Yeast Tolerance to Acidity
Yeast exhibits a considerable tolerance to extremes of pH, being able to maintain an active fermentation in a 5% glucose solution in the pH range of 2.4 to 7.4, but ceasing activity at pH 2.0 or pH 8.0. For optimum results, good practice dictates that the pH of the fermenting medium be maintained within the range of about 4.0 to 6. A drop of more than 50% in fermentative activity has been observed at pH 3.5. More gradual declines in yeast activity were encountered at higher pH levels, with measurable effects showing up at pH values over 6.0.
The explanation for the yeast's ability to maintain a relatively constant activity over a 100-fold change in hydrogen ion concentration (pH 4 to 6) is found in the fact that the pH of the cell interior of the yeast remains quite constant at about pH 5.8, regardless of any relatively wide pH variations in the fermenting medium. The enzymes involved in fermentation thus operate in an optimum pH environment within the yeast cell that is largely unaffected by external changes in pH.
