Milk composition and its variation
What milk is made of, how species differ, what changes milk composition, its physical properties, and SNF and fat-corrected milk sums.
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Milk is a remarkable food: a liquid designed to supply everything a young mammal needs, combining water, energy-rich fat and lactose, high-quality proteins, minerals such as calcium and phosphorus, and vitamins. For dairy farmers and processors, its composition is also money: milk is often paid for by its fat and solids-not-fat content, and the amounts of fat, protein and casein decide how much butter, cheese or milk powder a factory can make.
Milk composition and the factors that affect it are core topics in dairy science and technology, and in animal science exams.
The main components
Cow's milk is roughly 87 per cent water and 13 per cent total solids (TS). The solids divide into fat and solids-not-fat (SNF): protein, lactose and minerals.
Milk fat
About 3.5 to 4.5 per cent in cow's milk, present as fat globules (mostly 1 to 10 micrometres) surrounded by the milk fat globule membrane, which keeps them dispersed. Milk fat is mostly triglycerides, with hundreds of fatty acids, including short-chain fatty acids (such as butyric acid) unique to ruminant milk fat, and fat-soluble vitamins A, D, E and K. Fat is the most variable component.
Proteins
About 3.2 to 3.5 per cent:
- Caseins (about 80 per cent of protein): αs1-, αs2-, β- and κ-casein, which form casein micelles with calcium phosphate. Caseins coagulate when milk is acidified to pH 4.6 or treated with rennet, the basis of cheese and fermented milks.
- Whey (serum) proteins (about 20 per cent): β-lactoglobulin, α-lactalbumin, serum albumin and immunoglobulins; they remain soluble at pH 4.6 but are denatured by heat.
Lactose
About 4.6 to 5.0 per cent: milk's sugar, made only in the mammary gland. Its concentration is the most constant component, because it controls the osmotic pressure of milk and so largely determines how much water is drawn into milk in the udder. Bacteria ferment it to lactic acid; people with low lactase activity may not digest it well.
Minerals and vitamins
About 0.7 per cent ash: calcium (about 1.2 g per litre), phosphorus, potassium, sodium, chloride and magnesium, much of the calcium and phosphate bound in casein micelles. Milk is a good source of riboflavin, vitamin B12 and, in its fat, vitamin A.
Factors that change composition
- Species and breed: Jersey and Guernsey cows give milk richer in fat and protein than Holsteins; buffalo milk often contains 6 to 8 per cent fat.
- Stage of lactation: colostrum, in the first days, is very rich in immunoglobulins and solids; fat and protein are lowest at peak yield and rise in late lactation.
- Feed: diets low in effective fibre or high in certain oils cause milk fat depression; energy supply affects protein.
- Season and temperature: heat stress reduces fat and protein.
- Milking: the first milk drawn is low in fat; the last (strippings) is much richer, so incomplete milking lowers fat.
- Age and parity: older cows tend to give milk with slightly lower fat and SNF.
- Udder health: mastitis raises somatic cell counts and lowers casein, lactose and cheese yield.
- Genetics, including variants such as A1 and A2 β-casein and κ-casein B, which improves cheese yield.
Physical properties
- Density of cow's milk: about 1.028 to 1.034 g/mL at 15 to 20 °C; lowered by added water or fat, raised by skimming.
- pH: about 6.6 to 6.8; titratable acidity about 0.13 to 0.16 per cent lactic acid in fresh milk.
- Freezing point: about −0.52 to −0.54 °C, very constant, so a higher freezing point indicates added water.
- Colour: white from light scattering by casein micelles and fat globules; cream colour from carotene in cow's milk fat (buffalo and goat milk fat is whiter).
Solids-not-fat and fat-corrected milk
Richmond's formula estimates total solids from fat and the corrected lactometer reading (CLR), which is (density − 1) × 1,000 at the standard temperature:
TS (%) = 0.25 × CLR + 1.22 × fat (%) + 0.72
A sample has CLR 28 and 4.0 per cent fat:
- TS = 0.25 × 28 + 1.22 × 4.0 + 0.72 = 7.0 + 4.88 + 0.72 = 12.6 per cent.
- SNF = TS − fat = 12.6 − 4.0 = 8.6 per cent.
Fat-corrected milk (FCM) compares cows producing milk of different fat content on a common basis. A widely used 4 per cent FCM formula is:
4% FCM (kg) = 0.4 × milk (kg) + 15 × fat (kg)
A cow producing 25 kg of milk at 3.4 per cent fat (0.85 kg fat):
- 4% FCM = 0.4 × 25 + 15 × 0.85 = 10 + 12.75 = 22.75 kg.
Key points
- Cow's milk: about 87% water, 3.5–4.5% fat, 3.2–3.5% protein (80% casein, 20% whey), 4.6–5.0% lactose, about 0.7% minerals.
- Fat is the most variable component; lactose the most constant.
- Caseins form micelles that coagulate with acid (pH 4.6) or rennet; whey proteins are denatured by heat.
- Composition varies with species, breed, lactation stage, feed, season, milking and udder health.
- Physical constants: density 1.028–1.034, pH 6.6–6.8, freezing point about −0.52 to −0.54 °C.
- TS = 0.25 CLR + 1.22 fat + 0.72; 4% FCM = 0.4 milk + 15 fat.
What the exams ask
- School-leaving and entrance papers: composition of milk and its nutritional value.
- Degree and research-fellowship entrances: milk fat, protein and lactose chemistry, casein micelles, factors affecting composition, physical properties and calculations.
- Dairy technology examinations: milk testing and payment systems, standardisation and the effect of composition on product yields.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why is buffalo milk thicker than cow's milk?
It contains more fat, protein and total solids, often around 16 to 18 per cent solids against about 13 per cent in cow's milk, and its fat globules are larger.
Why is colostrum different from normal milk?
It is rich in antibodies, protein, vitamins and minerals that protect and nourish the newborn, and is lower in lactose. It changes to normal milk over the first few days.
What causes low milk fat?
Usually diets too low in effective fibre or too high in rapidly fermentable starch or unsaturated oils, which change rumen fermentation. Breed, heat stress and incomplete milking also matter.
Sources
- Walstra, P., Wouters, J. T. M. and Geurts, T. J. Dairy Science and Technology, 2nd edition. CRC Press, 2006.
- Fox, P. F., Uniacke-Lowe, T., McSweeney, P. L. H. and O'Mahony, J. A. Dairy Chemistry and Biochemistry, 2nd edition. Springer, 2015.
- FAO. Milk and Dairy Products in Human Nutrition. FAO, Rome, 2013.
- Gaines, W. L. The Energy Basis of Measuring Milk Yield in Dairy Cows. University of Illinois Agricultural Experiment Station Bulletin 308, 1928.
- Claeys, W. L. et al. "Consumption of raw or heated milk from different species: an evaluation of the nutritional and potential health benefits." Food Control 42, 188–201 (2014).
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