Listeria monocytogenes survival in refrigerator dill pickles.

Listeria monocytogenes can survive and grow in refrigerated foods with pH values of approximately 4.0 to 5.0 and salt concentrations of 3 to 4%. Home-fermented refrigerator dill pickles fit this description. Contamination of this product with L. monocytogenes could cause serious problems because these items are not heated prior to consumption. L. monocytogenes survival and growth patterns were investigated in refrigerator dill pickles at 1.3, 3.8, and 7.6% salt concentrations. Pickling cucumbers were dipped into an inoculum of L. monocytogenes, brine mixtures were added, and cucumbers were held at room temperature for 1 week and then refrigerated for up to 3 months. The pH, NaCl percentage, titratable acidity percentage, and total populations of Listeria and aerobic, psychrotrophic, and lactic acid bacteria were measured at the addition of brine, after 2, 4, and 7 days of storage at room temperature, and then weekly during refrigerated storage. The initial Listeria population was 5.4 to 5.6 log CFU/cm2 on cucumber surfaces and 3.9 to 4.6 log CFU/g internally. There was an approximate 0.3- to 1-log increase during room temperature fermentation followed by a population decline during refrigerator storage, with a greater decrease in the brines with the highest NaCl concentration. Up to 49 days, the internal tissue of pickles with 1.3, 3.8, or 7.6% salt concentrations were presumptively positive for L. monocytogenes by the enrichment method, and at 91 days the surfaces of such pickles were still positive for L. monocytogenes. Populations of total aerobes and lactic acid bacteria increased during room temperature storage and decreased gradually during refrigerated storage.

Effect of pH, NaCl content, and temperature on growth and survival of Arcobacter spp.

Growth and survival of six human isolates of the pathogenic Arcobacter spp. in the presence of selected environmental factors were studied. Four strains of Arcobacter butzleri and two strains of Arcobacter cryaerophilus were exposed to pH levels of 3.5 to 8.0. Most strains grew between pH 5.5 and 8.0, with optimal growth of most A. butzleri and A. cryaerophilus strains at pH 6.0 to 7.0 and 7.0 to 7.5, respectively. The 24-h optimal growth range in the presence of NaCl was 0.5 to 1.0% for A. cryaerophilus. However, after 96 h, the optimum was between 0.5 and 2.0% NaCl. The optimum range for growth of A. butzleri strains was 0.09 to 0.5% NaCl after 96 h. The upper growth limits were 3.5 and 3.0% NaCl for A. butzleri and A. cryaerophilus, respectively. Survival at 25 degrees C in up to 5% NaCl was noted for A. butzleri 3556 and 3539 and A. cryaerophilus 3256. Decimal reduction times (D-values) at pH 7.3 in phosphate-buffered saline for three A. butzleri strains were 0.07 to 0.12 min at 60 degrees C, 0.38 to 0.76 min at 55 degrees C, and 5.12 to 5.81 min at 50 degrees C. At pH 5.5, decreased thermotolerance was observed, with D-values of 0.03 to 0.11 min at 60 degrees C, 0.30 to 0.42 min at 55 degrees C, and 1.97 to 4.42 min at 50 degrees C. Calculated z-values ranged from 5.20 to 6.28 degrees C. D-values of a three-strain mixture of A. butzleri in raw ground pork were 18.51 min at 50 degrees C and 2.18 min at 55 degrees C. Mild heat (50 degress C) followed by cold shock (4 or 8 degrees C exposure) had a synergistic lethal effect, reducing more cells than with an individual 50 degrees C treatment or with cold shock temperatures of 12 or 16 degrees C.