Drying process of milk using drum dryer

The main drying process of skim milk powder and whole milk powder is spray drying. However, drum drying and fluid-bed drying are used for special purposes.

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.
Drying process of milk using drum dryer

Skim of low-fat milk

Dry milk was referred to as milk powder until the mid 1960s, when the designation was changed by the American Dry Milk Institute to dry milk in the United States.

Large quantities of skim of low fat milk are dried. This may be done by spraying atomized droplets of milk into chamber through which heated air is circulated (spray drying).

The atomizers used in this process include mainly rotary and pressure types. The final articles size of the dried powder produced by utilizing a rotary atomizer and the basic plant is greater than 1000 microns.

The milk also may dry by allowing it to flow over the surface of two heated metal drum that rotate toward each other.

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 dried milk then is scraped from the drum surface, as they rotate, by metal scraper.

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.

Dried milk (usually the spray dried type that contains about 5% of moisture) may be re-humidified to slightly higher moisture content after drying.

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.
Skim of low-fat milk