Food Protection with Chemicals, and by Biocontrol 305
S. aureus and a spoilage Clostridium (putrefactive anaerobe [P.A.] 3679). With the latter, a noninhibitory concentration of nitrite and sorbate was bactericidal.
162
The widest use of sorbates is as a fungistat in products such as cheeses, bakery products, fruit juices,
beverages, salad dressings, and the like. In the case of molds, inhibition may be due to inhibition of
the dehydrogenase enzyme system. Against germinating endospores, sorbate prevents the outgrowth
of vegetative cells (Figure 13–1).
As lipophilic acids, sorbate, benzoate, and propionate appear to inhibit microbial cells by the same
general mechanism. The mechanism involves the proton motive force (PMF). Briefly, hydrogen ions
(protons) and hydroxyl ions are separated by the cytoplasmic membrane, with the former, outside
the cell, giving rise to acidic pH and the latter, inside the cell, giving rise to pH near neutrality. The
membrane gradient thus created represents electrochemical potential that the cell employs in the active
transport of some compounds such as amino acids. Weak lipophilic acids act as protonophores. After
diffusing across the membrane, the undissociated molecule ionizes inside the cell and lowers intracellular pH. This results in a weakening of the transmembrane gradient such that amino acid transport
is affected adversely. This hypothesis has been supported by research on P.A. 3679 where sorbate
inhibited phenylalanine uptake, decreased protein synthesis, and altered phosphorylated nucleotide
accumulation.
162,163 Although alteration of the PMF by lipophilic acids has wide support, other factors
may be involved in their mode of action.
52 For example, a H
+ -ATPase in the plasma membrane of
S. cerevisiae aids in maintenance of cell homeostasis by exporting protons. The efficacy of this plasma
membrane appears to be responsible, at least in part, for the adaptation of S. cerevisiae cells to sorbic
acid.
85 With respect to safety, sorbic acid is metabolized in the body to CO 2 and H 2 O in the same
manner as fatty acids normally found in foods.
44
THE PROPIONATES
Propionic acid is a three-carbon organic acid with the structure CH 3 CH 2 COOH. This acid and its
calcium and sodium salts are permitted in breads, cakes, certain cheese, and other foods, primarily as
a mold inhibitor. Propionic acid is employed also as a “rope” inhibitor in bread dough. The tendency
toward dissociation is low with this compound and its salts, and they are consequently active in lowacid foods. They tend to be highly specific against molds, with the inhibitory action being primarily
fungistatic rather than fungicidal.
With respect to the antimicrobial mode of action of propionates, they act in a manner similar to that
of benzoate and sorbate. The pK of propionate is 4.87 and at a pH of 4.00, 88% of the compound is
undissociated, whereas at a pH of 6.0, only 6.7% remains undissociated. The undissociated molecule
of this lipophilic acid is necessary for its antimicrobial activity. The mode of action of propionic acid
is noted above with benzoic acid. See also the section on medium-chain fatty acids and esters in this
chapter, and the review by Doores
47 for further information.
SULFUR DIOXIDE AND SULFITES
Sulfur dioxide (SO 2 ) and the sodium and potassium salts of sulfite (=SO 3 ), bisulfite (–HSO 3 ), and
metabisulfite (=S 2 O 5 ) all appear to act similarly and are treated together here. Sulfur dioxide is used
in its gaseous or liquid form or in the form of one or more of its neutral or acid salts on dried fruits, in
lemon juice, molasses, wines, fruit juices, and others. The parent compound has been used as a food
preservative since ancient times. Its use as a meat preservative in the United States dates back to at
S. aureus and a spoilage Clostridium (putrefactive anaerobe [P.A.] 3679). With the latter, a noninhibitory concentration of nitrite and sorbate was bactericidal.
162
The widest use of sorbates is as a fungistat in products such as cheeses, bakery products, fruit juices,
beverages, salad dressings, and the like. In the case of molds, inhibition may be due to inhibition of
the dehydrogenase enzyme system. Against germinating endospores, sorbate prevents the outgrowth
of vegetative cells (Figure 13–1).
As lipophilic acids, sorbate, benzoate, and propionate appear to inhibit microbial cells by the same
general mechanism. The mechanism involves the proton motive force (PMF). Briefly, hydrogen ions
(protons) and hydroxyl ions are separated by the cytoplasmic membrane, with the former, outside
the cell, giving rise to acidic pH and the latter, inside the cell, giving rise to pH near neutrality. The
membrane gradient thus created represents electrochemical potential that the cell employs in the active
transport of some compounds such as amino acids. Weak lipophilic acids act as protonophores. After
diffusing across the membrane, the undissociated molecule ionizes inside the cell and lowers intracellular pH. This results in a weakening of the transmembrane gradient such that amino acid transport
is affected adversely. This hypothesis has been supported by research on P.A. 3679 where sorbate
inhibited phenylalanine uptake, decreased protein synthesis, and altered phosphorylated nucleotide
accumulation.
162,163 Although alteration of the PMF by lipophilic acids has wide support, other factors
may be involved in their mode of action.
52 For example, a H
+ -ATPase in the plasma membrane of
S. cerevisiae aids in maintenance of cell homeostasis by exporting protons. The efficacy of this plasma
membrane appears to be responsible, at least in part, for the adaptation of S. cerevisiae cells to sorbic
acid.
85 With respect to safety, sorbic acid is metabolized in the body to CO 2 and H 2 O in the same
manner as fatty acids normally found in foods.
44
THE PROPIONATES
Propionic acid is a three-carbon organic acid with the structure CH 3 CH 2 COOH. This acid and its
calcium and sodium salts are permitted in breads, cakes, certain cheese, and other foods, primarily as
a mold inhibitor. Propionic acid is employed also as a “rope” inhibitor in bread dough. The tendency
toward dissociation is low with this compound and its salts, and they are consequently active in lowacid foods. They tend to be highly specific against molds, with the inhibitory action being primarily
fungistatic rather than fungicidal.
With respect to the antimicrobial mode of action of propionates, they act in a manner similar to that
of benzoate and sorbate. The pK of propionate is 4.87 and at a pH of 4.00, 88% of the compound is
undissociated, whereas at a pH of 6.0, only 6.7% remains undissociated. The undissociated molecule
of this lipophilic acid is necessary for its antimicrobial activity. The mode of action of propionic acid
is noted above with benzoic acid. See also the section on medium-chain fatty acids and esters in this
chapter, and the review by Doores
47 for further information.
SULFUR DIOXIDE AND SULFITES
Sulfur dioxide (SO 2 ) and the sodium and potassium salts of sulfite (=SO 3 ), bisulfite (–HSO 3 ), and
metabisulfite (=S 2 O 5 ) all appear to act similarly and are treated together here. Sulfur dioxide is used
in its gaseous or liquid form or in the form of one or more of its neutral or acid salts on dried fruits, in
lemon juice, molasses, wines, fruit juices, and others. The parent compound has been used as a food
preservative since ancient times. Its use as a meat preservative in the United States dates back to at