Food Microbiology by Karl R. Matthews

Author:Karl R. Matthews
Language: eng
Format: epub
Publisher: Wiley

Susceptibility and Tolerance

Y. enterocolitica is unusual because it can grow at temperatures below 4°C. The doubling time at the optimum growth temperature (ca. 28 to 30°C) is ∼34 min, which increases to 1 h at 22°C, 5 h at 7°C, and ∼40 h at 1°C. Y. enterocolitica readily withstands freezing and can survive in frozen foods for extended periods even after repeated freezing and thawing. Y. enterocolitica generally survives better at room temperature and refrigeration temperature than at intermediate temperatures. Y. enterocolitica persists longer in cooked foods than in raw foods, probably due to increased availability of nutrients in cooked foods. Also, the presence of other psychrotrophic (growing between 5 and 35°C) bacteria, including nonpathogenic strains of Y. enterocolitica, in unprocessed food may restrict bacterial growth. The number of viable Y. enterocolitica organisms may increase more than a 1 million-fold on cooked beef or pork within 24 h at 25°C or within 10 days at 7°C. Growth is slower on raw beef and pork. Y. enterocolitica can grow at refrigeration temperature in vacuum-packed meat, boiled eggs, boiled fish, pasteurized liquid eggs, pasteurized whole milk, cottage cheese, and tofu (soybean curd). Growth also occurs in refrigerated seafoods, such as oysters, raw shrimp, and cooked crabmeat, but at a lower rate than in pork or beef. Yersiniae can also persist for extended periods in refrigerated vegetables and cottage cheese.

Y. enterocolitica and Y. pseudotuberculosis can grow over a pH range of approximately 4 to 10, with an optimum pH of ca. 7.6. They tolerate alkaline conditions extremely well, but their acid tolerance is less apparent and depends on the acidulant used, the environmental temperature, the composition of the medium, and the growth phase of the bacteria. The acid tolerance of Y. enterocolitica is enhanced by the production of urease, which hydrolyzes urea to release ammonia and elevates the cytoplasmic pH.

Y. enterocolitica and Y. pseudotuberculosis are susceptible to heat and are easily killed by pasteurization at 71.8°C for 18 s or 62.8°C for 30 min. Exposure of surface-contaminated meat to hot water (80°C) for 10 to 20 s reduced bacterial viability by at least 99.9%. Y. enterocolitica is also easily killed by ionizing and ultraviolet (UV) radiation and by sodium nitrate and nitrite added to food, although it is relatively resistant to these salts in solution. The pathogen can also tolerate NaCl at concentrations as high as 5%. Y. enterocolitica is susceptible to organic acids, such as lactic and acetic acids, and to chlorine. However, some resistance to chlorine occurs among yersiniae grown under conditions that are similar to natural aquatic environments.

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