The tide is currently turning against chemical-based bactericides for food use, opening up opportunities for alternatives from natural sources. The reasons for this are manifold and include general consumer preferences for natural foods, legislative changes, and the isolation of antibiotic resistant pathogens. Katie Fisher and Carol Philips of the University of Nottingham's School of Health, UK, believe that citrus essential oils (EOs) show strong potential to be used instead, not least because they are already recognised as safe and have already been seen to have an inhibitory affect against both gram-positive and gram-negative bacteria. In a review of the evidence to support this use published in Trends in Food Science and Technology, the researchers looked at the body of evidence to recommend their use, noting that the antimicrobial properties of citrus EOs have only started to be explored quite recently. However in order for their use to be considered in all seriousness, there are certain limitations that need to be bourn in mind. Fisher and Philips note that there is some evidence that they could kill off other organisms in the human gastrointestinal tract - not just unhelpful bacteria. "Should EOs be applied to food they may be able to inhibit a wide range of organisms, but they could also cause an imbalance in gut microflora," they wrote. Thus, while more research is conducted on the effect of certain EOs throughout the whole intestinal tract, they recommend that a good starting point for the food industry would be to look at using those citrus oils that are already being used as food flavours. This would include the likes of orange, lemon, lime and grapefruit. The flip side of this, however, is that citrus EOs can affect the organoleptic properties of a foodstuff. "Finding an oil/component/vapour that has the greatest sensory effect at the lowest concentration is essential," said Fisher and Philips. They suggest this be done by sensory tests, ideally using trained individuals. They also noted that the exact mechanism for citrus essential oils' antimicrobial properties is not known, although there are several good theories. These include morphological changes - and indeed some studies have indicated that essential oils could cause the outer membrane of some bacteria to disintegrate; while others have noted major thickening and disruption of the cell wall. The lack of a clear mechanism could pose a problem, however, since there is a change that resistant pathogens could arise. The researchers highlighted some of the antimicrobial uses of citrus essential oils that have already been investigated for foods. For example: Seafood For fresh seafood, which typically has a shelf life of just three days, citral and linalool were seen to reduce the microflora on the gills, skin and intestines (of carp) when stored at 20 degrees Celsius for 48 hours (Kim, Marshall, Cornell, Preston & Wei, 1995). Meat For beef meatballs, orange and lemon dry powders used at 5 per cent have been seen to be effective against LAB load over 12 days (Fernandez-Lopez et al, 2005). Conversely in chicken, lemon, orange and bergamot essential oils (and their components) were seen to have an insufficient effect on chicken skin inoculated with L. monocytogenes, B. cereus, S. aureus or E. coli, after 10 minutes exposure (Fisher & Philips, 2005). Dairy In dairy products, taste panels found that only orange, lemon and grapefruit were acceptable for use in milk, but terpineol was seen to be the most effective against Salmonella senftenberg, E. coli, S. aureus and Pseudomonas spp. An experiment that incorporated terpineol and orance oils in pasteurised milk at a concentration of 1000 micro-litres per litre for 52 days at 4 degrees Celsius found that the results depended on fat content. Orange was seen to be effective only in skimmed milk with a 1 - 2 log reduction (Dabbah, Edwards & Moats, 1970) Source: Trends in Food Science and Technology 19 (2008) 156-164 DOI: 10.1016/j.tifs.2007.11.006 Title: "Potential antimicrobial uses of essential oils in food: is citrus the answer?" Authors: Katie Fisher and Carol Philips
