1 Principles of Operation. Spray drying has become the most important method for dehydration of fluid foods in the Western world. The development of the process has been intimately associated with the dairy industry and the demand for drying of milk powders. However, the technology has been expanded to cover a large food group which now is being successfully spray dried.

2 Applications

  Spray Drying Applications
  Bananas Egg (whole) Proteins (animals)
  Blood  Egg (white) Proteins (milk)
  Cake mixes Egg (yolk) Proteins (plants)
  Citrus Juice Fish concentrates Shortening (bakery)
  Coffee Infant formulas Starch derivatives
  Corn syrup Milk (whole) Tea
  Cream Milk (skim) Tomato puree
  Creamers (coffee) Milk (replacers) Yeast
  Cremes (pharmac.) Potatoes Yogurt

3 Unit Operations. Spray drying consists of the following unit operations:

• Pre-concentration of liquid

• Atomization (creation of droplets)

• Drying in stream of hot, dry air

• Separation of powder from moist air

• Cooling

• Packaging of product

Relatively high temperatures are needed for spray drying operations. However, heat damage to products is generally only slight, because of an evaporative cooling effect during the critical drying period and because the subsequent time of exposure to high temperatures of the dry material may be very short. The typical surface temperature of a particle during the constant drying zone is 45-50 C. For this reason, it is possible to spray dry some bacterial suspensions without destruction of the organisms. The physical properties of the products are intimately associated with the powder structure which is generated during spray drying. It is possible to control many of the factors which influence powder structure in order to obtain the desired properties.

4 Typical Spray Drying Systems. The diagram shows a schematic representation of a typical spray drying system for milk powder. For spray drying, it is usual to pump a concentrate of the liquid product to the atomizing device where it is broken into small droplets. These droplets meet a stream of hot air and they loose their moisture very rapidly while still suspended in the drying air. The dry powder is separated from the moist air in cyclones by centrifugal action. The centrifugal action is caused by the great increase in air speed when the mixture of particles and air enters the cyclone system. The dense powder particles are forced toward the cyclone walls while the lighter, moist air is directed away through the exhaust pipes. The powder settles to the bottom of the cyclone where it is removed through a discharging device. Sometimes the air-conveying ducts for the dry powder are connected with cooling systems which admit cold air for transport of the product through conveying pipes. Cyclone dryers, such as shown here have been designed for large production schedules capable of drying ton-lots of powder per hour.

4.1 Cyclone Spray Dryer. The following is a diagram of a typical spray drying operation utilizing a centrifugal atomizer and a cyclone separator