DAIRY TECHNOLOGY: MILK PROCESSING
Diagram
Milk powder and liquid milk side by side
Milk powder
Milk with the water dried out: a light powder that keeps for months and must be mixed with water.
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Liquid milk
Milk as it comes: fresh, perishable, and ready to drink.
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The core difference
Water. Powder is milk minus water; liquid milk is milk with its water still in it.
Figure 1: Milk powder compared with liquid 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. 8, Long-Life Dairy Food: Powdered Milk, p. 196.
Introduction
Milk powder and liquid milk are the same food in two forms. One has had its water removed so it can travel and keep; the other is the fresh milk we pour from a carton.
The core difference is water and keeping. Milk powder is about 96 to 97 percent solids, so a kilo of powder stands for about 8 litres of milk, and it keeps for months without a fridge. Liquid milk is about 87 percent water, keeps for days in the fridge, and is ready to drink at once.
Quick comparison
Table 1: Milk Powder compared with Liquid Milk
| Aspect | Milk Powder | Liquid Milk |
|---|---|---|
| Water content | About 3 to 4 percent water | About 87 percent water |
| Total solids | About 96 to 97 percent solids | About 12 to 13 percent solids |
| How it is made | Milk concentrated, then spray dried into powder | Milk pasteurized and packed, no drying |
| Shelf life | Months, often 6 to 12 months in a sealed pack | Days to 2 weeks in the fridge |
| Storage | Cool dry place, no fridge needed | Must be kept cold at all times |
| Weight to carry | Light, about 1 kg of powder stands for about 8 litres of milk | Heavy, 1 litre weighs about 1 kg |
| Preparation | Must be mixed with clean water before drinking | Ready to drink at once |
| Flavour | Slightly cooked flavour from drying | Fresh flavour |
| Nutrition | Nearly the same, small losses of heat-sensitive vitamins | Full fresh nutrition |
| Main use | Reconstituted drinking milk, baking, confectionery, aid supply | Daily drinking, tea, coffee and fresh products |
Scientific explanation
Definition
Milk powder is milk from which almost all the water has been removed by evaporation and spray drying, leaving a dry powder of milk solids.
Liquid milk is milk in its natural fluid form, pasteurized and packed for drinking, with its water still in it.
Working principle
Drying works because microbes need water to grow. Removing the water to under 4 percent stops microbial growth and slows chemical change, so the powder keeps for months.
Spray drying works because the milk is sprayed into tiny droplets in hot air. Each droplet dries in seconds, which limits heat damage and leaves a powder that dissolves again in water.
Apparatus
A spray dryer: a tall chamber where milk is sprayed through nozzles into hot air, with a cyclone to collect the powder.
For liquid milk, a pasteurizer and a filler. For reconstitution, a clean jug and a whisk or a stirrer.
Measurement procedure
For milk powder, pasteurize the milk, concentrate it by evaporation to about half its water, spray dry it in hot air, cool the powder, and pack it at once in a moisture-proof pack.
To reconstitute, mix the powder with clean water and stir until dissolved. A common ratio is about 100 g of whole milk powder to 900 ml of water, giving about 1 litre of milk.
Factors affecting readings
The drying temperature decides the flavour: hotter drying gives a more cooked flavour and lower solubility.
Moisture is the enemy of the powder: a torn pack lets damp air in, and the powder cakes, goes stale and can grow mould. Clean water matters for reconstitution, because dirty water spoils the milk.
Formulas and calculations
Reconstitution follows a simple balance: Powder needed (g) = (Solids wanted (g) divided by solids in powder (percent)) x 100.
Worked example: to make 1 litre of milk at 12.5 percent solids, the solids wanted are 125 g. With powder at 96 percent solids, the powder needed is 125 divided by 96, times 100, which is about 130 g of powder, mixed with about 870 ml of water. In practice the common home ratio of about 100 g of powder to 900 ml of water gives a slightly thinner milk.
Industrial applications
- Milk powder: reconstituted drinking milk where fresh milk is scarce or costly to carry.
- Milk powder: baking, confectionery, chocolate and ice cream, where a dry milk solid is wanted.
- Milk powder: food aid, emergency supply and infant formula base.
- Liquid milk: daily drinking, tea and coffee.
- Liquid milk: fresh yoghurt, cheese and cultured products.
Advantages and limitations
- Milk powder advantage: light to carry and keeps for months without a fridge.
- Milk powder limitation: must be mixed with clean water, and it tastes slightly cooked.
- Liquid milk advantage: fresh flavour and ready to drink at once.
- Liquid milk limitation: heavy to carry and spoils in days without refrigeration.
FAQ
How do I mix milk powder?
Stir about 100 g of whole milk powder into 900 ml of clean water until dissolved. Use it the same day and keep it cold, like fresh milk.
Is milk powder as nutritious as fresh milk?
Nearly. Protein, fat, lactose, calcium and minerals are the same; small amounts of heat-sensitive vitamins are lost in drying.
Why does milk powder keep so long?
Microbes need water to grow, and the powder holds only 3 to 4 percent water, so they cannot grow in a sealed dry pack.
Can I bake with milk powder?
Yes. It is widely used in baking, chocolate and ice cream, where a dry milk solid is easier to handle than liquid milk.
Why does reconstituted milk taste different?
The drying heat gives a slight cooked flavour, and the powder must be fully dissolved or it tastes chalky.
References and further reading
- A. K. M. Humayun Kober, Milk and Dairy Foods: Nutrition, Processing and Healthy Aging, CRC Press, 2024, Ch. 8, Long-Life Dairy Food: Powdered Milk, p. 196 (Google Books page read)
- FTN books on disk: Condensed and Dried Milk, Dairy Processing Handbook by Bylund (agrimoon dairy series).
