Inhibition of Listeria monocytogenes by oregano, cranberry and sodium lactate combination in broth and cooked ground beef systems and likely mode of action through proline metabolism E. Apostolidis, Y.-I. Kwon 1 , K. Shetty ⁎ Laboratory of Food Biotechnology, Department of Food Science, University of Massachusetts, Amherst, MA 01003, USA abstract article info Article history: Received 7 November 2007 Received in revised form 31 August 2008 Accepted 16 September 2008 Keywords: Listeria monocytogenes Phenolics Oregano Cranberry Sodium lactate Lactic acid Proline Meat Antimicrobial Listeria monocytogenes is a food safety challenge in various food systems including fresh and frozen meat and poultry. Natural antimicrobials from plant sources in combination with salts of organic acids have the potential to control L. monocytogenes. In this research the efficiency of water soluble phenolic extracts of oregano and cranberry in combination with sodium lactate was evaluated for control of L .monocytogenes. In both broth and cooked meat studies, the results indicated that the combination of water soluble extracts of oregano and cranberry, at a ratio of 50:50 and a concentration of 750 ppm, with 2% sodium lactate had the best inhibitory effect in the tested strain. Based on the rationale that phenolics in oregano and cranberry behave as proline analogs, the potential recovery of pathogen growth in medium with the best inhibitor concentration and supplemented with 1 mM proline was evaluated. The results indicated that there was a proline-induced growth recovery, pointing to a possible mechanism of action of inhibitors, involving proline metabolism. These results confirm the potential of plant extracts to be antimicrobial, and when combined with sodium lactate, can be used as a nature constituent of multiple-barrier food preservation systems. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Listeria monocytogenes is a gram positive, psychrotropic, facultatively anaerobic, catalase-positive, oxidase-negative, non-spore forming bac- terium (Adams and Moss, 1995) and is the etiological agent of listeriosis, a severe food-borne disease (Glaser et al., 2001). It is an atypical disease of major public health concern because of the severity and the non enteric nature of the diseases (meningitis, septicemia, and abortion), with high mortality (Wong et al., 2000; Rocourt and Cossart, 1997). It frequently has a long incubation time and a predilection for severe infection in individuals who have underlying conditions which lead to impairment of T-cell-mediated immunity (Rocourt and Cossart, 1997). Since 1987 FDA has established a “zero tolerance” for L. monocytogenes. Due to its psychrotropic character L. monocytogenes is a pathogen of concern in refrigerated food products, such as meat (cooked and fresh), cheese and milk (Rocourt and Cossart, 1997; Cox, 1989). Antimicrobial strategies to overcome the low temperature toler- ance are essential and the use of natural phytochemicals in combination with lactic acid or lactate could be promising (McCue et al., 2005; Shetty and Lin, 2005; Lin et al., 2004). Many phenolic phytochemical containing fruit and herbal products have shown to possess antimicrobial activity against L. monocytogenes (Hao et al., 1998; Puupponen-Pimia et al., 2001; Chung et al., 1990; Sagun et al., 2006). Studies have shown the antimicrobial activity of oregano and two of its major components, carvacrol and thymol, along with enhanced synergy with high rosmarinic acid extract (Exarchou et al., 2002; Shetty, 1997; Seaberg et al., 2003; Ahn et al., 2004; Vattem et al., 2005a; Marwan and Nagel, 1986). Cranberry is a traditional fruit popular in the United States which contains a wide range of phenolic phytochemicals, and has been historically associated with positive health benefits (Vattem et al., 2005b). It is now believed that these positive health benefits, including antimicrobial effects, are a result of the constituent phenolic phytochemicals such as phenolic acids, biphenolics such as ellagic acid, flavanoids and proanthocyanidins (Vattem et al., 2005a; Vattem and Shetty, 2005). A recent study has shown that synergies of oregano and cranberry water extracts give enhanced hurdle for L. mono- cytogenes control, both in meat and fish systems (Lin et al., 2004). By using synergies of oregano and cranberry extracts the activities of the major phenolic compounds present, rosmarinic and ellagic acid, were combined in order to get greater antimicrobial efficacy. Salts of organic acids such as sodium lactate or potassium lactate and sodium diacetate are extensively used in meat and poultry International Journal of Food Microbiology 128 (2008) 317–324 ⁎ Corresponding author. Tel.: +1 413 545 1022; fax: +1 413 545 1262. E-mail address: kalidas@foodsci.umass.edu (K. Shetty). 1 Current address: Department Food & Nutrition, Hannam University, Daejeon, 305811, Republic of Korea. 0168-1605/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ijfoodmicro.2008.09.012 Contents lists available at ScienceDirect International Journal of Food Microbiology journal homepage: www.elsevier.com/locate/ijfoodmicro