LWT 41 (2008) 119–127 Antimicrobial activity of some plant extracts and essential oils against foodborne pathogens in vitro and on the fate of inoculated pathogens in chocolate P. Kotzekidou à , P. Giannakidis, A. Boulamatsis Laboratory of Food Microbiology and Hygiene, Department of Food Science and Technology, Faculty of Agriculture, Aristotle University of Thessaloniki, Box 250, 54124 Thessaloniki, Greece Received 6 July 2006; received in revised form 24 January 2007; accepted 24 January 2007 Abstract The efficacy of commercially available plant extracts and essential oils used extensively as flavour ingredients in confectionery products were used as antimicrobials in laboratory media against the following microorganisms: Escherichia coli O157:H7, Salmonella Enteritidis, Salmonella Typhimurium, Staphylococcus aureus, Listeria monocytogenes, and Bacillus cereus. Using the disc diffusion method, inhibition zones in diameter 420 mm were observed by adding 10 ml of each antimicrobial substance on the following microorganisms: lemon flavour applied on E. coli O157:H7, lemongrass essences against S. aureus, plum using a B. cereus strain and strawberry flavour using a L. monocytogenes strain. E. coli O157:H7 strains were the most susceptible microorganisms inhibited by 18 extracts, followed by S. Typhimurium and S. aureus which were inhibited by 17 extracts. Lemon flavour, lemongrass essences, pineapple and strawberry flavour inhibited the foodborne pathogens at the lowest concentration (5 ml/100 ml). Plant extracts and essential oils with potent antimicrobial activities were tested in chocolate held at different temperatures (7 and 20 1C) in dry or humidified environment, which resulted in different a w values of the product (i.e. 0.340, 0.450, and 0.822), in order to determine their efficacy on the fate of the inoculated pathogens. The most inhibitory action was observed by lemon flavour applied on chocolate inoculated with E. coli cocktail culture after storage at 20 1C for 9 days. Plant extracts tested on chocolate show an enhanced inhibitory effect during storage at 20 1C indicating that their application may provide protection in case of storage at the above temperature or even higher. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Antimicrobials; Chocolate; Essential oils; Foodborne pathogens; Plant extracts 1. Introduction Many naturally occurring compounds found in plants, herbs, and spices have been shown to possess antimicrobial functions and serve as a source of antimicrobial agents against foodborne pathogens (Deans & Ritchie, 1987). As essential oils and their constituents are extensively used as flavour ingredients in a wide variety of foods, beverages, and confectionery products, their application in controlling pathogens could reduce the risk of foodborne outbreaks and assure consumers safe food products (Burt, 2004). Consumer demand for less use of synthetic preservatives has led to research and use of ‘‘naturally derived’’ antimicrobials. Certain plants and their extracts used as flavouring agents are known to possess antimicrobial activity offering a potential alternative to synthetic preservatives (Gould, 1996). In modern food industries mild processes are applied in order to obtain safe products which have a natural or ‘‘green’’ image (Burt, 2004). Under these conditions the antimicrobial effects of plant extracts and essential oils intend to reduce the proliferation of foodborne pathogens. The antimicrobial activity of plant extracts used as flavouring agents in foods is due to their essential oil fraction (Conner, 1993). Plant essential oils and their components have broad-spectrum activity against both Gram-negative and Gram-positive foodborne pathogens ARTICLE IN PRESS www.elsevier.com/locate/lwt 0023-6438/$30.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2007.01.016 à Corresponding author. Fax: +30 2310 991632. E-mail address: kotzekid@agro.auth.gr (P. Kotzekidou).