Sulfur Dioxide(SO2) and Sulfites(SO3) Sulfur dioxide, Sodium sulfite(Na2SO3), Sodium bisulfide(NaHSO3), and Sodium metabisulfite(Na2S2O5) are used to control microorganisms and insects in soft fruits, lemon juices, beverages, sausages, pickles, and fresh shrimp. Currently, these additives are not permitted in U.S.A. in meat, as they destroy vitamin B1. They are more effective against molds and yeasts than bacteria.
Among bacteria, the aerobic Gram-negative rods are the most susceptible. The antimicrobial action is produced by the undissociated sulfurous acid that rapidly enters the cell and reacts with the thiol groups in structural proteins, enzymes and cofactors as well as with other cellular components. The fungicidal effect is more pronounced at low pH and low Aw. The concentrations range is between 200-300 ppm for antimicrobial uses. SO2 and sulfites are also used as antioxidants in fresh and dried fruits and vegetables to prevent browning. People with respiratory problems can be allergic to sulfites.
Hydrogen Peroxide(H2O2) A solution of H2O2(0.05-0.1%) is recommended as an antimicrobial agent in raw milk to be used in cheese processing to control growth of psychrotrophic Gram- negative bacteria that produce heat-stable enzymes. H2O2 is also used in liquid egg to facilitate destruction of Salmonella by low-heat pasteurization, and in aseptic packaging of foods, and food processing equipment. In raw milk and liquid egg, catalase is used before pasteurization to hydrolyze H2O2 to H2O and O2. H2O2 is strong oxidizing agent and the germicidal action is associated with this property. It causes damage to cellular membranes.
Epoxides(Ethylene Oxide, Propylene Oxide) Ethylene oxide and propylene oxide are used as fumigants to destroy microorganisms and insects in grains, cocoa powder, gums, nuts, dried fruits, spices, and packaging materials. They are effective against cells, spores, and viruses. Epoxides are alkylating agents and react with various groups(SH, NH2, OH) in cellular macromolecules e.g., enzymes adversely affecting their functions. They react with chlorides and form toxic compounds.
Butylated Hydroxyanisol(BHA) BHA is used at 200 pmm or less as antioxidant to delay oxidation of unsaturated lipids. It has antimicrobial activity, it inhibits growth of many Gram-positive and Gram-negative bacteria, however, some species may be resistant to it. It prevents toxin production by molds and growth of yeast. It exerts its antimicrobial action through damaging the cell membrane and enzymes. Its antimicrobial action increases in the presence of sorbate and decreases in high lipid foods and at low temperature.
Ethylenediaminetetraacetate(EDTA) The sodium and calcium salts of EDTA at 100 pmm are approved for use in foods to chelate trace metals in order to prevent their adverse effect on food quality. Because of its ability to chelate divalent cations, it can destabilize the barrier functions of Gram-negative bacteria and, to some extent the cell wall of Gram-positive bacteria. In this way it enhances the antimicrobial action of other chemicals, especially those that are acting on membranes, such as surface active compounds, lysozymes and bacteriocines. EDTA is also inhibitory for germination and outgrowth of spores of C. botulinum.
Lysozyme The enzyme lysozyme(a muramidase) is present in large quantities in some foods such as egg white and shellfish, as well as in small quantities in milk and some plant tissues. It hydrolyzes the mucopeptide layer present in the cell wall of Gram-positive bacteria and in the middle membrane of Gram-negative bacteria. Gram- negative bacteria become sensitive to the lysozyme effect only after the barrier function of the outer membrane is destabilized by chemical(.e.g., EDTA) and physical(e.g., freezing or heating) stresses.
The antimicrobial action is manifested by the lysis of cells. Lysozyme is most effective at pH 6.0-7.0 and at Concentrations of 0.01-0.1%. It can be used directly to control Gram-positive bacteria and with EDTA to control Gram- negative bacteria. It has been used in some fermentation processes to prevent growth of undesirable lactic acid bacteria.
Monolaurin(Glycerol Monolaurate) Monolaurin, the ester of lauric acid and glycerol, is one of the effective bactericidal agents. Its effectiveness in deboned meat, chicken sausages, minced fish, and other foods has been observed against undesirable bacteria, particularly the anaerobes. It enhances the thermal inactivation of spores of Bacillus spp. The antimicrobial property of this lipophilic compound is enhanced with lactate, sorbate, ascorbate and nisin, but may be reduced by starchy and proteinaceous compoundas.
