Milk is made up of many different elements. Chemical analysis reveals a high diversity of ions and molecules which form an ordered mixture rather than a random association of substances. The slightest modification made to milk in terms of temperature or acidity affects this initial balance and opens the door to dairy transformations.
Journey to the centre of a drop of milk :
Water molecules are constantly in movement; salts, proteins and fats are dispersed in the water in milk in the form of particles of varying sizes from tiny metallic ions to fat globules. Caught up in the incessant quivering of water, diverse ions and molecules, soluble protein aggregates and casein micelles move about in a totally uncoordinated manner, each element moving in one direction and then abruptly changing direction.
Water accounts for 80% of the weight of the human body and is also the principal component in milk with 905g of water in every litre of milk. Via the blood, water carries oxygen and nutritive substances to all parts of the body. It also allows toxins to be eliminated by the kidneys and helps to regulate body temperature.
The water molecules roll and slide against each other at a speed of several hundred metres per second, colliding with and picking up all sorts of ions and molecules of unimaginable diversity and number, and endlessly sweeping them into their zigzagging journey.
In this way they constantly push the casein micelles back and forth. These casein micelles are 30 to 300 times larger than the lactose molecules but they never collide since they repel each other. The occasional fat globule, which is 100, 1000 or 10,000 times larger than a casein micelle, also appears in the midst of this turbulence.
These globules are much too large to be shoved around by the water molecules and so it is during their slow and unhindered rise to the surface of the milk that they meet and come together to form large clusters of fat.
These are chains of amino acids with different compositions and structures. In milk we can distinguish the caseins, which are suspended in the water molecules, and the soluble whey proteins.
- Caseins :
Caseins account for most of the protein in milk but they do not form a homogenous ensemble. There are several categories that differ in terms of their amino acid composition.
They tend to regroup into aggregates made up of hundreds or even thousands of molecules: it is the casein micelles that give milk its white colour; these macromolecules are extremely rich in mineral salts – especially in calcium – and each one carries a negative electric charges meaning that they mutually repel each other.
If the acid level in the milk increases the casein micelles are destabilised and they come together to form a casein network: this is what is happening when milk coagulates.
- Whey proteins :
These proteins are resistant to a slight increase in the acid levels of their environment and they also differ from caseins in terms of their composition and sensitivity to heat which alters their nature and structure: some classes of these proteins (immunoglobulins) are antibodies, i.e. the antibacterial substances present in milk.
Lactose is a milk carbohydrate, also called ‘milk sugar’, which is present in milk in a quantity of 49g per litre, i.e. the equivalent of 12 sugar cubes (nevertheless lactose is not the most sugary substance: it is 6 times less powerful than saccharose which is present in beetroots). In the intestines, lactose encourages the development of bacteria that inhibit pathogen microbes. It also contributes to the absorption of calcium.
The fat in milk is an extraordinarily complex mixture of different molecules; it is essentially made up of triglycerides, formed by the association of three fatty acids and glycerol alcohol. Milk contains more than one hundred and fifty different types of fatty acids.
The fat in milk is insoluble in water and therefore forms an emulsion which can be seen in the form of a multitude of spherical droplets. Each of these droplets is made up of a mass of fat surrounded by a membrane.
Since these droplets of fat are lighter that water they slowly rise to the surface of the milk and combine with other droplets on the way to form clusters. This natural but reversible separation forms milk cream, which is a combination of skimmed milk and pure fat.
Milk contains around 9g of minerals (1.3g of calcium, 1g of phosphorus, 1.6g of potassium, 1.1g of chloride, 0.5g of sodium, 0.14g of magnesium).
Milk is the best source of the calcium needed for bone and teeth formation since it contains ideal proportions of phosphorus and vitamin D required for the body’s assimilation of calcium.
Milk contains two categories of vitamins:
- Water-soluble vitamins, which are soluble in water and whey (vitamins B and C)
- Fat-soluble vitamins, which are soluble in fat and therefore in cream (vitamins A, D and E; therefore skimmed milk does not contain any fat-soluble vitamins). Milk contains almost all the vitamins needed for life except for vitamin C, which is contained in fruit and vegetables. Neither does it contain any iron. However this lack can be compensated by simply adding a small amount of cocoa powder to milk for example.
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