packaging. A significant portion of fresh
meats used as raw materials for processed
products can be contaminated with this psy-
chrotrophic pathogen and point to the
importance of preventing postprocessing
recontamination of ready-to-eat products.
Table 17–1 illustrates the incidence of
L. monocytogenes in post-heat processing
environments in 41 meat plants. Other viable
product contamination areas include slicers,
dicers, saws, lugs, tubs and other containers,
hand tools, gloves, aprons, packaging mate-
rials, packaging equipment, tables, shelves,
racks, and cleaning equipment. Other areas
where this pathogen may be hidden include
recesses, hollow rollers, motor housings,
switch boxes, rusted materials, cracked or
pitted hoses and door seals, walls that are
cracked or pitted or covered with inade-
quately sealed surface panels, vacuum/air
pressure pumps lines or hoses, air filters,
open bearings, and ice makers.
L. monocytogenesis often found around wet
areas and cleaning aids, such as floors, drains,
wash areas, ceiling condensate, mops and
sponges, brine chillers, and at peeler stations.
Biofilm formation is exacerbated through
older and unclean equipment with exposed
bolts and threads, and unsealed rivets. Thus,
control of Listeria organisms in processing
plants is essential to reduce the potential of
post-processing contamination. One cannot
control the growth of this pathogen through
refrigeration at 4 to 5ºC (a common storage
temperature) because this microbe can survive
at a 0ºC storage environment. Doyle (1987)
has suggested that the use of antimicrobial
agents, reduced temperature (<2ºC) storage,
reformulation of products (reduced minimum
water activity [Aw], pH, etc.), or post-process-
ing pasteurization of products may need to be
incorporated for the control of such psy-
chrotrophic pathogens in foods.
Frank et al. (2003) evaluated the effective-
ness of cleaning and sanitizing chemicals in
the removal of L. monocytogenesbiofilms
coated with soil of poultry origin and applied
under static conditions without heat applica-
tion. Alkaline and neutral cleaning com-
pounds were evaluated as well as sodium
hypochlorite, acidified sodium chlorite, per-
oxyacetic acid, peroxyacidic acid/octanoic
acid mixture, and quaternary ammonium
compound sanitizing agents. The alkaline
cleaning compound removed 99% of fat and
93% of protein within 30 minutes. The neutral
cleaning compound was equally effective at
removing fat, but eliminated only 77% of pro-
tein. The alkali cleaning compound also effec-
tively removed L. monocytogenes biofilm
coated with protein. Biofilm removal is more
successful if cleanup is initiated as soon as
possible after the production shift ceases.
More prompt cleaning after production302 PRINCIPLES OFFOODSANITATION
Table 17–1Incidence of Listeria monocytogenes in Meat Plant Post-heat Processing Environments
Location Percentage Positive for L. monocytogenesFloors 39
Drains 39
Cleaning aids 34
Wash areas 24
Casing peelers 22
Food contact surfaces 20
Condensate 7
Walls and ceilings 5
Compressed air 4