Mechanism of viability loss during fluidized bed drying of baker's yeast

Compressed Saccharomyces cerevisiae yeasts were dried in a modified Yamato Pulvis GA 22 fluidized bed dryer and also exposed to moist heat in classical thermal death time experiments. The results showed that dehydration and not moist heat, dry heat, or oxidation was responsible for viability decreases during fluidized bed drying. It was determined that the viabilities of the compressed yeasts placed in the dryer were not substantially affected by the drying temperature during the warming-up and constant-rate drying periods (when moisture contents were greater than 15%). However, at moisture contents below 15% when compressed yeasts were in the falling-rate drying period, viability decreased sharply. Phantom thermal death time curves were constructed in order to distinguish between the kinetics of death due to moist heat (classical thermal death time experiments) and the kinetics of death due to dehydration in the fluidized bed dryer. For both compressed yeasts below 50 degrees C, moist heat D (thermal decimal reduction time) values were found to be greater than D-d (dehydration decimal reduction time) values which indicates that at such temperatures, the death of both compressed yeasts due to moist heat is less vital than death due to dehydration. As temperatures increased above 50 degrees C, moist heat became more responsible for killing the compressed yeasts than dehydration. When compressed yeast was dried in the presence of trehalose (a known dehydration protectant), 30% higher viability was seen compared to the same yeast dried without trehalose. (C) 1998 Canadian Institute of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved.