Commercial baker's yeast stability as affected by intracellular content oftrehalose, dehydration procedure and the physical properties of external matrices

The effects of vacuum-drying and freeze-drying on the cell viability of a c ommercial baker's yeast, Saccharomyces cerevisiae, strain with different en dogenous contents of trehalose were analyzed. An osmotolerant Zygosaccharom yces rouxii strain was used for comparative purposes. Higher viability valu es were observed in cells after vacuum-drying than after freeze-drying. Int ernal concentrations of trehalose in the range 10-20% protected cells in bo th dehydration processes. Endogenous trehalose concentrations did not affec t the water sorption isotherm nor the T-g values. The effect of external ma trices of trehalose and maltodextrin was also studied. The addition of exte rnal trehalose improved the survival of S. cerevisiae cells containing 5% i nternal trehalose during dehydration. Maltodextrin (1.8 kDa) failed to prot ect vacuum-dried samples at 40 degreesC. The major reduction in the viabili ty during the freeze-drying process of the sensitive yeast cells studied wa s attributed to the freezing step. The suggested protective mechanisms for each particular system are vitrification and the specific interactions of t rehalose with membranes and/or proteins. The failure of maltodextrins to pr otect cells was attributed to the fact that none of the suggested mechanism s of protection could operate in these systems.