The U.S. Food and Drug Administration requires food processors to implement and validate processes that will result in significantly minimizing or preventing the occurrence of hazards that are reasonably foreseeable in food production. During production of fresh-cut leafy vegetables, microbial contamination that may be present on the product can spread throughout the production batch when the product is washed, thus increasing the risk of illnesses. The use of antimicrobials in the wash water is a critical step in preventing such water-mediated cross-contamination; however, many factors can affect antimicrobial efficacy in the production of fresh-cut leafy vegetables, and the procedures for validating this key preventive control have not been articulated. Producers may consider three options for validating antimicrobial washing as a preventive control for cross-contamination. Option 1 involves the use of a surrogate for the microbial hazard and the demonstration that cross-contamination is prevented by the antimicrobial wash. Option 2 involves the use of antimicrobial sensors and the demonstration that a critical antimicrobial level is maintained during worst-case operating conditions. Option 3 validates the placement of the sensors in the processing equipment with the demonstration that a critical antimicrobial level is maintained at all locations, regardless of operating conditions. These validation options developed for fresh-cut leafy vegetables may serve as examples for validating processes that prevent cross-contamination during washing of other fresh produce commodities.

Listeria spp. have been isolated from a wide variety of sources, and in many situations Listeria innocua is more commonly found than Listeria monocytogenes. Growth of three L. monocytogenes strains was studied when inoculated simultaneously with a rhamnose negative L. innocua strain into culture media and cheese sauce. Fraser broth (FB), Trypticase™ soy broth plus 0.6% yeast extract (TSB-YE), University of Vermont medium (UVM) modified Listeria enrichment broth, and cheese sauce were inoculated (ca. 102 cells per ml) and incubated for 24 h; FB, TSB-YE, and cheese sauce at 35°C, UVM at 30°C. Growth of four rhamnose-positive, L. innocua strains was also studied in culture media. Growth of L. monocytogenes was similar to that for L. innocua in TSB-YE or cheese sauce. However, in FB and UVM, L. innocua populations were significantly higher than L. monocytogenes. This occurred when media were inoculated individually or simultaneously. This may explain in part why L. innocua is isolated more frequently than L. monocytogenes from foods and environmental samples.

The growth parameters for Wisteria monocytogenes Scott A at various temperatures, pH values, water activity values, and in different carbohydrates were assessed. Growth was seen from 4°C through 45°C and at pH values from 4.7 through 9.2. Generation times were calculated for each parameter at which growth was observed. Optimum growth was defined as shortest generation time. L. monuqyfugenes grew optimally over the temper&re range 30-37°C. The optimum uH was 7.0. Growth was demonstrated in solutions of UD to 39.4% sucrose (a~ of 0.92). Carbohydrate fermentation was variable. Further studies involved examination of growth at optimal water activities (0.97) with suboptimal temperatures and pH vaIues, and at 7.0, with suboptimal temperatures and water activities. Growth was seen when these parameters were combined.

No abstract

Samples of the outer portions of fresh beets, broccoli, cabbage, carrots, cauliflower, corn, head lettuce, leaf lettuce, mushrooms, and potatoes, as well as frozen green beans, green peas, pea pods, and spinach were examined for the presence of Listeria monocytogenes using a 7‐day enrichment procedure at 30°C. Samples were streaked on modified McBrides agar, and isolates were tested for characterization reactions. Fresh carrots, lettuce, and mushrooms, and frozen spinach were also examined using a cold enrichment procedure at 4°C. Samples were taken weekly, streaked onto modified McBrides agar, and were characterized. It was determined that no L. monocytogenes was detectable in any of these samples.