In drum drying the milk is distributed on rotating, steam-heated drums, where the water evaporates.
Drum dryers were introduced into industries about 100 years ago and it starting with the double-drum dryer which features the feeding by nipping between two drums. J. A Just was one of the first inventors to receive –patent rights on a drum dryer with two rolls in 1902.
Drum dryer consists of one or more hollow metal cylindrical rolls or drums that are mounted to rotate on horizontal axes at a variable speed.
Using film evaporating systems, the milk is first pre-concentrated to 40-50% solids. A thin layer or film of product is dried over an internally steam-heated drum with steam pressure up to 620 kPa and 149 °C. Approximately 1.2 – 1.3 kg steam are required per kilogram of water evaporated.
The film dries as the drum rotates. The dried milk then is scraped from the drum surface, as they rotate, by metal scraper. The feeding materials can be slurries, paste, or solutions and final dried products are in the form of powders, flakes or chips.
The operating variables for a drum dryer include condensation of incoming product in an elevator, temperature of incoming product, steam pressure in drum, speed of drum, and height of product over drum.
In the process, relatively large particles are obtained. It has poor dissolving properties and where sold for domestic use is subjected to further instantisation, which agglomerates granules and leads to a faster dissolution time.
Drum drying processing of milk
