Principles of Drying

i. Roller Drying

The principle of the roller drying process is that the pre-concentrated milk is applied in a thin film upon the smooth surface of internally steam heated and continuously rotating cylindrical metal drum. As the water evaporates, a thin layer of dried milk is formed on the drum that is continuously scraped off by a stationary knife/doctor blade/scraper located opposite the point of application of the milk. The product is in the form of thin film, which is later pulverized and packed.

ii. Spray Drying

The basic principle of the spray drying consists of atomizing the concentrated milk into sprays of desired size (fog like mist) in a large-size drying chamber, where it is mixed with a current of hot air. Owing to their large surface area, milk sprays surrender their moisture practically instantaneously and dry into a fine powder. Dried powder is continuously removed after separation from the air by device such as cyclone separator or bag filter.