The thermal resistance of Listeria monocytogenes associated with a milkborne outbreak of listeriosis was determined in buffer and whole milk. Thermal resistance was stable over a 2-year period and could not be altered by selecting heat-stressed survivors. The rate of inactivation was linear and did not differ significantly between pH 5.5 and 9.0. When portions of whole milk containing 1 × 105 cells of L. monocytogenes/ml were heated at seven temperatures from 52.2 to 74.4°C, the D-values ranged from 1683.7 to 0.7 s, respectively. The zD-value was 6.3°C. The D-value at 71.7°C was 0.9 s. L. monocytogenes would not survive the pasteurization process.

The sterilization value (F,) and heat penetration factors (fh and j) were determined from time/temperature data as a function of container headspace, reel speed, and product consistency for cans of condensed cream of celery soup heated in an FMC Steritort. The product was formulated in the laboratory, and water was added to control consistency. Two commercial instruments were used to measure product consistency. Fill weight (headspace) was the most critical processing parameter for the simulated Sterilmatic (individual-serving-size cans) and Orbitort (institutional-size cans) processes. As the headspace bubble was eliminated, the product was heated by conduction in a manner similar to a still process. The degree of agitation of the product was also directly affected by reel speed and consistency. The integrated sterilization value (IS) was determined from the spore count reduction technique of process determination for selected processes and normally exceeded F,. The difference was as much as 6.9 min in the 603 x 700 can.

Seventeen strains of Salmonelfu enretitidl isolated from patients and various implicated products from 5 egg-associated outbreaks of salmoncllosis were investigated for plasmid content, phage type and thermal resistance. D values obtained at 57.2 and 60°C in liquid whole egg for all strains were with+ ranges reported by previous investigators. Sublethal heat shock at 40°C for 2 hr or 48°C for 30 min increased thermotolerance 2-to 3-fold. The induced thermotolerance decayed to control levels after 2-3 cycles of cell duplication at a lower temperature. Most strains contained a 39-Mda plasmid and were phage type 8. No correlation was observed between thermotolerance and presence or absence of plasmid or phage type.

Thermal resistance of intracellular and freely suspended Listeria monocytogenes that was associated with a milkborne outbreak of listeriosis was studied by using the sealed tube and slug flow heat exchanger methods. Test temperatures for the former method were 57.

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.