Effects of low temperatures (9-33 degrees C) and pH (3.3-5.7) in the loss of Saccharomyces cerevisiae viability by combining lethal concentrations of ethanol with octanoic and decanoic acids.

Octanoic and decanoic acids increase the rate of loss of Saccharomyces cerevisiae viability caused by lethal concentrations of ethanol, the specific death rate being an exponential function of the acid concentration. The highly liposoluble decanoic acid is the most effective. The fatty acids deleterious effect increases at pH below pKa (4.9) mainly due to the increase of the undissociated form concentration. The temperature effects (range 9 33 degrees C; at pH 3.9) on the kinetics of the toxin(s)-induced death suggest that the deleterious action of ethanol, octanoic acid and decanoic acid have the same biological target sites, probably related to transport processes across membranes, particularly the plasma membrane. In fact, the enthalpies of activation of octanoic acid- and decanoic acid-enhanced-ethanol-induced death were similar and close to the enthalpy of activation of ethanol-induced death. This average value (delta H++ = 11.4 +/- 2.7 kcal/mol) is of the order of magnitude of that of solute transport across plasma membranes. Results clearly suggest the important contribution of octanoic and decanoic acids, combined with ethanol, in the loss of yeast viability at the last steps of industrial ethanolic fermentations, particularly those carried out at low or intermediate temperatures. They also support the combination of lipophilic acids with low pH in food preservation.