FACTORS GOVERNING SURFACE MORPHOLOGY OF SPRAY-DRIED AMORPHOUS SUBSTANCES

Changes in particle morphology (size, shape, and appearance) have been monitored during drying of drops of foods and food-related materials. The apparatus produces a single stream of drops of uniform size, using a vibrating-orifice device for drop production. The drop size and the time-temperature history of the drops as they fall can be varied and controlled.

Qualitative observations are reported for drying of aqueous solutions of lactose, maltodextrin, skim milk, and coffee extract, with different feed concentrations. Particular emphasis is placed upon the tendency for development of folds upon the particle surface. A mechanistic model is developed, relating the tendency for folding to the extent of viscous flow of surface material in response to a surface-energy driving force. This model gives semi-quantitative agreement with observations for solutes of different molecular weight (and hence different viscosity) and for different feed concentrations.