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Spray drying is an encapsulation technique employed by the food and pharmaceutical industries. A substance to be encapsulated (the load) and an amphoteric carrier (usually some sort of modified starch) are as a suspension in water (the slurry). The slurry is then fed into a spray drier, usually a tower heated to temperatures well over the boiling point of water. As the slurry enters the tower, it is . Partly because of the high surface tension of water and partly because of the hydrophobic/hydrophilic interactions between the amphoteric carrier, the water, and the load, the atomized slurry forms micelles. The small size of the drops (averaging 100 micrometres in diameter) results in a relatively large surface area which dries quickly. As the water dries, the carrier forms a hardened shell around the load. Load loss is usually a function of molecular weight. That is, ligher molecules tend to boil off in larger quantities at the processing temperatures. Loss is minimized industrially by spraying into taller towers. A larger volume of air has a lower average humidity as the process procedes. By the osmosis principle, water will be encouraged by entropy to leave the micelles and enter the air. Therefore, the same percentage of water can be dried out of the particles at lower temperatures if larger towers are used. The application of the spray drying ecapsulation technique is to prepare "dehydrated" powders of substances which do not have any water to dehydrate. For example, instant drink mixes are spray dries of the various chemicles which make up the beverage. The technique was once used to remove water from food products; for instance, in the preparation of dehydrated milk. Because the milk was not being encapsulated and because spray drying cause thermal degredation, milk dehydration and similar processes have been replaced by other dehydration techniques.
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