[Malithi Weerakkody]

Age-old Wisdom - Neglected

Fermentation is one of the first food preservation techniques known to man. Since ancient times, it has been observed that the activity of microorganisms on certain fresh foods improved their taste, texture and aroma at the same time making the food last longer. However, with the advent of novel chemical and physical methods, fermentation lost its mark as a food preservation technique although it is used for the production of a wide variety of foods including dairy products, vegetable and fruit products and meat products.

The modern chemical and physical methods of food preservation are more effective than the traditional methods, nonetheless, not without drawbacks. Most chemical preservatives such as nitrites have shown to be toxic, and often accused of being carcinogenic. The physical treatments such as application of high temperature destroy the essential nutrients and they may also alter the organoleptic properties of foods. Hence the consumers demand for the foods minimally processed, drawing the attention back on the traditional food preservation techniques.

Food Biopreservation versus Fermentation

Biopreservation is more or less similar to fermentation in the sense that it uses microorganisms- endogenous or added- and/or their natural antimicrobial products to extend the shelf life of foods. In fact, many food-grade bacteria serve as biopreservatives as well as fermenters.

Nevertheless, the term biopreservation is used in a broader sense than fermentation. Some biopreservative microorganisms may not ferment foods although they may produce inhibitory substances against pathogenic or spoilage microflora. Moreover, the new trend of using bacteriophages as biopreservatives also places biopreservation a step further ahead traditional fermentation.

Biopreservation of foods is mainly achieved through two approaches;

1. the addition of protective cultures i.e. live microorganisms and
2. the addition of antimicrobial compounds produced by the microorganisms

Protective Cultures versus Starter Cultures

Some live microorganisms added to foods serve a bioprotective function by safeguarding the foods against undesirable microorganisms. These bacterial cultures are termed as Protective Cultures.

This strategy is based on the concept of microbial antagonism where microbes hinder the growth of other microorganisms either by competing for space and nutrients or by releasing inhibitory substances such as organic acids, hydrogen peroxide, and bacteriocins. In the case of bacteriophages, they control the undesirable bacteria by invading the cells by destroying them or hindering their metabolism.

In order to be considered as a protective culture, a selected bacterial species has to meet certain criteria.

1. They have to be nonpathogenic as well as non-toxigenic.
2. They have to inhibit the growth of undesirable microorganisms.
3. They should not impart any undesirable characteristics in the food.

These protective cultures do not necessarily have to ferment foods in order to produce a preserving effect. Thus they stand apart from the ‘starter cultures’ which are used in the fermentation processes that always cause a sensory alteration of the food.

Apart from hindering the growth of other spoilage and pathogenic microorganisms, some bacteria may serve as indicators of temperature abuse of some refrigerated foods. Types of bacteria used for this purpose cannot grow under refrigeration temperatures therefore an increase of temperature would be indicated by the increase of population of the said bacterial species.

Some Common Biopreservative Bacteria

Lactic Acid Bacteria are the most widely studied group of microorganisms for biopreservation of foods. Many species of lactic acid bacteria are considered GRAS (Generally Regarded as Safe) and are currently used as biopreservatives. Lactic acid bacteria of the genera Lactococcus, Lactobacillus, and Pediococcus have successfully demonstrated their potential of controlling pathogens such as Clostridium botulinum, Salmonella, and Staphylococcus aureus in milk, meat and seafood products.

Moreover, certain yeasts, including strains of Saccharomyces cerevisiae have also been reported to produce antimicrobial proteins suggesting the possibility of using them as biopreservatives.

Furthermore, bacteriophages have also proposed as a potential biopreservatives against bacteria such as E. coli O157:H7, Salmonella and Listeria monocytogenes which are common culprits of food spoilage and food-borne illnesses.

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 as 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 aceti 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 bacterial species causing food spoilage.



Sources

1. Ananou, S., Maqueda, M., Martínez-Bueno, M., & Valdivia, E. (2007). Biopreservation, an ecological approach to improve the safety and shelf-life of foods. Communicating current research and educational topics and trends in applied microbiology, 1, 475-486.

2. Garcia, P., Martinez, B., Obeso, J. M., & Rodriguez, A. (2008). Bacteriophages and their application in food safety. Letters in applied microbiology, 47(6), 479-485.