The effects of vacuum-drying and freeze-drying on the cell viability of a commercial baker's yeast, Saccharomyces cerevisiae, strain with different endogenous contents of trehalose were analyzed. An osmotolerant Zygosaccharomyces rouxii strain was used for comparative purposes. Higher viability values were observed in cells after vacuum-drying than after freeze-drying. Internal concentrations of trehalose in the range 10-20% protected cells in both dehydration processes. Endogenous trehalose concentrations did not affect the water sorption isotherm nor the T-g values. The effect of external matrices of trehalose and maltodextrin was also studied. The addition of external trehalose improved the survival of S. cerevisiae cells containing 5% internal trehalose during dehydration. Maltodextrin (1.8 kDa) failed to protect vacuum-dried samples at 40 degreesC. The major reduction in the viability during the freeze-drying process of the sensitive yeast cells studied was attributed to the freezing step. The suggested protective mechanisms for each particular system are vitrification and the specific interactions of trehalose with membranes and/or proteins. The failure of maltodextrins to protect cells was attributed to the fact that none of the suggested mechanisms of protection could operate in these systems.