal’s diet and their composition can vary not only from diet but also from the genetics of the same being.

Milk proteins can be divided into two large categories: caseins and whey proteins.

Caseins are proteins that contain phosphorus and precipitate from milk when it reaches a pH value around 4.6.

Conversely, whey proteins do not contain phosphorus and remain in solution even at pH 4.6. In cow’s milk it is estimated that 82% of proteins are caseins while the remaining part are whey proteins (6).

Caseins can be of different types (α-s1, α-s2, β and k) and each of them has its own amino acid composition and different properties.

Caseins are found in milk in the form of spherical structures called micelles and thanks to their high phosphorus concentration, they manage to capture within them minerals such as calcium and magnesium.

Their presence allows milk to have a higher concentration of calcium compared to what would happen if calcium were simply dissolved in the aqueous phase of milk in their absence. This makes milk an excellent source of calcium for its consumers.

k caseins are glycoproteins, that is a protein with also a carbohydrate portion inside, located near the surface of the micelles (7).

Whey proteins are separable from caseins when milk coagulates, they are of very high nutritional and biological value but are very sensitive to temperature.

It is a family consisting of 50% β-lactoglobulin, 20% α-lactalbumin, and the remaining part of immunoglobulins, lactoferrin, transferrin and in smaller quantities enzymes.

As for caseins, these proteins also vary according to their amino acid composition, but do not contain phosphorus. On the contrary they contain amino acids rich in sulfur that allows the different protein chains to join with disulfide bonds (that is bonds between two sulfur atoms), thus forming compact spherical forms.

These bonds can be broken thus leading to protein denaturation. The role of whey proteins within milk has not yet been fully clarified.

β-lactoglobulin is thought to perform the function of transporting vitamin A, while α-lactalbumin plays a role in lactose synthesis. Lactoferrin and transferrin instead play an important role in iron absorption (8).

Vitamins and Minerals

Regarding vitamins instead, milk contains a whole series of vitamins (all those of group B, C, A and D) but their low content means that milk cannot be a source of them.

Different instead is the discussion on minerals. Milk in fact is considered a good source of calcium and magnesium, minerals that as mentioned are found in the aqueous phase thanks to bonds with caseins (9).

BIBLIOGRAPHY:

1- T. O. Oftedal, S. J. Iverson, Comparative Analysis of Nonhumans Milk A. Phylogenetic Variation in the Gross Composition of Milks, Handbook of Milk Composition, Volume A, Food Science and Technology, 1995, 749-789; T. Huppertz, V. K. Kelly, A. Y. Tamime, Constituents and Properties of Milk from different species, Blackwell Publishing, 2006, 1-42.
2- R. G. Jensen, The Composition of Bovine Milk Lipids, J. Dairy Sci., 2002, 85, 295-350; G. Hillbrick, M. A. Augustin, Milk Fat Characteristics and Functionality: Opportunities for Improvement, Aus. J. Dairy Technol., 2002, 57, 45-51.
3- C. Leray, Lipids and Health, Nutrition-Health, 2020, 27.
4- J. Bitman, D. I. Wood, Changes in Milk Fat Phospholipids during Lactation, J. Dairy Sci., 1990, 73, 1208-1216; M. Smoczynski, Role of Phospholipid Flux during Milk Secretion in the Mammary Gland, J. Mammary Gland Biol Neoplasia, 2017, 22, 117-129.
5- G. Haase, T. A. Nickerson, Kinetic Reactions of Alpha and Beta Lactose, J. Dairy Sci., 1966, 49, 127-132.
6- H. H. D. Meyer, K. Gellrich, S. Wiedemann, Composition of Major Proteins in Cow Milk differing in mean Protein Concentration, Czech J. Anim. Sci., 2014, 3, 97-106; G. Bobe, G. L. Lindberg, A. E. Freeman, D. C. Beitz, Composition of Milk Protein and Milk Fatty Acids is Stable for Cows Differing in Genetic Merit for Milk Production, J. Dairy Sci., 2007, 90, 3955-3960; J. J. Murphy, F. O’Mara, Nutritional manipulation of milk protein concentration and its impact on the dairy industry., Livest Prod Sci, 1991, 35, 117–134; J. B. Coulon, B. Remond, Variations in milk output and milk protein content in response the level of energy supply to the dairy cows: a review, Livest Prod Sci, 1991, 29, 31–47.
7- M. Corredig, P. K. Nair, Y. Li, H. Eshpari, Z. Zhao, Understanding the Behavior of Casein in Milk Concentrates, J. Dairy Sci., 2019, 102, 4772-4782.
8- L. Sawyer, β-Lactoglobulin, Advanced Dairy Chemistry I (3rd edn), ed. by Fox PF and P. McSweeney P. Kluwer, Amsterdam, 2003, 319–386; I. Grunnet, J. Knudsen, Medium-chain fatty acid synthesis by goat mammary-gland fatty acid synthetase, Biochem J, 1983, 209, 215–222; L. M. Sordillo, K. L. Streicher, Mammary gland immunity and mastitis susceptibility. J Mammary Gland Biol, 2002, 7, 135-146.
9- S. Zamberlin, N. Antunac, J. Havranek, D. Samarzija, Mineral elements in milk and dairy products, Mljekarstvo, 2012, 62, 111-125.

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