Food Biopreservatives of Microbial Origin, Bacteriocin, and Nanotechnology ◾ 213
Propionibacterium spp. They are on the generally regarded as safe (GRAS) list and are used in
many foods as additives to enhance flavor and shelf life and as safety precautions against undesirable microorganisms. These acids and their salts are used in foods at approximately a 1%–2%
level 3,4 (also see Chapter 36).
Acetic acid, its salts, and vinegar (which contains 5%–40% acetic acid and many other compounds that give it the characteristic aroma) are used in different foods for inhibiting growth and
reducing the viability of Gram-positive and Gram-negative bacteria, yeasts, and molds. Acetic
acid is generally bacteriostatic at 0.2% but bactericidal above 0.3% and more effective against
Gram-negative bacteria. However, this effect is pH dependent, and the bactericidal effect is more
pronounced at low pH (below pH 4.5). It is added to salad dressings and mayonnaise as an antimicrobial agent. It is permitted to be used as a carcass wash.
Propionic acid and its salts are used in food as a fungistatic agent, but they are also effective
in controlling growth and reducing viability of both Gram-positive and Gram-negative bacteria.
Gram-negative bacteria seem to be more sensitive at pH 5.0 and below, even at acid levels of
0.1%–0.2%. Propionic acid is used to control molds in cheeses, butter, and bakery products and
to prevent growth of bacteria and yeasts in syrup, applesauce, and some fresh fruits.
Lactic acid and its salts are used in food more for flavor enhancement than for their antibacterial effect, especially when used above pH 5.0. However, recent studies have shown that they have
a definite antibacterial effect when used in foods at 1%–2% levels even at or above pH 5.0. Growth
of both Gram-positive and Gram-negative bacteria is reduced, indicating increased bacteriostatic
action. Below pH 5.0, lactic acid can have a bactericidal effect, especially against Gram-negative
bacteria. It may not have any fungistatic effect in the food environment. It is used in many processed meat products and has also been recommended as a carcass wash.
The antimicrobial effect of these three acids is considered to be a result of their undissociated
molecules. The dissociation constants (pKa) are 4.8 for acetic, 4.9 for propionic, and 3.8 for lactic
acid. Thus, at most food pH (5.0 and above), the undissociated fractions of the three acids can be
quite low, the lowest being for lactic acid. The lower antimicrobial effectiveness of lactic acid is
probably a result of its low pKa. The antimicrobial action of the undissociated molecules is produced by dissociation of the molecules in the cytoplasm following their entry through the membrane. H + released following dissociation initially reduces the transmembrane proton gradient and
neutralizes the proton motive force and then reduces the internal pH, causing denaturation of
proteins and viability loss. However, other studies have suggested that these weak acids produce
antimicrobial action through the combined effects of undissociated molecules and dissociated
ions. The acids can also induce sublethal injury of the cells and increase their chance of viability
loss. Undissociated molecules as well as dissociated ions can induce cellular injury. 3,4
Several pathogenic bacterial strains, such as some strains of Salmonella Typhimurium, and
Escherichia coli O157:H7, have been found to be relatively resistant to low pH because of their ability
to overproduce some proteins induced by the acid environment. These proteins, also called stress proteins, enable the cells to withstand lower internal pH (see Chapter 10). The presence of such a strain
in food, which is freshly acidified at a 1%–2% level to control microorganisms, may pose a problem.
Diacetyl
Diacetyl is produced by several species of lactic acid bacteria in large amounts, particularly through
the metabolism of citrate (see Chapter 12). 5 Several studies have shown that it is antibacterial
against many Gram-positive and Gram-negative bacteria. Gram-negative bacteria are particularly
sensitive at pH 5.0 or below. Diacetyl is effective at approximately 0.1%–0.25%. Recent studies
Propionibacterium spp. They are on the generally regarded as safe (GRAS) list and are used in
many foods as additives to enhance flavor and shelf life and as safety precautions against undesirable microorganisms. These acids and their salts are used in foods at approximately a 1%–2%
level 3,4 (also see Chapter 36).
Acetic acid, its salts, and vinegar (which contains 5%–40% acetic acid and many other compounds that give it the characteristic aroma) are used in different foods for inhibiting growth and
reducing the viability of Gram-positive and Gram-negative bacteria, yeasts, and molds. Acetic
acid is generally bacteriostatic at 0.2% but bactericidal above 0.3% and more effective against
Gram-negative bacteria. However, this effect is pH dependent, and the bactericidal effect is more
pronounced at low pH (below pH 4.5). It is added to salad dressings and mayonnaise as an antimicrobial agent. It is permitted to be used as a carcass wash.
Propionic acid and its salts are used in food as a fungistatic agent, but they are also effective
in controlling growth and reducing viability of both Gram-positive and Gram-negative bacteria.
Gram-negative bacteria seem to be more sensitive at pH 5.0 and below, even at acid levels of
0.1%–0.2%. Propionic acid is used to control molds in cheeses, butter, and bakery products and
to prevent growth of bacteria and yeasts in syrup, applesauce, and some fresh fruits.
Lactic acid and its salts are used in food more for flavor enhancement than for their antibacterial effect, especially when used above pH 5.0. However, recent studies have shown that they have
a definite antibacterial effect when used in foods at 1%–2% levels even at or above pH 5.0. Growth
of both Gram-positive and Gram-negative bacteria is reduced, indicating increased bacteriostatic
action. Below pH 5.0, lactic acid can have a bactericidal effect, especially against Gram-negative
bacteria. It may not have any fungistatic effect in the food environment. It is used in many processed meat products and has also been recommended as a carcass wash.
The antimicrobial effect of these three acids is considered to be a result of their undissociated
molecules. The dissociation constants (pKa) are 4.8 for acetic, 4.9 for propionic, and 3.8 for lactic
acid. Thus, at most food pH (5.0 and above), the undissociated fractions of the three acids can be
quite low, the lowest being for lactic acid. The lower antimicrobial effectiveness of lactic acid is
probably a result of its low pKa. The antimicrobial action of the undissociated molecules is produced by dissociation of the molecules in the cytoplasm following their entry through the membrane. H + released following dissociation initially reduces the transmembrane proton gradient and
neutralizes the proton motive force and then reduces the internal pH, causing denaturation of
proteins and viability loss. However, other studies have suggested that these weak acids produce
antimicrobial action through the combined effects of undissociated molecules and dissociated
ions. The acids can also induce sublethal injury of the cells and increase their chance of viability
loss. Undissociated molecules as well as dissociated ions can induce cellular injury. 3,4
Several pathogenic bacterial strains, such as some strains of Salmonella Typhimurium, and
Escherichia coli O157:H7, have been found to be relatively resistant to low pH because of their ability
to overproduce some proteins induced by the acid environment. These proteins, also called stress proteins, enable the cells to withstand lower internal pH (see Chapter 10). The presence of such a strain
in food, which is freshly acidified at a 1%–2% level to control microorganisms, may pose a problem.
Diacetyl
Diacetyl is produced by several species of lactic acid bacteria in large amounts, particularly through
the metabolism of citrate (see Chapter 12). 5 Several studies have shown that it is antibacterial
against many Gram-positive and Gram-negative bacteria. Gram-negative bacteria are particularly
sensitive at pH 5.0 or below. Diacetyl is effective at approximately 0.1%–0.25%. Recent studies