Control by Low pH and Organic Acids ◾ 477
can adversely affect the ionic bonds of the macromolecules and thus interfere with their threedimensional 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 carbon 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. 2–4
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 .
influencing Factors
Nature of Acids
The weak organic acids used in food vary in antimicrobial effectiveness because of their differences in pK. 4 An acid with higher pK has proportionately higher amounts of undissociated molecules at a food pH and is more antimicrobial. Limited studies have revealed that, in general,
under similar conditions, the antimicrobial effectiveness of four acids follows the order acetic
> propionic > lactic > citric. Similarly, at lower pH and higher concentrations, an acid is more
antimicrobial. Solubility of the acids in water is also important for the desirable effect. Acetate,
propionate, lactate, and citrate are very soluble in water, whereas benzoate (50% g), sorbate
(0.16% g), and paraben (0.02%–0.16% g) are poorly soluble in water and, thus, at the same
concentration, have different effectiveness. In many studies, the antimicrobial effectiveness of
these acids against microorganisms is studied on a percentage basis (g in 100 mL). However,
they vary in molecular weight; thus, at the same concentration, they have different numbers
of molecules and produce different concentrations of undissociated molecules as well as dissociated ions. For a comparison, it is better to use the acids on a molar concentration basis (see
Chapter 17).
Organic acids also differ in their lipophilic properties, which, in turn, regulate their ease in
entering the cells. Acetic and propionic acids are more lipophilic than lactic acid and have more
antimicrobial effectiveness than lactic acid. Citrate is transported through the membrane by a
specific transport mechanism (citrate permease) and is less effective than lipophilic acids. Many
microorganisms can metabolize the anions of weak acids, such as acetate, lactate, and citrate. Use
of salts of these acids may decrease the antimicrobial effect at higher pH. Some acids show synergistic effects when used in suitable combinations (e.g., acetic and lactic acids; propionic and sorbic
acids) or with another preservative (e.g., benzoic acid with nisin; propionic, acetic, or lactic acid
with nisin or pediocin AcH; propionate or benzoate with CO 2 ). 1,4,5
can adversely affect the ionic bonds of the macromolecules and thus interfere with their threedimensional 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 carbon 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. 2–4
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 .
influencing Factors
Nature of Acids
The weak organic acids used in food vary in antimicrobial effectiveness because of their differences in pK. 4 An acid with higher pK has proportionately higher amounts of undissociated molecules at a food pH and is more antimicrobial. Limited studies have revealed that, in general,
under similar conditions, the antimicrobial effectiveness of four acids follows the order acetic
> propionic > lactic > citric. Similarly, at lower pH and higher concentrations, an acid is more
antimicrobial. Solubility of the acids in water is also important for the desirable effect. Acetate,
propionate, lactate, and citrate are very soluble in water, whereas benzoate (50% g), sorbate
(0.16% g), and paraben (0.02%–0.16% g) are poorly soluble in water and, thus, at the same
concentration, have different effectiveness. In many studies, the antimicrobial effectiveness of
these acids against microorganisms is studied on a percentage basis (g in 100 mL). However,
they vary in molecular weight; thus, at the same concentration, they have different numbers
of molecules and produce different concentrations of undissociated molecules as well as dissociated ions. For a comparison, it is better to use the acids on a molar concentration basis (see
Chapter 17).
Organic acids also differ in their lipophilic properties, which, in turn, regulate their ease in
entering the cells. Acetic and propionic acids are more lipophilic than lactic acid and have more
antimicrobial effectiveness than lactic acid. Citrate is transported through the membrane by a
specific transport mechanism (citrate permease) and is less effective than lipophilic acids. Many
microorganisms can metabolize the anions of weak acids, such as acetate, lactate, and citrate. Use
of salts of these acids may decrease the antimicrobial effect at higher pH. Some acids show synergistic effects when used in suitable combinations (e.g., acetic and lactic acids; propionic and sorbic
acids) or with another preservative (e.g., benzoic acid with nisin; propionic, acetic, or lactic acid
with nisin or pediocin AcH; propionate or benzoate with CO 2 ). 1,4,5