How does pH effect yeast fermentation?

Bakers yeast (also known as Brewer's yeast -- it's the same organism) is a single-celled microorganism. It thrives when the pH is slightly acidic -- pH 4.5 is best. However, it will grow over a wide range, even when the pH is slightly basic. However, it will slow down as the pH gets farther from 4.5. If the pH gets too acidic or too basic, the yeast just won't grow. For bakers, the last point is the most important. A neutral to slightly acid pH will give the fastest rate of fermentation, all other things being equal. Luckily, for breadmakers, this isn't a problem unless you start adding exotic ingredients to the mix. Yeast does just fine with the pH of most bread recipes. For breadmaking, the temperature of the rise is more important than the pH.