A survey of frozen milk product (FMP) plants in California was conducted to determine prevalence of Listeria spp. among plants and within specific areas in plants. Association of possible factors contributing to the presence of Listeria was analyzed. Of 922 samples, 111 (12%) were positive for Listeria spp. Listeria monocytogenes and L. innocua were the only species isolated. Of 39 plants sampled, L. monocytogenes was the only species recovered from 5 (12.8%) plants and L. innocua was the only species recovered from 13 (33.3%) plants. Both species were isolated from 9 (23.1%) plants. No Listeria was isolated from 12 (30.8%) plants. A minimum of five colonies was identified from each positive site. Only one species was identified from 105 of the 111 positive sites (94.6%). There was a statistically significant association of culture results from selected sites in plants with corresponding drain cultures; however, from 9 of 106 (8.5%) site/drain combinations no Listeria was isolated from the drain when the selected sites were positive. Type of product received and type of pasteurization did not influence recovery of Listeria from a plant. There was a significant association with size of the plant and recovery of Listeria. Level of sanitation and extent of environmental contamination control program (ECCP), individually, were not associated with recovery of Listeria from a plant. However, the rate of recovery of Listeria from plants with above average sanitation and excellent or moderate ECCP (N=7) was 6.81%, whereas the recovery rate from plants with below average sanitation and slight or no ECCP (N=7) was 27.5%. The highest recovery rates of Listeria in FMP plants were in batch flavoring, freezing and ingredient blending, and packaging filling areas.

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