Spray drying is the process of mixing and drying a slurry (a kind of suspension) to form a homogeneous mixture of powders. The powders are mixed with a liquid (like water), then the mixture is sprayed into hot dry air, so that the liquid evaporates leaving small spherical granuales of the mixed powders.

Use in the food industry

Spray drying is sometimes used in the food industry, in milk, egg, fruit juice and dairy products, as well as coffee (although it has been mostly displaced by freeze drying in this aspect).

The spray dryer operates by atomising a stream of the solution using compressed air. The atomisation breaks the solution into small droplets, thereby incresing the surface area and thus the rate of evaporation. The small size of the drops (averaging 100 micrometres in diameter) results in a relatively large surface area which dries quickly. Although the air dry-bulb usually reaches 200 degrees celsius, the air wet-bulb rarely exceeds 55 C and the dried particles are removed from the drier usually within 30 seconds. The temperatures of the particles during drying can range from the wet-bulb temperature of the inlet air to above 100 C as they exit in the dry state. Although these temperatures are high compared to other drying methods available, the moisture near the end is usually near the BET monolayer and the time of exposure is very short, resulting in relatively high nutrient preservation when used in food processing.

Sources

• Nutritional evaluation of food processing second edition (1975), Robert S. Harris, Ph.D. and Endel Karmas Ph.D. (eds)

Food industry Chemical processes | Unit operations

Sprühtrocknung