Collapse temperature of bacterial suspensions: the effect of cell type and concentration

The characterisation of the physical state of frozen and freeze dried biological products delivers powerful information for freeze-drying process optimisation. The influence of lactic acid bacterial cell size, shape and concentration on collapse temperature of concentrated bacterial suspensions was investigated. Lactobacillus bulgaricus (long rods), and Streptococcus thermophilus (small spherical cells) were used as cellular models for this study. Whatever the strain, when lactic acid bacterial cells were added to protective solutions, the collapse temperature increased, thus allowing the use of higher sublimation temperatures during primary drying than expected from the protective medium alone. Moreover, the higher the cell concentration, the greater the effect, linear relationships existing between the collapse temperatures and the total dried matter. Cells of both strains gave a kind of robustness to the freeze-dried product, but the increase observed in collapse temperature was considerably higher (3 - 5 degree C) for L. bulgaricus compared to S. thermophilus. This result was ascribed to the different size and shape of the strains.