Very Small Plant Listeria Audits 

Audit of Post-Lethality Environment of Very Small Processing Plants for Listeria Species 

Dennis R. Buege and Steven C. Ingham 

Extension Meat Science and Extension Food Safety Program – University of Wisconsin-Madison 

Purpose of Audit: Prevention of post-lethality contamination of RTE meat and poultry products by the 

environmental pathogen, Listeria monocytogenes, has become a high priority for the industry and those agencies 

which regulate it. In October 2004 a USDA FSIS rule went into affect directing plants to sample food contact 

surfaces in their post-lethality environment at a frequency dependent on how RTE products are formulated and 

handled. From 2004 in-state training sessions for “very small” plants focusing on prevention of Listeria 

contamination of products and implementation of the Listeria rule, it was readily apparent that the vast majority 

of such plants had never conducted any environmental monitoring for this pathogen in their operations. The 

purpose of this audit was to sample food contact and non-food contact surfaces in the post-lethality environment 

to determine if and where Listeria contamination was a problem in smaller-scale plants. 

Benefit of Audit to Smaller-Scale Plants: The results of this audit of smaller-scale plants provides operators 

with a snapshot of the Listeria status of the post-lethality environment over a wide range of plants. Knowing 

where this pathogen is likely to be found in operations similar to their own will aid processors in addressing the 

challenge of controlling Listeria in the post-lethality environment. As this information was being collected it 

was shared with the participating plants and our state industry in general to provide just this type of assistance. 

How Audit Was Conducted: Between January and September, 2004, 31 State-Inspected and FSIS-Inspected 

Wisconsin smaller-scale plants were sampled for the presence of Listeria species in their post-lethality 

environments (438 food contact and non-food contact surface samples collected). Plants were invited to 

participate and arrangements were made to sample when the handling and packaging of RTE product was taking 

place. In a few instances production was not going on at the time of sampling, but samples were still collected 

from key locations. Where possible, as large a surface sample as possible was collected with commercially 

available pre-moistened sterile sponges to detect the possible presence of the pathogen. Usually the surface area 

sampled far exceeded the one square foot sampling area recommended in the FSIS Listeria rule. Collected 

samples were held under refrigeration and delivered to a commercial microbiological testing laboratory within 

24 hours, for determination of Listeria species only. Results were shared with participating plants after they 

became available. 

Findings of the Audit: An accompanying table summarizes the findings of the this audit. Three and one half 

percent of food contact surfaces were positive for Listeria species. Positive samples were found on slicers used 

for wholesale products (2/23), surfaces of packaging tables/carts (2/36), the internal cavity of chamber vacuum 

packaging equipment (1/17) and tubs or lugs which held RTE product (1 of 29). Because Listeria is so widely 

distributed in the environment, it is very unlikely that it would never be found in even very sanitary 

environments. The relatively low incidence found on food contact surfaces suggests these plants are doing 

reasonably well in sanitation and employee practices which directly affect the RTE product. The two positive 

samples found on wholesale product slicer were both from clean slicers not in use at the time of sampling. This 

points to the difficulty in cleaning that piece of equipment. In addition to thorough breakdown and cleaning of 

slicers, it is recommended that slicers and other post-lethality food contact surfaces be re-sanitized directly 

before use when such equipment is not used every day (frequently the case with small establishments). 

The audit results on non-food contact surfaces reveals a particularly high rate of incidence on floors and drains. 

This has been reported before for larger plants, but serves as a strong admonishment for smaller plants to pay 

close attention to these areas. Floor and drains will always have a higher rate of incidence than food contact 

surfaces, and fortunately floor and drains are somewhat removed from direct product contact. However, the 

frequent presence of Listeria organisms in floors and drains does create an ever-present potential threat of cross 

contamination to food contact surfaces in the post-lethality environment. It is recommended that processors 

work with sanitation suppliers to develop a sound floor and drain sanitation program if they don’t already have 

one in place. Such a program may include more routine scrubbing of floors and drains, application of high 

levels quat sanitizers (800+ ppm – leaves an active residue), limiting use of high pressure hoses (which create 

aerosols which transport bacteria) and altering/limiting employee foot traffic where possible. 

auditsummary.s124.wpd 
2004 Audit for Listeria Species in Very Small Plants in Wisconsin 

Number of Plants Sampled = 31 (January 14 to September 2, 2004) 

Sampling Site 

No. Samples 

Collected 

No. Positive 

Samples 

% 

Postive 

Food Contact Surfaces

 Wholesale Slicer 23 2 8.7 

Packaging tables/carts 36 2 5.6 

 Product cavity of vacuum packager 17 1 5.9 

Tubs, lugs, carts which hold RTE product 29 1 3.4 

Bare or gloved hands of person doing packaging 22 0 0.0 

 Retail Slicer 19 0 0.0 

Knives/scissors used on RTE product 12 0 0.0 

Shelves in cooler which hold RTE product 5 0 0.0 

 Small link casing peeler 4 0 0.0 

Chain link or belt conveyors 4 0 0.0 

Non-food Contact Surfaces

 Drain by Smokehouse 31 13 41.9 

 Floor in front of smokehouse 32 9 28.1 

 Floor in or adjacent to packaging room 18 4 22.2 

 Floor in RTE cooler 14 3 21.4 

Drain in packaging room 20 3 15.0 

 Door frames/handles in RTE product area 36 2 5.6 

 Lower frame of smokehouse cart 28 1 3.6 

Control panel of vacuum packager 31 0 0.0 

 Walls of RTE cooler 28 0 0.0 

 Side frame of equipment in packaging area 25 0 0.0 

Brushes, mops, squeegees in RTE 4 0 0.0 

Food contact surface = 171 samples = 6 positives = 3.5% 

Non-food contact surfaces = 267 samples = 35 positives = 13.1% 

Floors and drains = 115 samples = 32 positives = 27.8% 

“Listeria.Tested”, Misc. #11, 4-15-05