Effect of Variation in Food Components and Composition on the Antimicrobial Activity of Oregano and Clove Essential Oils in Broth and in a Reformulated Reduced Salt Vegetable Soup Product

The objective of this study was to determine and quantify the influence of various food components (carbohydrates, fat, protein or NaCl contents) or chemical properties (pH, water activity levels) on the antimicrobial efficacy of oregano and clove essential oils (EOs). Growth of Listeria innocua or Escherichia coli treated with oregano or clove EOs was monitored following separate addition of various food components. Antimicrobial activity of EOs was enhanced in presence of NaCl (? 0.5 g/100 ml), or in media with low pH values (? 5.0), especially when adjusted with organic acids. Enhanced antimicrobial activity was observed following reduction in water activity, which appeared related to the nature of solute used. Antibacterial activity of EOs was reduced in presence of vegetable oil (? 1 ml/100 ml), protein (? 1 g/100 ml) or starch (? 10 g/100 ml). Based on data obtained, the composition of vegetable soup was altered to optimise the efficacy of EOs, by lowering the pH to 5.0 using citric acid. A combination of oregano EO and acidification appeared to control growth of L. innocua and E. coli during storage at 4 or 10 ºC. Thus, reformulation treatments including EO addition should be considered to improve the shelf-life of chilled ready meals.