Antimicrobial activity of some plant extracts and essential oils against foodborne pathogens in vitro and on the fate of inoculated pathogens in chocolate

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 >20 mm were observed by adding 10 μl 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 °C) 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 °C for 9 days. Plant extracts tested on chocolate show an enhanced inhibitory effect during storage at 20 °C indicating that their application may provide protection in case of storage at the above temperature or even higher.