ENZYME INACTIVATION IN A DROPLET FORMING A BUBBLE DURING DRYING

Alkaline phosphatase activity, moisture content and bubble volume in s pray-dried condensed skim milk were measured experimentally. The activ ity and moisture content decreased and the bubble volume increased, wi th increasing outlet-air temperature. Bubble volumes were sometimes la rger than the dry solids volume. Scanning electron micrographs contain ed some particles that were hollow spheres, and others that were caved -in. Mathematical models from the literature which included bubble for mation were used to simulate the experiments Predictions generally fel l within the range of the experimental results. Activity was predicted to increase with decreasing air temperature and droplet size. In the simulation, most inactivation occurred in a critical region of 70-80% solids, wherein both the moisture content and droplet temperature were sufficiently elevated. Allowance for bubble formation during drying d id not significantly affect the predicted moisture content or activity .