Thermal Food Processing

pH-Assisted Thermal Processing 571
Apart from the free proton concentration itself (that is, the pH value), microorganisms
are affected by the concentration of nondissociated weak acid, which is
also dependent on the pH value. It has been shown that both the ionized and the
nonionized forms of an organic acid can contribute to its inhibitory effect,
although the undissociated form is generally more inhibitory. 19
Some weak acids (acetic and lactic acids, for example) are lipophilic in their
undissociated form and pass readily through the plasma membrane, entering the
bacterial cell. In the cytoplasm at a pH of approximately 7, the acid molecules
dissociate, releasing protons that cannot pass across the lipid bilayer (only by active
transport) and accumulate in the cytoplasm, thus lowering the internal pH. 5 Acidified
cytoplasm in turn inhibits metabolism and consequently growth. Moreover, the dissociated
form of these acids may have inhibitory effects for the cell at high concentrations.
Other weak acids do not pass through the bacterial membrane, and hence
do not acidify the cytoplasm, or as proposed for, sorbic acid 20 could act as membraneactive
compounds. The mechanism for strong acids, such as hydrochloric or phosphoric,
is to decrease considerably the external pH, providing a high environmental
concentration of protons that determines the acidification of the cytoplasm of the cell.
Some acids are very effective inhibitors of microbial growth and are intentionally
added to many foods as preservatives. This is the case of benzoic and
sorbic acids. 16 Others, such as acetic, fumaric, propionic, or lactic, simply prevent
or delay the growth of pathogenic and spoilage bacteria. 16
Hsiao and Siebert, 21 after studying 11 physical and chemical properties of
17 organic acids commonly found in food systems, attributed the differences in
their inhibitory effect to fundamental properties of them, including polar groups,
number of double bonds, molecular size, and solubility in nonpolar solvents.
However, these same authors found different patterns of acid resistance when
studying the effects of the same acids on six test bacteria. This could mean that
although the effect of different acids can be predictable for some known bacterial
species (in function of their molecular properties), the susceptibility of a new
bacteria should have to be investigated before any conjecture can be established.
Still, acetic followed by lactic acids are commonly found as the most inhibitory
acids for many bacteria, 15,22–24 and this could be related to their low molecular
weight and high dissociation constant (pK a) values. Probably, these are the most
important properties of weak acids, 25 because they determine the capability of
acids to enter bacterial cells in an undissociated form. Acids are only able to enter
the cell when they are in the undissociated form. Once inside the cell, they can
dissociate and play their roles inside. 26 The amount of undissociated acid present
in a system depends on its pH, the pK a of the acid, and its concentration. At a
given pH value, acids with the higher pK a would show the higher proportion
of undissociated acid. 25 When an acid has several acid groups, the values of
pK a1 …pK an are the dissociation constants of the protons and the value pK a1 (the
lowest pK a value) corresponds to the first proton, that is, to the balance of undissociated
(the form in which the acid may enter the cell)/dissociated acid. Acids
with a higher pK a1 value, such as acetic or lactic, would have higher proportions
of the undissociated molecule at higher pH values, hence being more effective.