Spray Drying - Process Summary

The liquid food is generally preconcentrated by evaporation to economically reduce the water content. The concentrate is then introduced as a fine spray or mist into a tower or chamber with heated air. As the small droplets make intimate contact with the heated air, they flash off their moisture, become small particles, and drop to the bottom of the tower and are removed. The advantages of spray drying include a low heat and short time combination which leads to a better quality product.

Diagram of a 2 stage dryer

Diagram of a spray dryer

Principal components include: 

  • a high pressure pump for introducing liquid into the tower
  • a device for atomizing the feed stream
  • a heated air source with blower
  • a secondary collection vessel for removing the dried food from the airstream
  • means for exhausting the moist air
  • usually includes a preconcentration step i.e. MVR evaporation

Atomizing devices are the distinguishing characteristic of spray drying. They provide a large surface area for exposure to drying forces:

1 litre = 12 billion particles = >300 ft2 (30m2)

The exit air temperature is an important parameter to monitor because it responds readily to changes in the process and reflects the quality of the product. Generally, we want it high enough to yield desired moisture without heat damage. There are two controls that may be used to adjust the exit air temperature:

  • altering feed flow rate
  • altering inlet temperature

If heat damage occurs before the product is dried, the particle size must be reduced; smaller particle dries faster, therefore, less heat damage. This can be accomplished in three ways: 

  • smaller orifice
  • increase atomizing pressure
  • reduce viscosity - by increasing feed temperature or reducing solids