harsh elimination procedures in food processing environments. As these
procedures,includingdifferent disinfectionactions,havenot alwayssucceeded
in theirtask therehas beena needfor novelapproachesin the eliminationofL.
monocytogenes. Molecular typingmethodshaveofferednewtoolsin the fight
againstL. monocytogenesas they haveenabled researchers to tracethe organism
in the foodprocessingenvironments and point out sitesof contamination, which
in turn has enabledthe targetingof preventivemeasures.
33.3.1 CharacteristicsofListeriamonocytogenescontaminationof food
processingenvironments
Listeria monocytogenescontamination can be dividedinto two majorevents: (1)
contamination of the foodprocessingenvironmentand (2) contaminationof the
product (Autioet al., 2004).The contamination of the processingenvironment
can furtherbe dividedinto:(a) contaminationoriginatingfromthe outsideof the
plant by raw materials and (b) persistentcontaminationoriginatingfromthe
insideof the plant.
Contamination of the food processing environment can occur through
differentways,which are highlighted in Fig. 33.1. Rawmaterials are frequently
contaminatedwithL. monocytogenes(Johanssonet al., 1999;Herediaet al.,
2001;Miettinenet al., 2001b)and are probablythe mostimportantsource of
contamination (Lawrence and Gilmour, 1995; Berrang et al., 2002). It is
therefore not surprising to findL. monocytogenesduringprocessing in the raw
areaof the plant. The presenceofL. monocytogenesin the raw arearequires
strict hygienebarriersbetween the rawarea and the high hygiene areas,
especiallythe post-heattreatment area,in orderto stopthe spreadof con-
tamination. Poor separation or no separation has been shown to increase
contamination in the post-heattreatment area in meatprocessingplants (Lunde¬n
et al., 2003a).
Althoughthe initialcontaminationof the raw areain foodprocessingplants
appearsto be causedby contaminatedraw materials,the contaminationof the
processingenvironment and especially the post-heat treatmentarea is more
Fig. 33.1 Listeriamonocytogenescontaminationin foodprocessingplants.Block
arrowsindicatecontaminationroutesofL. monocytogenes.544 Handbookof hygiene controlin the foodindustry