Inhibitory Effect of Decanoic Acid on Yeast Growth at Various pHs and Ethanol Concentrations

A multiplex batch calorimeter was used to monitor the heat evolution during incubation of yeast cultures in the presence of decanoic acid at various concentrations. The changes observed in the recorded growth thermograms when the concentration of decanoic acid increased were quantitatively described using the values of the growth rate constantμ determined for each culture from the calorimetric recording. This made possible the calculation of the 50% inhibitory concentration (K μ) and the 100% inhibitory concentration (MIC μ) of decanoic acid in relation to the growth of four yeast strains at 30°C. By using adequate control cultures, it was possible to distinguish the separate inhibitory effect of decanoic acid and ethanol, as well as their combined effect. Other experiments with a strain of Saccharomyces cerevisiae showed that the “apparent” Kμ and MIC μ values (calculated on the basis of the total decanoic acid added to the cultures) increased with the pH, while the corresponding K μ and MIC μ values calculated on the basis of the amount of undissociated decanoic acid remained relatively constant, indicating that the toxic action is due mainly to the undissociated form of the acid. The “apparent” K μ and MIC μ values of decanoic acid at pH 5.5 also depended on the ethanol concentration in the medium, showing a marked decrease when the initial ethanol concentration varied from 0.33% to about 1.0% (v/v), and then decreasing only slightly when the concentration of ethanol increased to 6.25% (v/v).