In an egg pasteurizing plant, in-line filters removing solids from raw blended whole egg were sampled on a daily basis for 5 months for the presence of Listeria species. Two selective enrichment procedures (the Food and Drug Administration and U.S. Department of Agriculture [USDA] protocols) were assessed along with three selective plating media, Oxford Listeria selective agar, modified Vogel Johnson agar, and lithium chloride phenylethanol moxalactam agar. Overall, 173 samples were studied, with 125 (72%) being Listeria positive and the USDA method with Oxford agar proving most efficient. The only species isolated were Listeria innocua (62.2%) and Listeria monocytogenes (37.8%).
To estimate the numbers of listeriae present in the blended raw egg, samples were taken from a sampling point immediately prior to the pasteurizer and subjected to selective enrichment in USDA broth. A most probable number counting experiment was employed to study egg samples in quintiplicate, and Listeria spp. were detected using Tecra Listeria ELISA kits, which had been previously evaluated for their sensitivity and ease of use. Samples from 9 successive days' production showed a mean level of Listeria spp. of 1 organism per ml. Hence, Listeria spp. were frequently present at low levels in raw egg before pasteurization. A total of 500 daily samples of pasteurized product were also studied, and all proved to be negative for Listeria, confirming the safety of the pasteurization process with regard to listeriae.