Purpose: The purpose of this study was to evaluate susceptibilities of Escherichia coli ATCC 35218, Salmonella Enteritidis MA44, Saccharomyces cerevisiae KE 162 and Zygosaccharomyces bailii NRRL7256 to mixtures of potassium sorbate (KS), vanillin (V) and citral (C) in laboratory media and agarized mango and melon purées (pH= 4.5, 13.0°Brix).
Methods: Antimicrobial mixtures were evaluated in laboratory media using the Berembaum experimental design. For each microorganism, fourteen systems were prepared using fractions of individual minimal inhibitory concentrations (MIC) previously determined (MICKS:150-5000 ppm; MICV:1000–3500 ppm; MICC:375–750 ppm). Systems were poured into 150 mm Petri dishes, swabbed with the inoculum, incubated up to 10 days at 30/37°C and daily examined for growth. Fractional inhibitory concentration (FIC) indexes and isobolograms were obtained. Several of these mixtures were validated in agarized fruit purees. Additionally, for some V/C combinations, KS inhibitory concentration was determined (0 ppm-512 ppm) by the spiral gradient endpoint (SGE) method.
Results: In laboratory media, inhibitory antimicrobial combinations were all additive (FIC index = 1), excepting some ternary mixtures that included fractions of antimicrobials between 1/12 MIC and 1/3 MIC which were synergistic (FIC index < 0.6) for the yeasts. For all microorganisms, all inhibitory Berembaum mixtures were confirmed in both agarized fruit purées.
When the SGE method was used (0 ppm < KS < 512 ppm), several inhibitory ternary mixtures were found (V < 1150 ppm; C < 250 ppm). The lowest values of KS concentration, for a given V/C combination, corresponded to bacteria in melon purée. Z bailii was not inhibited at any condition.
Significance: Some synergistic antimicrobial combinations could be useful in reducing the amounts to inhibit growth and diminishing consumer concerns about the use of preservatives.