Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY LOW PH AND ORGANIC ACIDS 485
port protons through the proton pump, which causes depletion in energy and a
decrease in pHi. The structures on the cell surface, outer membrane or cell wall,
inner membrane or cytoplasmic membrane, and periplasmic space are also exposed
to [H + ]. This can adversely affect the ionic bonds of the macromolecules and thus
interfere with their three-dimensional structures and some related functions. At pH
< 5.0, the undissociated molecules of some acid can be considerably high. Being
lipophilic (except citric), they enter freely through the membrane as a function of
the concentration gradient. Because the pHi is much higher than the pK of the acid,
the molecules dissociate, releasing protons and anions. Some anions (e.g., acetate
and lactate) are metabolized by several microorganisms as a C source. If they are
not metabolized, the anions are removed from the cell interior. However, the [H + ]
will reduce the internal pH and adversely affect the proton gradient. To overcome
this problem, the cells pump out the excess protons by expending energy. At lower
pHo (pH 4.5 or below), this represents an expense of a large amount of energy that
cells may not be able to generate. As a result, the internal pH drops, adversely
affecting the pH gradient. The low pH can also act on the cellular components (such
as proteins) and adversely affect their structural (by interfering with the ionic bonds)
and functional integrity. 1,3,4,7
These changes can interfere with the nutrient transport and energy generation,
and in turn interfere with microbial growth. In addition, low pH can reversibly and
irreversibly damage cellular macromolecules, which can subsequently inflict sublethal
as well as lethal injury to cells.
Low pH can alter the ionic environment of the spore coat by replacing its ions
(e.g., Ca 2+ ) with H + and make the spores unstable toward other environmental
stresses, such as heat and low A w. 1–6
A. Nature of Acids
\
Table 35.1 Influence of pH on the Amount (%) of Dissociated Ions of Weak Organic
Acids
Acid pK
4
% Dissociated at pH
5 6
Acetic 4.8 15.5 65.1 94.9
Propionic 4.9 12.4 58.3 93.3
Lactic 3.8 60.8 93.9 99.3
Citric 3.1 81.1 99.6 >99.1
Sorbic 4.8 18.0 70.0 95.9
Benzoic 4.2 40.7 87.2 98.6
Parabena 8.5