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Spray-drying

By definition, spray drying is the transformation of feed from a fluid state into a dried form by spraying the feed into a hot drying medium. The process is a one step continuous operation. The feed can be either a solution, suspension or a paste. The spray dried product conforms to powder consisting of single particles or agglomerates, depending upon the physical and chemical properties of the feed and the dryer design and operation. During the last three decades spray drying has undergone an intensive research and development, so that modern spray drying equipment can meet the requirements to produce a powder with tailor-made specifications required by the end-user.

Spray-drying is carried out in two phases. In the first phase the pre-treated milk is evaporated to a dry matter content of 45-55%. In the second step the concentrate is pumped to a drying tower for final drying. This process takes place in three stages:

  • dispersion of the concentrate in very fine droplets
  • mixing of the finally dispersed concentrate into a stream of hot air which quickly evaporates the water
  • separation of the dry milk particles from the drying air

Evaporation is a necessary production stage for high-quality powder. Without prior concentration the powder particles will be very small and will have a high air content, poor wettability and short shelf-life. The process will then be also uneconomical. Falling film evaporators are generally used for concentration which is carried out in two or more stages to a dry slurry content of 45-60%. The equipment is the same as that used in the production of condensed milk.

Spray-drying in the dairy industry dates back to around 1800, but it was not until 1850 that it became possible in major scale to dry milk. All processes, however, required addition of sugar, sulphuric acid or alkali, so that the end product could not be considered pure.

One of the first spray drying patents was applied for in 1901 by the German Mr. Stauf, who sprayed the milk by nozzles into a chamber with warm air. The first real break-through, however, was in USA in 1913, when the American Mr. Grey and the Dane Mr. Jensen developed a nozzle spray dryer and started to produce and sell drying installations on a commercial scale.

The first rotary atomiser was developed by the German Mr. Kraus in 1912, but not until 1933, when the Danish engineer Mr. Nyrop filed his world patent, which was the real break-through of atomisation.
The basis for the modern dry milk industry, formed by these pioneers lead to new developments, and the spray drying equipment installed nowadays is in most cases very sophisticated and involves a highly technical and technological design

 
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