Protective cultures: An alternative to chemical preservatives

Biopreservation is defined as the technique of extending the shelf life of food by using natural or controlled microbiota or antimicrobials. The fermentation metabolites as well as antagonistic bacteria are generally selected to limit spoilage and inactivate pathogens in food, thus enhancing food safety and security.?With the growing trend of consumers becoming more aware of the ingredients in their food, the call for organic, chemical preservative and additive free foods are on the rise.?At manufacturing level, the strive for a “clean label”, while preventing spoilage and ensuring food safety, further highlights the need for novel preservative and/or processing methods. At present, biopreservation is considered as a popular alternative to conventional preservation methods in food, to ensure product quality and safety.

Biopreservation relies on the use of protective cultures, such as lactic acid bacteria (LAB), which produce extracellularly excreted peptide-based bacteriocins which display antimicrobial activity. Bacteriocins are mainly produced at the end of, or during the exponential growth phase of the protective culture (Fig.1).?As a result, there is no requirement for it to be labeled as opposed to the addition of bacteriocins in purified or semi-purified form. The benefit of biopreservation is the continued production of bacteriocin throughout production and storage, even at reduced temperatures. Due to the peptide nature of the bacteriocins, these molecules are easily degraded by proteolytic enzymes in the intestinal tract upon ingestion reducing interference with native gut bacteria.

Along with bacteriocin production, the protective culture also competes for nutrients and produces other inhibitory compounds like organic acids, hydrogen peroxide, and enzymes which can inhibit the growth of spoilage- and pathogenic bacteria. Bacteriocin’s mode of action against bacteria can be divided into two distinct groups; firstly, bacteriocins resulting in a bactericidal effect (with or without cell lysis) mostly targeting the cell envelope-associated mechanisms, and secondly, bacteriocins exerting a bacteriostatic effect that results in growth inhibition. The mechanisms vary among the class of bacteriocin produced and?include: (1) targeting the Lipid II (intermediate in the peptidoglycan biosynthesis machinery) which leads to the inhibition of peptidoglycan synthesis; (2) Using Lipid II as a docking molecule to facilitate pore formation resulting in permeabilization and eventually cell death; (3) killing target cells by binding to the cell envelope-associated mannose phosphotransferase system (ManPTS) which also leads to pore formation of the membrane; and (4) killing their target cells by inhibition of gene expression and protein production.?The anti-fungal mechanism involves complex and synergistic interactions between antifungal metabolites and cell-to-cell interaction which is still being researched.

Bacteriocins can be purified and added during production, however, its use is limited in the food industry. While these bacteriocins are generally recognized as safe (GRAS) only two purified bacteriocins, nisin and pediocin, have been allowed in food due to strict global food regulations. Other factors limiting the use of purified bacteriocins include its inability to provide effective protection and high costs. As a result, bacteriocins are mainly applied in the form of protective cultures.

The use of protective cultures in cheese has especially been useful as it prevents the late blowing of cheese caused by Clostridium spores.?Bacteriocins has been effective against L. monocytogenes and other Listeria spp. in cheeses.?It also has shown to have antimicrobial activity against the indigenous bacteria, non-starter LAB, Staphylococcus, Bacillus as well as anti-fungal properties against mould. Interestingly, some bacteriocin-producing LAB have been shown to improve cheese maturation and flavour by inducing the lysis of the starter or non-starter LAB by the release of intracellular proteinases and peptidases which leads to accelerated cheese ripening and prevents Clostridium growth.?

In situ production of bacteriocins by protective cultures is thus the most widespread, popular, and legal use of bacteriocins to achieve a product that is of good quality and safe.?Protective cultures can therefore be a good alternative to chemical preservatives and additives.