Milk, a biological fluid produced by mammals, is a complex chemical system. Approximately 87% of milk is water, which serves as the continuous phase for its dissolved, suspended, and emulsified components. Chemically, milk is a blend of an aqueous solution, a colloidal suspension, and an oil-in-water emulsion coexisting in a stable state. This physicochemical architecture allows it to deliver nutrients, including proteins, lipids, carbohydrates, and minerals.
Milk as a Complex Physical System
The opaque, white appearance and fluid texture of milk result from how its components are physically dispersed in the water phase. This dispersion involves two distinct physical states: a colloidal suspension and an emulsion. Milk fat, which comprises almost 99% triglycerides, is present as an oil-in-water emulsion. These lipid molecules are gathered into microscopic fat globules, suspended throughout the aqueous phase.
A thin barrier called the Milk Fat Globule Membrane (MFGM) surrounds each fat droplet, stabilizing the emulsion. This membrane, composed primarily of phospholipids and proteins, prevents the hydrophobic fat globules from clumping together and separating from the water (coalescence). Milk proteins, mainly caseins, form the second physical structure: a colloidal suspension. Casein proteins aggregate with calcium phosphate to create large, spherical structures called casein micelles, dispersed throughout the milk serum.
These micelles are too large to be dissolved like sugar but too small to settle out, defining them as a colloid. The stability of this colloidal system is maintained by a negatively charged, hydrophilic outer layer, rich in kappa-casein. This layer creates a repulsive force between micelles. This balance of physical forces gives milk its characteristic stability and physical properties.
The Chemical Components Proteins and Lipids
Milk proteins are differentiated into two main groups: caseins and whey proteins. Caseins account for about 80% of the total protein content and are characterized by their flexible structure and high concentration of phosphoserine residues. These phosphate groups are essential for binding calcium and forming the colloidal calcium phosphate nanoclusters that hold the micelle together. Caseins are heat-stable and precipitate readily when the milk’s pH drops to their isoelectric point of approximately 4.6.
The remaining proteins, known as whey proteins, primarily consist of beta-lactoglobulin and alpha-lactalbumin. These globular proteins are soluble in the milk serum, unlike caseins. Whey proteins are susceptible to denaturation (unfolding) when exposed to heat, such as during pasteurization. This heat-induced denaturation can cause them to interact with kappa-casein on the micelle surface, altering the milk’s processing properties.
Milk lipids are composed overwhelmingly of triglycerides, constituting approximately 98% of the milk fat. These triglycerides are esters formed from a glycerol backbone and three fatty acid chains, exhibiting diversity in chain length and saturation. Milk fat is characterized by a high proportion of saturated fatty acids, such as palmitic and stearic acid. It also contains oleic acid, the most abundant unsaturated fatty acid, and short-chain fatty acids like butyric acid, which contribute to the distinct flavor profile.
The Chemical Components Lactose and Minerals
Lactose is the primary carbohydrate in milk, typically making up between 4.7% and 5.2% of the fluid. Chemically, it is a disaccharide formed by a molecule of glucose linked to a molecule of galactose via a beta-1,4-glycosidic bond. This sugar is water-soluble and is the least sweet of the common dietary sugars. The presence of a hemiacetal group classifies lactose as a reducing sugar, allowing it to participate in non-enzymatic browning reactions like the Maillard reaction.
Minerals are dissolved or suspended in the milk serum, influencing its chemical environment. Calcium and phosphate are the most significant minerals, playing a dual role in nutrition and structural stability. A substantial portion of calcium and phosphate is bound within the casein micelles as colloidal calcium phosphate nanoclusters. This colloidal mineral phase helps cross-link the casein proteins, stabilizing the micelle structure. Other electrolytes, including potassium, sodium, and chloride, are present in true solution and contribute to the slight acidity of fresh milk (pH 6.6 to 6.8).
Chemical Transformations in Milk
Milk is susceptible to various chemical changes, the most common being souring. Souring is initiated by lactic acid bacteria naturally present in the milk. These bacteria metabolize lactose through fermentation, converting it into lactic acid. This transformation results in a rapid drop in the milk’s pH.
As the acidity increases, the milk begins to curdle due to the destabilization of casein micelles. The drop in pH neutralizes the negative surface charge of the kappa-casein, removing the electrostatic repulsion that kept the micelles apart. Without this stabilizing force, the casein molecules aggregate into visible clumps (curds), separating from the whey. Processing methods also induce changes, such as pasteurization, where heat causes whey protein denaturation, making them less soluble. Homogenization breaks fat globules into smaller sizes, changing the surface chemistry by replacing the native MFGM with a new membrane made of adsorbed casein and whey proteins.