Coating with phenolic branched-chain fatty acid reduces Listeria innocua populations on apple fruit.

ABSTRACT

An antimicrobial coating was produced by mixing phenolic branched-chain fatty acid (PBC-FA) with glycerol and a carboxymethyl cellulose solution (CMC) at pH 7. The resulting PBC-FA-CMC solution formed an emulsion with an average droplet size of 77 nm. The emulsion in the coating solution was stable for at least 30 days at 20 °C. The in vitro antimicrobial activity of the film formed from the PBC-FA emulsion was tested against a mixture of 3 strains of Listeria innocua (7 log CFU/mL). Film with a concentration of 1000 µg/mL of PBC-FA effectively reduced the population of L. innocua below the limit of detection (<1.48 log CFU/mL) in vitro. The effect of the 1000 µg/mL PBC-FA-CMC coating formulation was then evaluated against L. innocua inoculated on "Gala" apples. Results showed that compared with the non-coated control, the coating reduced L. innocua populations by ~2 log CFU/fruit and ~6 log CFU/fruit on the apple when enumerated on tryptic soy agar and selective media (PALCAM), respectively, indicating that PBC-FA applied as a coating on apples resulted in the sub-lethal injury of bacterial cells. When L. innocua was inoculated onto PBC-FA-coated apples, the L. innocua population decreased by ~4 log CFU/fruit during 14 days of shelf-life at 20 °C. The PBC-FA coating lowered the moisture loss but did not affect the color, firmness, or soluble solids content of apples during the 14-day at 20 °C. Overall, this study revealed that there is a potential that PBC-FA can be used as an antimicrobial coating to inactivate Listeria and preserve the quality of apples.