Anti-listerial effects of essential oils and herbs in fresh-cut produce: opportunities and limitations

The potential anti-listerial benefits of essential oils and herbs in fresh-cut produce systems were investigated. Interactions with modified atmospheres and product types were examined in detail, including effects on quality. A strong anti-listerial response from rosemary herb was discovered during maceration and the chemical basis of this determined for future exploitation. The anti-listerial properties of essential oils (thyme, oregano and rosemary), under a range of storage atmospheres (air, 5%CO2/2%,O2/93%N2 and 20%CO2/1% O2/79%N2) and temperatures (4 and 8°C), were examined using a model vegetable system. Effectiveness was in the order thyme EO> oregano EO> rosemary EO, and greatest under 20%CO2/1%O2/79%N2 and at 4°C. When headspace volatiles from the EOs were tested there was little anti-listerial effect, suggesting that the EOs needed to be in direct contact with cultures. When applied to modified atmosphere packaged fresh-cut vegetables (lettuce, cabbage, coleslaw mix and carrots), the effectiveness was in the order thyme EO > oregano EO > rosemary EO > basil EO. Applying undiluted EOs directly to the fresh produce had a detrimental effect on appearance. A product effect was seen with EOs and herbs having increased anti-listerial activity on shredded cabbage and carrot. Diluting the EOs, or using them diluted in combination, reduced adverse sensory effects, but also eliminated the anti-listerial effects. In general, use of the fresh herb equivalents of the EOs was ineffective. However, while commercial rosemary EO was relatively ineffective, freshly macerated rosemary herb had very strong anti-listerial effects. To investigate this further, the chemical composition and anti-listerial activity of rosemary oil obtained by different extraction methods (CO2 extraction, hydrodistillation, solvent extraction) was determined. Gas chromatography-mass spectroscopy identified the main components present as camphor, verbenone, borneol, bornyl acetate and caryophyllene. All of these individual components showed anti-listerial activity, however principal component analysis showed verbenone to be highly correlated with anti-listerial kill rate. The hydrodistillate, which had the highest antilisterial activity, contained the highest levels of verbenone. The extract of macerated rosemary herb had more than twentyfold the level of verbenone found in the unmacerated rosemary extract. When headspace analysis was carried out on uncut, freshly chopped and macerated rosemary herb, the relative levels of verbenone were 0, 6 and 118ppm. The strong anti-listerial activity of the macerated rosemary may be explained by the higher concentration of verbenone present in these extracts. Simulation of maceration by stomaching in industrial production of rosemary EOs is likely to greatly enhance their anti-listerial effectiveness.