Thermal Resistance of Listeria spp. in Milk.

The thermal resistance of one strain each of Listeria ivanovii , L. seeligeri , and L. welshimeri and three L. monocytogenes strains was determined in raw and sterile milk. Listeria spp. suspended in milk at concentrations of 1 × 105 cells/ml were heated at temperatures ranging from 52.2 to 71.1°C for various contact times. The heat resistance of L. monocytogenes appeared somewhat greater than that of the other Listeria spp. in both milks, but the difference was not statistically significant (α = 0.05). High-temperature, short-time processing is adequate for pasteurization of raw milk.

Growth of Salmonella enteritidis in Yolk of Shell Eggs from Normal and Seropositive Hens.

The growth of Salmonella enteritidis inoculated into the yolks of shell eggs from normal and seropositive hens was determined at various temperatures. All eggs were inoculated with approximately 1 colony-forming unit (CFU)/g of yolk. In eggs from normal hens, the organism multiplied with a generation time of 25 min, reaching a density of about 108 CFU/g in 12 h at 37°C. A generation time of 3.5 h was observed in eggs incubated at 15.5°C, a temperature frequently used for commercial storage of eggs. Cell density of >107 CFU/g was reached in 4 d at 15.5°C. No multiplication was observed in eggs incubated at 7°C for 94 d. When inoculated eggs from seropositive birds were incubated at 37°C, the organism multiplied with a generation time of 35 min, reaching a cell density of >106 CFU/g in 12 h. Raw egg white was detrimental to cells, reducing cell viability 50% in 4 h at 37°C. The limulus amoebocyte lysate test gave a positive reaction with whole liquid egg containing <103 CFU/g. A protocol is suggested for possible application of this test in epidemiological studies that screen grade A shell eggs for Salmonella contamination.

High-Temperature Short-Time Pasteurization Inactivates Listeria monocytogenes.

Full scare commercial pasteurization equipment operated at 72-73°C with a holding time of 15-16 s was used to determine the ability of commercial thermal processing to inactivate Listeria monocytogenes strain Scott A. Three methods of providing L. monocytogenes concentration in raw milk were employed: freely suspended (extra-cellular), inside bovine phagocytes (in vitro procedure), and inside bovine phagocytes in experimentally infected cows (in vivo). Three enrichment methods were used to assay for L. monocytogenes after pasteurization: cold enrichment (4°C, 28 d), selective enrichment of Lovett et al. (FDA procedure) (17), and the USDA-FSIS two stage enrichment procedure. In addition, a 1-L sample taken just before the vacuum breaker was incubated undiluted in the original sample container (4°C, 4 weeks). None of the four assay methods could detect Listeria in the pasteurized milk.

Thermal Inactivation of Phosphatase by the AOAC-V Method.

Thermal inactivation of phosphatase in raw whole milk was determined by the AOAC-V method. D-values in duplicate experiments at temperatures of 63.3, 66.1, 68.9, and 71.7°C were 332.0 and 338.0 s, 91.6 and 90.2 s, 31.4 and 27.6 s, and 9.9 and 8.9 s, respectively. The ZD-value was 5.39°C. Results confirmed phosphatase activity in milk heated at 71.7°C for 15 s and the need to heat raw whole milk for at least 19 s at 72.2°C to obtain a negative test by the AOAC-V.