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

Abstract

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°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°C and for compressed yeast no. 2 it was 27.5°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 ~200-fold differences in D values at the same temperature.