Fundamental Food Microbiology, Third Edition - Fuad Fathir

FOOD BIOPRESERVATIVES OF MICROBIAL ORIGIN 229
thiocyanate (SCN – ). In the presence of H 2O 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 Gramnegative
bacteria that are especially susceptible. This system 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,
catalase (0.1 to 0.5 g/1000 lb [455 Kg]) is added to remove the residual H 2O 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.
D. Reuterine
Some strains of Lactobacillus reuteri, found in the gastrointestinal tract of humans
and animals, produce a small molecule, reuterine (b-hydroxypropionaldehyde;
CHO–CH 2–CH 2OH), which is antimicrobial against Gram-positive and Gram-negative
bacteria. 6 It produces an antibacterial action by inactivating some important
enzymes, such as ribonucleotide reductase. 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
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IV. BACTERIOCINS OF LACTIC ACID BACTERIA
AS FOOD PRESERVATIVES
The term bacteriocin is currently used to refer to a group of bioactive peptides
produced by many bacterial strains from Gram-positive and Gram-negative groups.
The bacteriocins produced by many strains of lactic acid bacteria and some propionic
acid bacteria are of special interest in food microbiology because of their bactericidal
effect normally to different Gram-positive spoilage and pathogenic bacteria and
under stressed conditions to different Gram-negative bacteria important in food.
Chemically, bacteriocin peptides are ribosomally synthesized, cationic, amphipathic,
have a-helical or b-sheet structures, or both, and can have thioethers, disulfide
bridges, or free thiol groups. The presence of an amphipathic a-helical structure
with opposing polar and nonpolar faces along the long axis enables bacteriocins to
interact with both the aqueous and lipid phases when bound to the surface of the
membrane of a sensitive bacterial cell, leading to its functional destabilization and
death of the cell. This aspect is discussed later. 8