Q. Effect on Microbial Growth of pH?
Every microorganism has a minimal, a maximal, and an optimal pH for growth. In general, bacteria grow in the pH range of 6.0-8.0, yeasts 4.5-6.0 and filamentous fungi 3.5-4.0. Molds can grow over a wider range of pH than most yeasts and bacteria, and many molds grow at acidities too high for yeasts and bacteria. Most fermentative yeasts grow well in pH range of about 4.0 to 4.5, eg. fruit juices, and film yeasts grow well on acid foods, such as sauerkraut and pickles. On the other hand, most yeast does not grow well in alkaline foods and thus do not have a significant role to play in the spoilage of food products with high pH. However, large number of yeasts grows well in near neutral pH.
There are some exceptions for example some bacteria can grow in moderate acidity particularly those bacteria that produce acids as a result of their activities like lactobacilli and acetic acid bacteria. These have pH optima between 5.0 and 6.0 and others like the proteolytic bacteria can grow in foods with a high (alkaline) pH, as found in the stored egg white. Bacteria are more sensitive to pH than molds and yeasts, with the pathogenic bacteria being the most sensitive amongst them.