MILK constituents

Milk Composition

The figures below are typical averages drawn from standard dairy-chemistry references. Milk is a natural product, so every value moves with breed, stage of lactation, feed and season — treat them as working reference points, not specification limits.

Typical composition of whole cow’s milk

Milk composition by species (per 100 g)

Major minerals in cow’s milk (mg / 100 g)

Physical Properties of Milk

Milk is a colloidal suspension — fat globules and casein micelles dispersed in a serum of dissolved lactose, whey proteins and minerals. Its measurable physical properties are used throughout processing for standardisation, heat treatment and fault detection.

Quality, Faults & Testing

Routine quality control covers the physical, chemical and microbiological condition of milk at reception and through processing. The main tests:

• Microbiological: Total Plate Count (TPC) and targeted pathogen detection for safety and shelf-life.
• Fat: Gerber or infrared (MIR) analysis — for payment, standardisation and consistency.
• Protein: Kjeldahl or Dumas (reference), MIR (routine) — for payment and formulation.
• Sensory / organoleptic: taste, odour, appearance and texture — to catch off-flavours, rancidity and taints.
• Cryoscopy: freezing-point depression to detect added water (see below).
• Antibiotic residues: rapid inhibitor tests at reception — a positive load is rejected.

Milk Adulteration & Detection

As a traded commodity, milk is open to adulteration — substances added to increase volume or mask quality. Common forms and how they are detected:

• Water dilution — raises volume, lowers solids; detected by freezing point and specific gravity.
• Added skimmed milk powder — inflates apparent protein/solids; flagged by composition anomalies.
• Preservatives / neutralisers (e.g. hydrogen peroxide, formalin, urea, detergents) — identified by specific chemical and residue tests.
• Undeclared other-species milk (buffalo, goat, sheep) — confirmed by species-specific DNA or protein analysis.

Deviation from the expected composition or freezing point is the usual first signal; chromatography, electrophoresis and DNA methods confirm specific adulterants.

Possible Contaminants

Contaminants can enter milk from feed, environment or handling, and are controlled by withdrawal periods, feed control and reception testing:

• Antibiotic residues — from failure to observe withdrawal periods.
• Pesticides — via contaminated feed or pasture.
• Heavy metals (lead, cadmium, arsenic, mercury) — from contaminated soil, water or feed.
• Mycotoxins (e.g. aflatoxin M1) — carried over from mould-contaminated feed.
• Pathogens (Salmonella, E. coli, Listeria, Campylobacter) — controlled by hygiene and pasteurisation.
• Environmental pollutants (PCBs, dioxins) — from environmental exposure.

Testing requirements and limits are set by the food-safety regulations applicable in each market — confirm the current limits that apply to your supply.

Freezing Point of Milk

The freezing-point depression of milk is the standard check for added water, whether accidental (rinse water, plant residue) or deliberate. Because dissolved lactose and salts sit in a narrow natural band, extra water shifts the freezing point in a measurable, repeatable way.

The freezing point of milk is usually in the range −0.512 °C to −0.550 °C, averaging about −0.522 °C. A cryoscope freezes a small sample for a fast, accurate reading. Correct interpretation needs an understanding of the natural factors affecting it — freezing point varies with stage of lactation, animal health, breed and feed.

Milk pricing is more layered than a simple price per litre. Farmgate milk is usually quoted in pence per litre, but the value sits in the constituents: contracts pay primarily on butterfat and protein (and sometimes total solids or SNF), adjusted by quality and hygiene bands — somatic cell count, Bactoscan / total bacterial count and antibiotic screening — and by seasonality and volume terms. Processors also value different fractions: cheesemakers focus on fat and protein, powder makers on total solids, and liquid-milk plants standardise fat and SNF. Accurate constituent and freezing-point data therefore underpin both payment and processing. See the milk supply contracts page for how pricing mechanisms and quality specifications are actually structured.

Glossary of Terms

• Acid: added as an acid; also, lactose is converted into lactic acid by bacteria.
• Acid curd: curd formed by the action of bacteria or by adding an acid, e.g. citric acid.
• Albumin: a water-soluble protein, a component of whey.
• Annatto: orange-red dye used to colour cheese and butter.
• Bacteriophage: a virus that relies on a bacterial host for reproduction.
• Brine: a solution of salt and water.
• Casein: the main protein of milk.
• Colostrum: the first milk secreted after giving birth.
• Colony: a mass of individual cells.
• Disaccharide: e.g. lactose, a sugar composed of two monosaccharides.
• Homogenise: to break down the fat globules in whole milk and distribute them evenly so the cream does not separate.
• Hypochlorite: (a bleach) chemical solution used after cleaning utensils to destroy micro-organisms.
• Lactation: period during which milk is secreted.
• Lactose: milk-sugar.
• Lactic acid fermentation: the production of lactic acid from lactose by the action of micro-organisms.
• Lipolytic: the property of splitting or hydrolysing fat. Lipases are lipolytic enzymes; lipolytic bacteria break down fat.
• Mastitis: inflammatory condition of the udder.
• Mesophiles: micro-organisms with optimum growth temperatures between 25° and 45°C.
• Thermophilic bacteria (thermophiles): spore-forming micro-organisms with optimum growth temperatures between 60° and 70°C, but which can also grow less rapidly at 20° and 80°C.
• Organoleptic: testing the effects of a substance on the senses, especially taste and smell.
• HTST pasteurisation: heating milk to 72°C for 15 seconds and cooling rapidly to less than 7°C.
• Pathogenic bacteria: bacteria that cause disease or illness.
• Rennet: enzyme used to coagulate milk in cheesemaking.
• Polysaccharide: e.g. starch, cellulose. A complex carbohydrate of high molecular weight composed of many monosaccharide molecules.
• Proteolytic: protein-splitting. Proteases are proteolytic enzymes; proteolytic bacteria break down proteins.
• Psychrotrophs: micro-organisms capable of growth at 5°C or below, though their optimum may be similar to mesophiles (25°–45°C).
• Starter: bacterial culture of selected lactic acid bacteria used to produce the required acid and flavour development in fermented dairy products.

Sources for reference data

• Fox, P.F. & McSweeney, P.L.H. (2015). Dairy Chemistry and Biochemistry, 2nd ed. Springer.
• Walstra, P., Wouters, J.T.M. & Geurts, T.J. (2006). Dairy Science and Technology, 2nd ed. CRC Press.
• FAO — Dairy production and products: Milk composition (fao.org).
• Jenness, R. & Patton, S. Principles of Dairy Chemistry.

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