1-20. ACIDITY AND ALKALINITY
a. Foods and the pH Scale. Most bacteria are sensitive to changes in the
acidity or alkalinity of their environment. Acidity and alkalinity are determined by the
concentration of the hydrogen ion. The symbol for the logarithm of the reciprocal of the
hydrogen ion concentration is pH. The pH scale extends from 0 to 14 with a pH of 7.0
being neutral. A solution is acidic because of an excess of positively charged hydrogen
ions or it is alkaline because of an excess of negatively charged hydroxyl ions.
Numbers below 7.0 on the pH scale indicate an acid condition with 1.0 or less being the
most acid. Numbers above 7.0 on the pH scale indicate alkalinity with 14.0 being the
Foods have been categorized according to their pH as follows:
pH below 3.7
Low- or non-acid foods
pH over 5.3
(2) The approximate pH values for selected foods are listed below. The pH
values are only approximations, since there is considerable variation in the pH of some
foods and the pH of foods and the pH of food products can change during ripening,
processing, or spoilage.
b. The Optimum pH Range for Bacteria. Bacteria have an optimum range in
which they grow and function best. For many, this is near the neutral point or in a pH
range of from 6.0 to 8.5. Some bacteria can live in or even require a very acid
environment such as a pH of 3.0 or less. Some bacteria prefer an alkaline pH of 8.0 or
higher. An important phase of bacterial physiology is the determination of the optimum,
minimum, and maximum pH for organisms being investigated.
c. The Optimum pH Range for Molds and Yeasts. You will recall that molds
can grow over a wider range of pH values than can yeasts, as evidenced by the
difference in range of ph values: molds from 2.0 to 8.5, and yeasts from 5.5 to 6.5.
Bacterial growth is favored by a near neutral pH. An example of the importance of
knowledge about pH values and food microbiology is that the food organism Clostridium
botulinum prefers a near neutral pH for growth and its growth is inhibited by increased