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Milk powder is a dairy product that can be preserved for a long time thanks to water removal. In correct storage conditions (cool, dark, dry and air tight) milk powder can be preserved for years. By moisture removal its weight is about 1/7 of the weight of milk.
|Production milk powder|
|Temperatures and properties|
Milk powder is often used in bakery-, chocolate production and ready-to-consume meals.
Before the water is removed, the milk is :
- homogenised and
- pasteurised or sterilised.
After thickening the milk has a dry matter content of 50%. Further thickening is not possible as the flow characteristics at this high dry matter content is bad. Multistage evaporation installations consume less energy. The vapour of the first stage is used in the second stage and so on. Evaporation with this installation is in many cases more economical than spray drying. Evaporation is performed under vacuum as than the boiling point of milk is lowered to 70°C. Sometimes the temperature of the last stage is lower than 65°C.
The milk droplets in the atomiser can be produced in two ways:
- High pressure atomization: the droplets are formed by pressing the milk through small nozzles at very high pressure. This way gives an uniform droplet size, and also the powder has an uniform grain size.
- Disc atomization: the droplets are formed by injecting the milk on a disc turning at high speed (larger than 6000 rpm). The milk is atomized. The droplet size varies from large to very small. Powder of coarser and smaller size is obtained.
Grains of milk powder are agglomerated by moistening with vapour. The powder is moving upstairs in a fluidized bed, lumps together and grains are formed.
The process occurs at different temperatures. At high temperatures the moisture is vaporizing more quickly , but the milk powder is losing properties that the milk possesses. At lower temperatures the evaporation proceeds slower, but the milk powder maintains better the properties of milk.
- Low-heat milk powder:
Low pasteurised milk: at evaporation the temperature is maintained under 70ºC. The temperature of the milk droplets during spray drying is low. Hence the temperature of the exit powder is low. The milk powder contains no denatured milk protein, making the powder curdling during cheese prepartion and good soluble. As the powder is diluted to milk, there are no problems with taste.
- High-heat milk powder:
The pasteurisation of the milk is 5 minutes at 90ºC or 1 minute at 120ºC. With bad hygienic design thermophilic micro-organisms can develop at these temperatures and continuous contamination can occur. Through denaturation of the milk protein high-heat milk powder is badly soluble. The high temperatures during this process gives the milk powder a boiling smell and taste, especially with full fat milk powder. Curdling of the powder during cheese production is bad. But for addition to baby food this is an advantage. High-heat milk powder can be easily applied in soured products (like yoghurt). As the milk own bacteria inactivating system is inactivated, fermentation mixtures are not inactivated. High-heat milk powder gives to yoghurt extra high viscosity.