since pH is a logarithmic scale differences in pH of 1, 2 and 3 units
correspond to 10-, 100- and 1000-fold differences in the hydrogen ion
concentration.
The acidity or alkalinity of an environment has a profound effect on
the activity and stability of macromolecules such as enzymes, so it is not
surprising that the growth and metabolism of micro-organisms are
influenced by pH. Plotting microbial growth rate against pH produces
an approximately symmetrical bell-shaped curve spanning 2–5 pH units,
with a maximum rate exhibited over a range of 1–2 units.
In general, bacteria grow fastest in the pH range 6.0–8.0, yeasts 4.5–
6.0 and filamentous fungi 3.5–4.0. As with all generalizations there are
exceptions, particularly among those bacteria that produce quantities of
acids as a result of their energy-yielding metabolism. Examples import-
ant in food microbiology are the lactobacilli and acetic acid bacteria with
optima usually between pH 5.0 and 6.0.
Most foods are at least slightly acidic, since materials with an alkaline
pH generally have a rather unpleasant taste (Table 3.2). Egg white, where
the pH increases to around 9.2 as CO 2 is lost from the egg after laying, is
a commonplace exception to this. A somewhat more esoteric example,
which many would take as convincing evidence of the inedibility of
Table 3.2 Approximate pH ranges of some common food commodities
Chapter 3 25