CHANGES IN PARTICLE MORPHOLOGY DURING DRYING OF DROPS OF CARBOHYDRATE SOLUTIONS AND FOOD LIQUIDS .1. EFFECTS OF COMPOSITION AND DRYING CONDITIONS

Morphological changes of drops of sucrose and maltodextrin solutions, coffee extract, and skim milk have been monitored during drying. One method involved videotaping drops suspended in a stream of hot air. A second method was to sample a stream of drops of uniform initial size falling through a column with a controlled temperature profile. Suspended drops show a first period of near-spherical shrinkage, closely following predictions for a voidless sphere. This is followed by a second period of rapid inflate-deflate cycling (boiling) and a third and final period during which the drop grows and/or shrinks to reach a solidified morphology. The effects of composition, air temperature, initial solute concentration, and presence or absence of dissolved gases were determined. Particles of coffee extract from the falling-drop dryer evidence surface blowholes, replicating what is observed in commercial spray-dried coffee. This phenomenon is rationalized in terms of viscous resistance to sealing flows. © 1990, American Chemical Society. All rights reserved.