DAIRY TECHNOLOGY: MILK COMPOSITION
Diagram
Whole milk and skim milk side by side
Whole milk
Milk with its natural fat left in: about 3 to 4 percent fat, creamy and full flavoured.
↓
Skim milk
The same milk with the fat removed: under 0.5 percent fat, thin and light.
↓
The core difference
Fat. Skim milk is whole milk minus the cream, so it has fewer calories and a thinner body.
Figure 1: Whole milk compared with skim milk. Drawn in our own style from A. K. M. Humayun Kober, Milk and Dairy Foods: Nutrition, Processing and Healthy Aging, CRC Press, 2024, Ch. 2 and Ch. 5.
Introduction
Whole milk and skim milk come from the same milk. The difference is one step: the cream is taken out of skim milk and left in whole milk.
The core difference is fat. Whole milk carries about 3 to 4 percent fat, which gives it body, flavour and calories. Skim milk carries under 0.5 percent fat, so it is lighter, thinner and lower in calories, while its protein, lactose and calcium stay almost the same.
Quick comparison
Table 1: Whole Milk compared with Skim Milk
| Aspect | Whole Milk | Skim Milk |
|---|---|---|
| Fat content | About 3 to 4 percent fat | Under 0.5 percent fat |
| How it is made | Milk as it comes, standardised to a set fat level | Milk passed through a cream separator that spins the fat out |
| Calories | Higher, about 60 to 65 kcal per 100 ml | Lower, about 35 kcal per 100 ml |
| Protein | About 3.2 to 3.5 percent | About 3.4 to 3.6 percent, slightly higher per 100 ml |
| Lactose and calcium | Present in normal amounts | Present in normal amounts, almost unchanged |
| Fat-soluble vitamins | Carries vitamins A and D in the fat | Mostly lost with the fat, often added back by fortification |
| Body and mouthfeel | Creamy, full, coats the mouth | Thin, watery, light |
| Colour | White with a slight yellow tint | Bluish white, because the fat is gone |
| Cooking behaviour | Rich sauces, creamy texture | Thinner sauces, can curdle more easily in hot acidic dishes |
| Who it suits | Children and people who want full flavour | People cutting fat and calories who still want milk protein and calcium |
Scientific explanation
Definition
Whole milk is milk with its natural fat content kept, usually standardised to about 3 to 4 percent fat.
Skim milk, also called skimmed milk, is milk from which almost all the fat has been removed by separation, leaving under 0.5 percent fat.
Working principle
Fat is lighter than the water part of milk, so a fast spinning separator throws the skim milk to the outside and lets the lighter cream out through the centre. The fat globules are the part that carries flavour, body and fat-soluble vitamins.
Removing the fat removes most of the calories but leaves the protein, lactose, minerals and water-soluble vitamins behind, so skim milk keeps most of the nutrition of whole milk.
Apparatus
A cream separator, a fast spinning centrifuge that splits milk into cream and skim milk.
A milk standardising unit that blends cream and skim milk back together to reach the exact fat level wanted for whole milk.
Measurement procedure
Warm the milk slightly, feed it into the separator, and collect the skim milk and the cream from their separate outlets.
Measure the fat of the skim milk with the Gerber method or a milk analyser. For whole milk, blend cream back into skim milk until the fat reads the target level, then pasteurize and pack.
Factors affecting readings
The temperature of the milk and the speed of the separator decide how completely the fat is removed.
The fat of the starting milk varies with breed, feed and season, so the blending must be checked with a fat test on every batch.
Formulas and calculations
Fat removed can be worked out by a simple balance. If 100 kg of whole milk at 4.0 percent fat is separated into cream and skim milk, the fat in the skim milk is found from: Fat in skim (kg) = Fat in whole milk (kg) minus fat in cream (kg).
Worked example: 100 kg of milk at 4.0 percent fat holds 4.0 kg of fat. If the separator takes out 3.6 kg of fat as cream, the skim milk keeps 4.0 minus 3.6 = 0.4 kg of fat. If the skim milk weighs about 90 kg, its fat is 0.4 divided by 90, times 100, which is about 0.44 percent.
Energy can be estimated from the fat: each gram of fat gives about 9 kcal, so removing about 3.5 g of fat per 100 ml removes about 31 kcal per 100 ml.
Industrial applications
- Whole milk: drinking milk, tea, coffee and hot chocolate where a creamy flavour is wanted.
- Whole milk: rich sauces, custards and baking where fat gives tenderness.
- Skim milk: drinking milk for people cutting fat and calories.
- Skim milk: the base for skim milk powder and for low-fat yoghurt and cheese.
- Both milks: standardised milk of any fat level, made by blending cream and skim milk.
Advantages and limitations
- Whole milk advantage: full flavour and creamy body, and it carries vitamins A and D in its fat.
- Whole milk limitation: higher in calories and saturated fat.
- Skim milk advantage: low in fat and calories while keeping protein, lactose and calcium.
- Skim milk limitation: thin and watery, with less flavour, and its fat-soluble vitamins are mostly lost.
FAQ
Is skim milk just watered down whole milk?
No. It is whole milk with the fat removed by a separator. Nothing is added, and no water is added.
Does skim milk have the same calcium as whole milk?
Yes, about the same. Calcium sits in the water part of milk, not in the fat, so removing fat does not remove calcium.
Why does skim milk look bluish?
The fat globules scatter light and give whole milk its white colour. Without them, the milk looks thinner and slightly blue.
Can I cook with skim milk instead of whole milk?
Yes, but sauces will be thinner and the milk can curdle more easily in hot acidic dishes, because there is less fat to protect the protein.
Is skim milk better for everyone?
No. It suits people cutting fat and calories, but children and people who need energy often do better with whole milk.
References and further reading
- A. K. M. Humayun Kober, Milk and Dairy Foods: Nutrition, Processing and Healthy Aging, CRC Press, 2024, Ch. 2 and Ch. 5 (Google Books page read)
- FTN books on disk: Chemistry of Milk, Market Milk, Dairy Processing Handbook by Bylund (agrimoon dairy series).
