214 ◾ Fundamental Food Microbiology
have shown that in combination with heat, diacetyl is more bactericidal than when used alone.
Diacetyl has an intense aroma, and thus its use is probably limited to some dairy-based products
in which its flavor is not unexpected. Also, it is quite volatile and thus may lose its effectiveness
in foods that are expected to have a long storage life. Under reduced conditions, it is converted
to acetoin, which may have reduced antibacterial effects. This will pose difficulties in its use in
vacuum-packaged products. The antibacterial action is probably produced by deactivating some
important enzymes. The dicarbonyl group (–CO–CO–) of diacetyl reacts with arginine in the
enzymes and modifies their catalytic sites. 5
Hydrogen Peroxide
Some lactic acid bacteria produce H 2 O 2 under aerobic conditions of growth, and, because of
the lack of intracellular catalase, pseudocatalase, or peroxidase, they release it into the environment to protect themselves from its antimicrobial action. Some strains can produce, under proper
growth conditions, enough H 2 O 2 to induce bacteriostatic (6–8 μg/mL) but rarely bactericidal
action (30–40 μg/mL). 6 It is a strong oxidizing agent and can be antimicrobial against bacteria,
fungi, and viruses (also bacteriophages). Under anaerobic conditions, very little H 2 O 2 is expected
to be produced by these strains. In refrigerated raw milk, the antibacterial action produced by
adding nongrowing cells of mesophilic lactic acid bacteria is thought to be a result of the ability
of H 2 O 2 of bacteria to activate the lactoperoxidase-thiocyanate system in raw milk. 7 Raw milk
contains lactoperoxidase enzymes and thiocyanate (SCN – ). In the presence of H 2 O 2 , lactoperoxidase generates a hypothiocyanate anion (OSCN – ), which, at milk pH, can be in equilibrium with
hypothiocyanous acid (HOSCN). Both OSCN – and HOSCN are strong oxidizing agents and can
oxidize the –SH group of proteins, such as membrane proteins of Gram-negative bacteria that are
especially susceptible. In addition, this system has inhibitory activity against other bacteria, fungi,
and viruses. It is inactivated by pasteurization.
Hydrogen peroxide is permitted in refrigerated raw milk and raw liquid eggs (ca. 25 ppm) to
control spoilage and pathogenic bacteria. Before pasteurization of these foods, catalase (0.1–0.5 g/
1000 lb [455 kg]) is added to remove the residual H 2 O 2 . Its antibacterial action is attributed to its
strong oxidizing property and its ability to damage cellular components, especially the membrane.
Because of its oxidizing property, it can produce undesirable effects in food quality, such as discoloration in processed meat, and thus has limited use in food preservation. However, its application
in some food processing and equipment sanitation is being studied.
Reuterine
Some strains of Lactobacillus reuteri, found in the gastrointestinal tract of humans and animals,
produce a small molecule, reuterine (β-hydroxypropionaldehyde; CHO–CH 2 –CH 2 OH), which
is inhibitory to many Gram-positive (Listeria monocytogenes, Staphylococcus aureus) and Gramnegative (Salmonella enterica serovar Choleraesuis, Yersinia enterocolitica, Aeromonas hydrophila
and Campylobacter jejuni) bacteria. 6,8 It produces an antibacterial action by inactivating some
important enzymes, such as ribonucleotide reductase. Reuterin is active over a wide range of pH
and resistant to a variety of proteolytic and lipolytic enzymes present in foods. However, reuterine
is produced by the strains only when glycerol is supplied in the environment, which limits its use
in food preservation. In limited studies, food supplemented with glycerol and inoculated with
reuterine-producing Lab. reuteri effectively controlled growth of undesirable bacteria. Addition of
reuterine to certain foods also effectively controlled growth of undesirable bacteria. 6
have shown that in combination with heat, diacetyl is more bactericidal than when used alone.
Diacetyl has an intense aroma, and thus its use is probably limited to some dairy-based products
in which its flavor is not unexpected. Also, it is quite volatile and thus may lose its effectiveness
in foods that are expected to have a long storage life. Under reduced conditions, it is converted
to acetoin, which may have reduced antibacterial effects. This will pose difficulties in its use in
vacuum-packaged products. The antibacterial action is probably produced by deactivating some
important enzymes. The dicarbonyl group (–CO–CO–) of diacetyl reacts with arginine in the
enzymes and modifies their catalytic sites. 5
Hydrogen Peroxide
Some lactic acid bacteria produce H 2 O 2 under aerobic conditions of growth, and, because of
the lack of intracellular catalase, pseudocatalase, or peroxidase, they release it into the environment to protect themselves from its antimicrobial action. Some strains can produce, under proper
growth conditions, enough H 2 O 2 to induce bacteriostatic (6–8 μg/mL) but rarely bactericidal
action (30–40 μg/mL). 6 It is a strong oxidizing agent and can be antimicrobial against bacteria,
fungi, and viruses (also bacteriophages). Under anaerobic conditions, very little H 2 O 2 is expected
to be produced by these strains. In refrigerated raw milk, the antibacterial action produced by
adding nongrowing cells of mesophilic lactic acid bacteria is thought to be a result of the ability
of H 2 O 2 of bacteria to activate the lactoperoxidase-thiocyanate system in raw milk. 7 Raw milk
contains lactoperoxidase enzymes and thiocyanate (SCN – ). In the presence of H 2 O 2 , lactoperoxidase generates a hypothiocyanate anion (OSCN – ), which, at milk pH, can be in equilibrium with
hypothiocyanous acid (HOSCN). Both OSCN – and HOSCN are strong oxidizing agents and can
oxidize the –SH group of proteins, such as membrane proteins of Gram-negative bacteria that are
especially susceptible. In addition, this system has inhibitory activity against other bacteria, fungi,
and viruses. It is inactivated by pasteurization.
Hydrogen peroxide is permitted in refrigerated raw milk and raw liquid eggs (ca. 25 ppm) to
control spoilage and pathogenic bacteria. Before pasteurization of these foods, catalase (0.1–0.5 g/
1000 lb [455 kg]) is added to remove the residual H 2 O 2 . Its antibacterial action is attributed to its
strong oxidizing property and its ability to damage cellular components, especially the membrane.
Because of its oxidizing property, it can produce undesirable effects in food quality, such as discoloration in processed meat, and thus has limited use in food preservation. However, its application
in some food processing and equipment sanitation is being studied.
Reuterine
Some strains of Lactobacillus reuteri, found in the gastrointestinal tract of humans and animals,
produce a small molecule, reuterine (β-hydroxypropionaldehyde; CHO–CH 2 –CH 2 OH), which
is inhibitory to many Gram-positive (Listeria monocytogenes, Staphylococcus aureus) and Gramnegative (Salmonella enterica serovar Choleraesuis, Yersinia enterocolitica, Aeromonas hydrophila
and Campylobacter jejuni) bacteria. 6,8 It produces an antibacterial action by inactivating some
important enzymes, such as ribonucleotide reductase. Reuterin is active over a wide range of pH
and resistant to a variety of proteolytic and lipolytic enzymes present in foods. However, reuterine
is produced by the strains only when glycerol is supplied in the environment, which limits its use
in food preservation. In limited studies, food supplemented with glycerol and inoculated with
reuterine-producing Lab. reuteri effectively controlled growth of undesirable bacteria. Addition of
reuterine to certain foods also effectively controlled growth of undesirable bacteria. 6