In combination with monolaurin, the fungistatic activity of several antifungal compounds is enhanced. The antimicrobial activity of monolaurin is produced through its ability to destabilize the functions of the membrane. At lower concentrations, it is bacteriostatic by interfering with the uptake of nutrients. It can be used up to 500 ppm without affecting the taste of the food.
Antibiotics(Tetracyclines, Natamycin, and Tylosin) Tetracyclines(10 ppm) were approved by the food and Drug Administration(FDA) to extend the refrigerated shelf life of seafood and poultry. However, because of the possible increase in antibiotic-resistant bacteria, the use of these antibiotics in food was later banned. Natamycin is an antifungal agent. Tylosin is a bactericidal antibiotic that is more effective in Gram-positive than Gram-negative bacteria and inhibits outgrowth of germinated spores.
Spices Many spices, condiments, and plant extracts are known to contain antimicrobial compounds. Some of these include cinnamic aldehyde in cinnamon, eugenol in cloves, allspice, and cinnamon; and paramene and thymol in oregano and thyme. Because of the small amounts used as spices in foods, they do not produce antimicrobial effects. However, the antimicrobial components can be used in higher concentrations as oleoresins or essential oils. Garlic, onion ginger, cabbage, carrots and others may be used as natural preservatives, they need to be extinsively Studied.
Organic Acids Acetic Acid 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 bacteriostatic at 0.2% but bactericidal above 0.3%, and more effective against Gram-negative bacteria. Its effect is pH dependent, at lower pH it is more effective. It is added to salad dressings and mayonnaise as an antimicrobial agent.
Propionic Acid Propionic acid and its salts are used in food as a fungistatic agent, but they are also effective against both Gram-positive and Gram-negative bacteria. Propionic acid is used to control molds in cheeses, butter, and bakery products and to prevent growth of bacteria and yeasts in syrup, and some fresh fruits.
Lactic Acid 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. They have antibacterial effects in concentrations 1-2%. Growth of Gram-positive and Gram-negative bacteria is reduced, indicating increased bacteriostatic action. Below pH 5.0, lactic acid has a bactericidal effect, especially against Gram-negative bacteria.
Bacteriocins Bacteriocins are 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 and can have thioether, disulfide bridges, or free thiol groups. the presence of polar and nonpolar faces along the long axis of the molecule 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.
Bacteriocin-Producing Strains Different genera and species of lactic acid bacteria produce bacteriocins. some of the species include L.lactis, S. thermophilus, L. acidophilus, Leuconostoc mesenteroides, enterococcus faecalis. It appears that bacteriocin-producing strains of lactic acid bacteria are quite common in the food environment. Bacteriocins of lactic acid bacteria are bactericidal to sensitive cells, and death occurs very rapidly at a low concentration. Gram-positive bacteria are sensitive, Gram-negative bacteria become sensitive following damage of lipopolysaccharide by physical and chemical stresses.
Properties of Bacteriocin-Producing Lactic Acid Bacteria *A producer strain is immune to its own bacteriocin but can be sensitive or resistant to other bacteriocins. *A strain can produce more than one bacteriocin. *Strains from different species and genera can produce the same bacteriocins. *Strains from different subspecies can produce different bacteriocins. *Different species from a genus can produce different bacteriocins.
Examples of Bacteriocins Bacteriocin Producing Strain Nicin A Lactobacillus lactis Lactocin S Lactobacillus sake Pediocin AcH Pediococcus acidilactici Leucocin A Leoconostoc gelidum Plantaricin S Lactobacillus plantarum Enterocin enterococcus faecium Helveticin J Lactobacillus helvetecus
Antibacterial Actions of Bacteriocins of Lactic Acid Bacteria *They differ in the spectra of antibacterial activity against sensitive Gram-positive bacteria e.g.,nisin and pediocin have wider spectra than leucocin or sakasin. *Their MICs against a strain differ greatly e.g., pediocin is more potent than leucocin against L. monocytogenes. *Their bactericidal effeciency increases at acidic pH, higher temperature, in the presence of detergent, and during Logarithmic phase of bacterial growth. *A strain resistant to one bacteriocin can be sensitive to another one. *Injured bacteria become sensitive to a bacteriocin.
Applications of Bacteriocins Bacteriocins of food-grade lactic acid bacteria are considered safe food biopreservatives and have the potential to use them to kill sensitive Gram-positive food spoilage and foodborne pathogenic bacteria. In foods that contain Gram-negative injured cells, bacteriocins are effective in killing them. More than one bacteriocin can be used e.g., nisin and pediocin together. Bacteriocins can also have a topical therapeutic use.