Phytochemicals are components within a plant that often impart taste or flavor. Classified as non-nutritive, these factors can also be antimicrobial—a property that has been exploited throughout history. Many cultures use herbs and spices in both cooking and medicine, as much for the pleasant sensations they impart as for their antimicrobial actions against foodborne pathogens, food spoilage organisms and parasites.
Probably the most widely known phytochemicals used as antimicrobials in food preparation, are the essential oils. Found in herbs and spices, studies have shown that the active constituents of essential oils such as thyme and oregano oils, cinnamon and garlic are useful for controlling pathogens and spoilage microbes in food.
The most effective antimicrobial phytochemicals are phenolics, grouped into six families according to structural and chemical similarities, as shown below1.
These phytochemicals exist in a ring structure with varying degrees of substitutions that apparently govern antimicrobial activity. They act by damaging the cell membrane and by altering permeability. The best-known examples of these compounds include eugenol from clove oil, effective against bacteria and viruses, and caffeic acid found in thyme and tarragon, which is also effective against fungi.
Flavones and Flavonoids
Examples of these polyphenols include catechins in oolong and green tea, which show antiviral activity in vitro. They inhibit microbes by binding to their proteins and to bacterial cell membranes. Citrus flavonoids disrupt quorum sensing in Escherichia coli, thereby inhibiting biofilm formation and colony growth2.
Quinones also act against microbes through their binding activity, which inhibits protein actions and makes substrates unavailable. Research has shown that an active quinone found in Pergularia daemia (Forsk.) is active against food pathogens Bacillus subtilis, Staphylococcus aureus, and E. coli, as shown by in vitro testing3.
Tannins and Coumarins
Tannins have activity similar to quinones, in that they are effective against bacteria, yeasts and certain fungi. Other studies also find activity against viruses and methicillin-resistant Staphylococcus aureus (MRSA). Coumarins can be toxic in some species following ingestion. They have medicinal qualities in addition to antimicrobial activity against fungi, bacteria and viruses.
Essential oils, also known as terpenes, are volatile agents and secondary metabolites responsible for giving fragrance, taste and smell to plants. In addition to possessing medicinal properties such as combating inflammation and inhibiting tumor cells, essential oils show antioxidant, antifungal, antibacterial and antiviral activity. As such, they are under investigation for use in the food industry to prevent spoilage, reduce foodborne pathogens and extend shelf life. Researchers have found antimicrobial activity in various essential oils against Listeria monocytogenes, E. coli O157:H7, Salmonella enterica ser. Typhimurium ATCC® 14028, and Brochothrix thermosphacta.
Although alkaloids in general show limited antimicrobial activity, berberine appears to be effective against MRSA.
Often described as botanicals, phytochemicals are seen as safe additives for use in the food industry. Research into potential uses has shown that synergism between different phytochemicals could improve the antimicrobial action.
A study that combined cranberry extract with oregano found greater inhibition of Listeria monocytogenes using the additives in combination rather than alone4. The researchers found an optimal ratio for the two resulted in greater inhibition, observing this effect in food matrices such as meat and fish, as well as by agar plate diffusion test. The scientists also found that they could influence antimicrobial action by changing the food matrix environment, showing maximal effect at 4°C.
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1. Hintz, T. et al. (2015) “The Use of Plant Antimicrobial Compounds for Food Preservation“, BioMed Research International 2015, Article ID 246264 http://dx.doi.org/10.1155/2015/246264
2. Sultanbawa, Y. (2011) “Plant antimicrobials in food applications: Minireview”, in “Science against microbial pathogens: communicating current research and technological advances” A. Mendez-Vials [Ed.] pp. 1084-1093
3. Ignacimuthu, S. et al. (2009) “Antibacterial activity of a novel quinone from the leaves of Pergularia daemia (Forsk.), a traditional medicinal plant“, Asian Journal of Traditional Medicines 4 (pp.36–40)
4. Lin, Y.T. et al. (2004) “Inhibition of Listeria monocytogenes in Fish and Meat Systems by Use of Oregano and Cranberry Phytochemical Synergies“, Applied and Environmental Microbiology (pp.5672–78) DOI: 10.1128/AEM.70.9.5672–5678.2004
Nabavi, S.F. et al. (2015) “Review: Antibacterial Effects of Cinnamon: From Farm to Food, Cosmetic and Pharmaceutical Industries“, Nutrients 7 (pp.7729-48) doi:10.3390/nu7095359
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