Synergistic antimicrobial activity of caprylic acid in combination with citric acid against both Escherichia coli O157: H7 and indigenous microflora in carrot juice

The identification of novel, effective, and non-thermal decontamination methods is imperative for the preservation of unpasteurized and fresh vegetable juices. The aim of this study was to examine the bactericidal effects of caprylic acid+citric acid against the virulent pathogen Escherichia coli O157:H7 and the endogenous microflora in unpasteurized fresh carrot juice. Carrot juice was treated with either caprylic acid, citric acid, or a combination of caprylic acid+citric acid at mild heating temperature (45°C or 50°C). The color of the treated carrot juice as well as microbial survival was examined over time. Combined treatment was more effective than individual treatment in terms of both color and microbial survival. Caprylic acid+citric acid treatment (each at 5.0mM) at 50°C for 5min resulted in 7.46 and 3.07 log CFU/ml reductions in the E.coli O157:H7 and endogenous microflora populations, respectively. By contrast, there was no apparent reduction in either population following individual treatment. A validation assay using a low-density E.coli O157:H7 inoculum (3.31 log CFU/ml) showed that combined treatment with caprylic acid (5.0mM)+citric acid (2.5mM) at 50°C for >5min or with caprylic acid+citric acid (both at 5.0mM) at either 45°C or 50°C for >5min completely destroyed the bacteria. Combined treatment also increased the redness of the juice, which is a perceived indication of quality. Taken together, these results indicate that combined treatment with low concentrations of caprylic acid and citric acid, which are of biotic origin, can eliminate microorganisms from unpasteurized carrot juice.