Biopreservation refers to the use of natural nonpathogenic microflora and their antibacterial products to inhibit or destroy microorganisms to enhance food safety and extend its shelf life. In order to preserve food against food spoilage microbiota, food industry makes use of chemical preservatives or physical treatments. But these techniques have several drawbacks which include the toxicity of the chemical preservatives and alteration of the organoleptic and nutritional properties of foods. In order to achieve improved food safety, traditional means of preserving food in combination with other preservative techniques can be used to effectively control spoilage bacteria and can inhibit the activities of a wide spectrum of organisms without changing the physicochemical nature of the food.
Bio preservative agents and modes of action
Lactic acid bacteria (LAB)
The biopreservation of food utilizing lactic acid bacteria (LAB) and their metabolites that are inhibitory to food spoilage microbes has been practiced since early ages. Lactic acid bacteria have a major potential for use in biopreservation because of their antagonistic properties. They are safe to consume and during storage, they naturally dominate the microflora of many foods. Some LAB produces metabolites which include active antimicrobials such as lactic and acetic acid, hydrogen peroxide, and peptide bacteriocin nisin, which inhibit pathogenic bacteria.
Bacteriocins are peptides or complex proteins biologically active with antimicrobial action against other bacteria. On the basis of structure and mode of action LAB bacteriocins are divided into 4 major groups.
Bacteriocin possesses following characteristics that make them suitable candidates for use as food preservatives, such as:
• Protein nature
• Non-toxic to non-immunogenic
• Inactive against eukaryotic cells
• Thermo-resistant (can maintain antimicrobial activity after pasteurization and sterilization)
• Broad bactericidal activity, bacteriocin have the potential to kill most of the Gram-positive bacteria and some, damaged, Gram-negative bacteria including various pathogens such as Listeria monocytogenes, Bacillus cereus, Staphylococcus aureus, and Salmonella.
Nisin is an antibacterial peptide produced by Lactococcuslactis and used as a food preservative to extend the shelf life of food by suppressing Gram-positive spoilage and pathogenic bacteria. In foods, nisin is used at levels ranging from ~1-25 ppm, depending on the food type and regulatory approval.
The pathway of food preservation by lactic acid bacteria involving nisin
The pathway of food preservation by salt.
The hurdle effect of food preservation, such as by adding lactic acid bacteria and salt to a food product
In addition to lactic acid bacteria, certain yeasts including strains of Saccharomyces cerevisiae have been reported to have a Biopreservation effect due to their antagonistic activities, as well as the synthesis of a large class of antimicrobial compounds exhibiting a large spectrum of activity against food spoilage microorganisms. Several yeasts are present on the surface of fruits and vegetables have reported killing mold by producing some inhibitory proteins and enzymes (β-Gluconase), which binds to the mold mycelia and degrades its cell wall. Yeasts are commonly present on fruits and vegetables, they are not considered pathogenic, and thus can be used in place of fungicide.
Bacterial Metabolites as Biopreservatives
Antimicrobial substances produced mainly by lactic acid bacteria including organic acids, acetaldehyde, ethanol, hydrogen peroxide, carbon dioxide, diacetyl, reuterin, and bacteriocins are important as biopreservatives.
These substances may be produced by the viable bacterial cells added to protective cultures while some of them can be added independently to control spoilage and pathogenic flora of foods. Such natural preservatives include organic acids such as acetic and propionic acid which are produced by Acetobacter acetic and Propionibacterium spp. respectively. Acetic acid its salts are inhibitory against a broad range of bacteria- both Gram-positive and negative as well as yeasts, and moulds. Propionic acid and its salts mainly have a fungistatic effect.
Bacteriocins, a type of antimicrobial peptides, are another important group of biological preservatives. Nisin, a Class I bacteriocin produced by Lactococcus lactis, is commercially available as a food preservative in purified form and is widely used in products such as processed cheese, dairy products, and canned foods. It inhibits pathogens like L. monocytogenes and many Gram-positive bacteria species causing food spoilage.