Inactivation of Lactococcus lactis ssp. cremoris cells in a droplet during convective drying

Spray drying is a less costly alternative to freeze drying in the mass-production of active dry microorganisms, if the drying conditions could be optimized to preserve cell viability. As spray drying is akin to a black-box process, in this study we used an alternative approach of a single droplet drying to study how drying conditions affect the inactivation of bacterial cells. The inactivation histories of Lactococcus lactis ssp. cremoris were investigated at air temperatures of 70,90, and 110 degrees C. It was found that the viability of L cremoris cells could be maintained at approximately the original level for extended drying durations (60-210s), despite the high air temperatures. When plotted against droplet temperature T-d, the inactivation rate k(d) at six drying conditions formed a general trend. An inactivation model was proposed to describe different inactivation histories under varied drying conditions. The description closely followed the experimental data, reported for the first time in literature. k(d) increased rapidly after T-d passed a transition temperature range of 50-65 degrees C, coinciding with the onset temperature for denaturation of bacterial ribosomes. Other environmental parameters affecting inactivation are discussed to better understand the integrated effects of multiple stresses experienced by bacterial cells during convective drying. (C) 2013 Elsevier B.V. All rights reserved.