Fluidized bed drying of baker's yeast: moisture levels, drying rates, and viability changes during drying

Moisture levels, drying rates, and yeast cell viabilities (survival curves) were determined for two commercial compressed baker's yeasts dried in a modified fluidized bed dryer. Unlike the survival curves seen for microbes examined by classical thermal death procedures, the survival curves for both compressed yeasts stabilized-each at a different, distinct viability-as temperatures were changed in the fluidized bed dryer. The D and z values describing drying kinetics were determined from survival curves. The D values obtained for both compressed yeasts showed a linear relationship with drying temperature. Using air at 80 degrees C, compressed yeast no. 1 exhibited a D value of 4.20 minutes while the D value for compressed yeast no. 2 was 8.34 minutes. The z value for compressed yeast no. 1 was 28.4 degrees C and for compressed yeast no. 2 it was 27.5 degrees C. It was determined that the viability of the compressed yeasts in the fluidized bed dryer was not significantly affected by the drying temperature used during the warming-up and constant-rate drying periods (when moisture contents were greater than 15%). Mechanisms of death for fluidized bed drying appear to be very different from that reported for spray drying as evidenced by the similar to 200-fold differences in D values at the same temperature. (C) 1998 Published by Elsevier Science Ltd on behalf of the Canadian Institute of Food Science and Technology. All rights reserved.