The water in milk is partly evaporated, decreasing the volume and increasing the solids content from about 9% to 40-45%. The liquid milk is treated at a rate of 9000-11,000 litres an hour in a continuous process. Treatment under vacuum reduces the temperatures involved in the processing and any heat deterioration of the product. The concentrated milk is pumped into a spray drier. Filtered air is heated and taken into the spray drier. The milk is forced by high pressure pumps through small jets at 20,000kPa and atomised into a stainless steel chamber with the heated air. As the atomised concentrate enters the chamber, the water is evaporated and milk solids fall to the stainless steel floor.
Moisture-laden air passes out of the drier by way of aspirators or powder collectors. These powder collectors collect any remaining airborne powder.
The powder is automatically removed from the drying chamber floor onto a conveyor which takes it through a cooling cyclone and gyratory sifter into polythene-lined multi-wall paper bags. The powder is tested for solubility, scorched particles, moisture, fat content, acidity and bacteriological content before being passed for sale on local and export markets.
Powders produced in this way cover a wide range of products including full cream, skim milk, baker's special powder, buttermilk powder and special formulations.