Air drying optimization of Saccharomyces cerevisiae through its water-glycerol dehydration properties.

AIMS: This study describes the different stages of optimization in an original drying process for yeasts, which allows the retrieval of dried samples of Saccharomyces cerevisiae CBS 1171 with maximum viability. METHODS AND RESULTS: The process involves the addition of wheat flour to yeast pellets, followed by mixing and then air-drying in a fluidized bed dryer. The sensitivity to the osmotic stress was first studied in a water-glycerol solution and the observed results were then applied to the drying process. This study have shown that the yeast was quite resistant to osmotic stress and pointed out the existence of zones of sensitivity where viability dramatically decrease as function of final osmotic pressure and temperature of the treatment. Thus, for dehydration until low osmotic pressure (133 MPa, i.e. a(w) = 0.38) results have shown that viability was better when temperature of the treatment was less than 8 degrees C or higher than 25 degrees C. Moreover, kinetic of dehydration was found to greatly influence cells recovery. CONCLUSIONS: These observations allowed the choice of parameters of dehydration of yeasts with an original drying process which involve the mix of the yeasts with wheat flour and then drying in a fluidized bed. SIGNIFICANCE AND IMPACT OF THE STUDY: This process dried rapidly the yeasts to less than 220 MPa (aw < or = 0.2) with whole cell recovery and good fermentative capabilities.

Death of Escherichia coli during rapid and severe dehydration is related to lipid phase transition.

This study reports the effects of exposure to increasing osmotic pressure on the viability and membrane structure of Escherichia coli. Changes in membrane structure after osmotic stress were investigated by electron transmission microscopy, measurement of the anisotropy of the membrane fluorescent probe DPH (1,6-diphenyl-1,3,5-hexatriene) inserted in E. coli, and Fourier infrared spectroscopy (FTIR). The results show that, above a critical osmotic pressure of 35 MPa, the viability of the bacterium is drastically reduced (2 log decrease in survivors). Electron micrographs revealed a severe contraction of the cytoplasm and the formation of membrane vesicles at 40 MPa. Changes in DPH anisotropy showed that osmotic dehydration to 40 MPa promoted a decrease in the membrane fluidity of integral cells of E. coli. FTIR measurements showed that at 10-40 MPa a transition from lamellar liquid crystal to lamellar gel among the phospholipids extracted from E. coli occurred. Bacterial death resulting from dehydration can be attributed to the conjunction between membrane deformation, caused by the volumetric contraction, and structural changes of the membrane lipids. The influence of the latter on the formation of membrane vesicles and on membrane permeabilization at lethal osmotic pressure is discussed, since vesiculation is hypothetically responsible for cell death.