Biocontrol of Listeria monocytogenes in minimally processed brined refrigerated cucumbers

The purpose of the research has been to develop a biocontrol model system for the inhibition of Listeria monocytogenes (pathogen) in minimally processed fruits and vegetables to improve their safety. One application of this model was to evaluate the potential use of a bacteriocin-producer Lactococcus lactis or bacteriocin-nonproducer Lactobacillus curvatus as a biocontrol bacterium at refrigeration and at abuse temperatures in the food system. The research involved the characterization of the microflora growing at refrigeration temperatures in cucumbers held in air and brine, characterization of the microflora responsible for spoilage of the product at refrigeration and at abuse temperatures in cucumbers held in brine, and the potential use of the microflora (lactic acid bacteria) responsible for acidic spoilage of the product as biocontrol agents in this type of products. Isolates were tested for bacteriocin production against other lactic acid bacteria, Gram-negative spoilage microorganisms, and 5 strains of Listeria monocytogenes. Brining cucumbers allowed the growth of LAB at refrigeration and at abuse temperatures. LAB isolated and identified belonged to the lactocci, enterococci, leuconostoc, lactobacilli, and weisellae. Spoilage microorganisms were inhibited by refrigeration and brining. Some of the natural LAB isolated from the product showed to produce bacteriocin-like activity against other LAB and Listeria monocytogenes. The results showed that Listeria varied in sensitivity to the bacteriocin-like substance produced by some of the isolates, being serotype 1/2a and 1/2b more resistant than 4b. Blanching the cucumbers to reduce the initial microflora did not affect the predominance of LAB in the product, but it extended its shelf life. L. monocytogenes inoculated in the product survived at 5°C, but it did not grow. At abuse temperatures, Listeria grew when the natural LAB or the biocontrol agent were not present. This study demonstrates that the addition of a biocontrol bacterium able to produce a bacteriocin in this type of product will ensure its safety. However, more studies are needed to determine the best candidate and the best method for inoculation. LAB isolated from the product in this study could be used to help develop such biocontrol bacterium.