55 Essential Oils Mixtures as Inhibitors of Listeria monocytogenes, a Food- Borne Bacterium M.E. Cazar a , A. Jara and A. Cazar Universidad del Azuay Facultad de Ciencia y Tecnología Avda. 24 de Mayo 777 y Hernán Malo PO Box 01.01.981, Cuenca Ecuador Keywords: Listeria monocytogenes, essential oils, antibacterial activity Abstract Five essential oils obtained from common plants from the Ecuadorian Highlands were tested as inhibitors of Listeria monocytogenes. Oregano (Oreganum vulgare; Lamiaceae), laurel (Laurus nobilis; Lauraceae) and lemongrass (Cimbopogon citratus; Poaceae) yielded antibacterial essential oils, whose MIC was ranged from 25 to 12.5 l/ml. Mixture design was used as a strategy to enhance the antilisterial activity displayed by the bioactive essential oils. Mixtures of the three bioactive oils were prepared according to a simplex-lattice design. The binary mixture composed by oregano and laurel oil showed a better activity, compared with the pure components (6.25 l/ml). The bioactive mixture was applied as food preserver in the preparation of two ready-to-eat products. The shelf-life of the meat products was up to 60 days. Besides, the flavor, odor and appearance of the evaluated products had a good acceptance by a non-trained taster panel. INTRODUCTION The Gram-positive bacterium Listeria monocytogenes is a ubiquitous pathogen that thrives in diverse environments such as soil, water, various food products, humans and animals. The disease caused by this bacterium, listeriosis, is acquired by ingesting contaminated food products and mainly affects immunocompromised individuals, pregnant women and newborns. Listeriosis manifests as gastroenteritis, meningitis, encephalitis, mother-to-fetus infections and septicemia, resulting in death in 25-30% cases (Hamon et al., 2006). Ready-to-eat cooked meats are frequently contaminated with L. monocytogenes during post-processing steps. The contamination of cured and non-cured ready-to-eat cooked meats is a major safety concern for these products. The reason is the fact that this bacterium can proliferate to a threatening level during refrigerated storage, because of its ability to grow in the presence of curing salt at refrigerated temperature. Moreover, the emergence of multiple resistances in Listeria spp. has been reported, due to acquisition of a replicon from staphylococci (Zhu et al., 2005). Meat industry uses chemical additives in order to control the proliferation of microorganisms. In developing countries the use of nitrites, nitrates, trisodium phosphate, sorbic acid, p-hydroxibenzoate in meat processing is frequent (FAO, 2004). Due to safety concerns and consumer preference for natural products, the use of natural preservatives to replace synthetic forms is gaining importance. This trend has led to increased research on natural preservatives, in order to find bioactive plant-based antimicrobials. Although the antibacterial properties of essential oils have been recognized, the recent interest in alternative naturally derived antimicrobials has led to a renewed scientific interest in these substances. The use of a single essential oil as food preserver may alter the taste of food or exceed acceptable flavor thresholds (Gutierrez et al., 2008). A valid approach to overcome this problem is the formulation of essential oils mixtures and the testing of the effect in the preparation of meat products. a mcazar@uazuay.edu.ec Proc. IS on Medicinal Plants and Natural Products Eds.: J. Ghaemghami et al. Acta Hort. 1030, ISHS 2014