Milk is a complex biological fluid produced by mammals to nourish their young. It functions as both an emulsion and a suspension of various compounds in water, with water making up roughly 85% to 90% of the total volume. The remaining portion is a delicate balance of lipids, proteins, carbohydrates, vitamins, and minerals, which contribute to its nutritional profile and physical characteristics.
Milk Lipids and Fatty Acid Structure
The lipid fraction, known as milkfat, is the most chemically variable component and is a source of energy and fat-soluble nutrients. These lipids exist primarily as triglycerides, which account for over 98% of the total milkfat. Triglycerides are molecules composed of a glycerol backbone attached to three fatty acid chains.
The unique structure of milkfat is defined by the Milk Fat Globules (MFGs), which are tiny liquid fat droplets. Each MFG is encased in the Milk Fat Globule Membrane (MFGM), a protective layer rich in phospholipids and proteins. This membrane acts as a natural emulsifier, maintaining the stability of the fat within the milk.
Milkfat has a complex fatty acid profile, with over 400 individual fatty acids identified. Ruminant milkfat features short-chain fatty acids, such as butyric acid, which contribute to the characteristic flavor of milk products. The composition is predominantly saturated fatty acids (around 65%), with monounsaturated and polyunsaturated fatty acids making up the rest.
Protein Structure and Function
Milk proteins are categorized into two major groups: caseins and whey proteins. Caseins comprise the larger share, about 80% of the total protein content in cow’s milk. Caseins are phosphoproteins that do not exist as free molecules but are assembled into spherical structures called micelles.
Casein micelles are complex colloidal particles that transport large amounts of calcium and phosphate to the young mammal. The micelles are held together by calcium phosphate bridges, making them stable against heat, such as during pasteurization. This structure contributes to their slow digestion, providing a sustained release of amino acids.
The remaining protein content consists of whey proteins, representing about 20% of the total protein. Whey proteins, including beta-lactoglobulin and alpha-lactalbumin, are highly soluble in the milk serum. These globular proteins are more susceptible to denaturation by high heat but offer a rapid source of amino acids.
Lactose and Water Content
Lactose, or milk sugar, is the primary carbohydrate found in milk and acts as the main energy source. Chemically, lactose is a disaccharide, formed from two simpler sugar units: one molecule of glucose and one molecule of galactose. These two monosaccharides are linked together by a beta-glycosidic bond.
Lactose makes up approximately 4.7% to 4.9% of the total mass of cow’s milk. To utilize this sugar, the enzyme lactase is required to break the bond, separating lactose into glucose and galactose for absorption. When this enzyme is deficient, lactose remains undigested, leading to lactose intolerance.
Water constitutes about 87% of the milk’s volume and acts as the medium for all other components. Water serves as the solvent, carrying dissolved minerals, vitamins, and lactose throughout the fluid. It also provides the continuous phase for the suspension of casein micelles and the emulsion of milkfat globules.
Essential Micronutrients
Milk is a source of various micronutrients, including both fat-soluble and water-soluble vitamins. The fat-soluble vitamins (A, D, E, and K) are found primarily within the lipid fraction, associated with the milkfat globules. Water-soluble vitamins, such as B-group vitamins (B2, B12), are dissolved in the aqueous serum of the milk.
Among the minerals, calcium and phosphorus are present in significant amounts and exhibit high bioavailability. Much of the calcium and phosphorus is integrated into the structure of the casein micelles, which facilitates efficient absorption. This association, combined with the presence of lactose and vitamin D, contributes to the superior absorption rate of milk calcium, estimated between 30% and 35%.
Milk also provides trace minerals, including magnesium and zinc, which contribute to various bodily functions. The combination of these micronutrients, particularly calcium and phosphorus, plays a role in maintaining bone strength and overall growth.