Listeria innocua PFEI is a chloramphenicol- and erythromycin-resistant organism obtained by electotransforming L. innocua ATCC 33091 with the plasmid pGK12. L. innocua ATCC 33091 and L. innocua PFEI were more heat resistant between 56 and 66°C than Listeria monocytogenes F5069 and Scott A when evaluated in sterile phosphate buffer or milk. Decimal reduction times of each L. innocua strain were 1.5 to 3 times longer in either heating menstruum than were D-values of the most heat resistant L. monocytogenes strain studied (F5069). L. innocua PFEI retained the plasmid during heating so that, of 300 survivors evaluated, 100% were resistant to chloramphenicol and 98% were resistant to erythromycin. Thus, L. innocua ATCC 33091 or PFEI would be useful indicator organisms to evaluate the lethality of thermal processes with respect to L. monocytogenes. L. innocua PFEI has the advantages that it could easily be selected and enumerated among a large, complex, background microflora, but it may not be appropriate for application in a food processing environment.

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Microorganisms which survived and grew at 4" or 10°C in ultrapasteurized, aseptically packaged liquid whole egg were isolated and identified. Growth at 4" and lO"C, and inactivation from 50 to 7O"C, of selected isolates were evaluated in ultrapasteurized egg. Bacillus circulans and a Pseuriomonas species grew well at 4°C. with generation times of 1 .O and 0.3 days, respectively. Enterococcus fuecalis and B. circuhns were the most heat resistant isolates studied. Because B. circulans grew at 4°C and was relatively heat resistant it would be a useful test organism for many applications while E. faecalis would be useful since it was heat resistant and would grow rapidly when the storage temperature is less than ideal.

Thermal death times (F-values) for L. monocytogenes F5069 inoculated into sterile liquid whole egg were determined between 62 and 73°C by a submerged capillary tube procedure. The initial population was 5 × 106 to 2 × 107 CFU/tube (0.05 ml). High populations intentionally were selected to build in a safety factor. At each temperature, F-values were determined to be the shortest heating time which did not permit recovery of L. monocytogenes from six or more replicate tubes. L. monocytogenes were recovered by incubating the entire contents of the capillary tube in brain heart infusion broth at 25°C for 2 weeks. At 62°C, F = 16 min and at 69°C, F = 1.6 min. The zF-value was 7.1°C. Minimal pasteurization of egg would not result in product free from L. monocytogenes if initial populations were large. Ultrapasteurization processes may be designed to produce product free from L. monocytogenes and appropriate for prolonged refrigeration.

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