Milk is an emulsion of oil in water, meaning that there are oil droplets in a water solute (continuous water phase). Milk consists of 87% water. The oil is made up of 3.5% dissolved protein, 3.6% lipids (fats), 4.9% carbohydrates and 0.7% vitamins and minerals. Complex phospholipids and protein molecules surround the droplets of fat, this stabilises them so that the oil does not separate easily from the water. Although, if left to stand, cream will form on the surface of the milk and so the droplets are not completely stabilised. Lactose is the main carbohydrate in milk, this is a disaccharide, meaning it consists of two monosaccharide molecules. A monosaccharide is a carbohydrate that cannot be split by dilute acids. Lactose is sometimes referred to as ‘milk sugar’, it comprises of one glucose molecule linked to a galactose molecule. It is manufactured in the mammary gland of the cow and only occurs in milk.
Milk is a good source of calcium, which is important for growth and development. Calcium is a component of teeth and bones and is present in the blood as it is required for muscle contraction and other metabolic processes.
If milk is left to stand, fat molecules rise to the top and form a layer of cream. The thickness of the cream varies depending on weather, pasture that cow is grazing and lactation cycles. The layer of cream becomes rancid before the milk underneath and so the thicker the layer of milk the faster the milk will go bad. To reduce the cream layer the milk undergoes homogenisation.
Homogenisation is a process in which the fat is distributed throughout the milk by forcing it through tiny nozzles at high pressure. The large fat molecules of the cream are broken down from sizes of about two or three micrometres to roughly a tenth of that diameter. These smaller molecules are less buoyant and as they have a high surface area relative to their volume, there is a greater resistance to motion through the milk compared to the original fat molecules. The fat molecules also get coated with more proteins become more resistant to being squeezed together. This means that the smaller molecule are less likely to rise to the surface or clump together and so a layer of cream is not produced.
A fine dispersion of calcium caseinate suspended in the milk causes the colour to be white.