Antimicrobial properties of oregano oil (origanum vulgare) against listeria monocytogenes in alheira

Fermented meat products are part of the daily diet in rural areas of Portugal and have become very popular in urban centers. “Alheiras’’ are traditional, slightly smoked, naturally fermented meat sausages typical of the Northern regions (Trás-os-Montes) in Portugal. Essential oils (EOs) have been shown good antimicrobial properties, becoming a good natural alternative to the use of chemical preservatives. Oregano EO already demonstrated, in vitro, a good antimicrobial activity, essentially against Listeria monocytogenes. The aim of this study was to evaluate the ability of oregano EO to inhibit a cocktail of L. monocytogenes strains in paste of “alheira”, during 21 days. A cocktail of L. monocytogenes was inoculated at 10 7 CFU/mL in “paste of alheira” with different concentrations of oregano EO (4%, 1.5%, 0.5%, 0.195% and 0.0975%). Control samples without L. monocytogenes were also evaluated. Microbiological analyzes and determination of water activity and pH values were performed after 0, 3, 7, 15 and 21 days of storage at 4 ºC. The experiment was done in triplicate. The results showed that L. monocytogenes decrease gradually over time and at day 15, a 2-3 log decrease was observed for all concentrations investigated. When a concentration of 4% of oregano EO was used, L. monocytogenes was not detected after 7 days of storage. The lowest concentrations used (0.195 and 0.0975%) showed a good inhibition, with ~5 log reduction after 21 days. Counts of lactic acid bacteria were ~109 CFU/mL for all samples. No differences in the values of pH and a w were detected between samples. Although this EO have shown anti-listeria properties in low concentrations, sensory evaluation is necessary in order to guarantee its acceptance by the consumer.