🔄 Drying

Drying is the removal of almost all the water from milk to obtain a dry product that keeps for a long time. Drying is one of the oldest ways of preserving food: without water, microorganisms, including yeasts and moulds, cannot grow. According to Marco Polo’s accounts, the Mongols dried milk in the sun and obtained a powder (Tetra Pak).

At the plant, milk is first concentrated in evaporators: removing water this way is much cheaper, because evaporation needs only 10–12% of the energy that spray drying consumes. For whole milk powder the concentrate contains 50–52% solids. It is atomised by nozzles at a pressure of 50–300 bar or by a disc spinning at 5–25 thousand revolutions per minute. The air in the chamber is heated to 150–300 °C, but the droplets dry instantly: the temperature falls from about 200 to 70–90 °C, so the powder does not overheat.

The finest particles are captured by cyclones and bag filters, and the powder is finished and cooled in a fluid bed. Skimmed milk powder contains no more than 5% moisture and 1.5% fat. There are other methods too: roller (drum) drying on hot drums produces a partly caramelised powder valued in chocolate making, while freeze drying is hardly used for milk because of its high energy costs.

Drying or dehydration? Here drying means taking water out of milk to make it keep, not a kitchen dehydrator for fruit or a clothes dryer. At a plant, concentrated milk is sprayed into hot air, while nomads have long dried soured curd in the sun. Freeze drying is a different route and is hardly used for milk because of its energy needs. Roller drying on hot drums gives a partly caramelised powder, whereas spray drying keeps the droplets at 70–90 °C.

On a pack look for “skimmed milk powder” or “whole milk powder”, and check that the ingredient list holds only milk. Infant formula is a separate product with its own label. Milk powder is sold in tins, pouches and large bags at supermarkets, at shops for bakers and pastry cooks, and at chemists or baby shops. Keep an opened pack dry and tightly closed.

Milk powder is an ingredient for recombined milk, yoghurt and ice cream, baked goods, confectionery and infant formula. Ancient sun drying is still alive: it is used to make Central Asian kurt, Mongolian aaruul, Himalayan chhurpi, Jordanian jameed and Iranian kashk, dried fermented milk curd that keeps for months.