Fermentation is an anaerobic process in which sugar molecules are broken down into simpler molecules. Some of the commonly known products of the fermentation process are bread, beer, wine, etc.
The fermentation process is affected by a number of factors, including pH, temperature, nutrient concentration, and availability. Among the microorganisms, yeast and mold are acid-tolerant and prefer lower pH conditions for their growth. This means that fermentation by yeast and mold will be faster at lower pH values. In fact, the best pH range for yeast fermentation is 4.5-5.5.
One of the reasons, lower pH increases the fermentation rate is because the pH affects the shape of the enzymes (proteins) that help fermentation reactions. The amino acids that form the enzymes can be either acidic or basic depending on the functional group attached to them. It has been found that at lower pH, the acidic amino acids are present and that fermentation rate is faster as compared to higher pH conditions. In addition, it should be noted that most of the natural sugar from the fruits is generated in the acidic conditions. This means that yeast (and the associated enzymes for the fermentation process) have been naturally selected for working under the acidic conditions. And hence lower pH conditions are preferable for the fermentation process.