enoughto multiply, and contaminatethe product.The problemis exacerbated
when a processincludes deadspaces where productcan stagnate.
As an example,if a singlecell ofEscherichiacoliis trappedin a deadspace
filled with5 ml of a slightly viscouslow-acidfoodproduct at a temperatureof
approx. 25 ÎC, it couldtake less than24 hoursfor the numberof microbialcells
to increaseto 0.2 109 per ml, assuming theydouble every40 minutes
(Lelieveld,2000).If 1 ml per houris washed out fromthe deadspaceby the
passing product, thenthe product wouldbe contaminatedwith200 millionE.
colicellsper hour,by the end of the first day'sproduction.If the production
capacity of the line is 5 106 ml per hour,the average levelof E. coli
contamination wouldbe 200/5= 40 per ml. Manytraditionalprocess lineshave
much larger(oftenverycontaminated)deadspacesand growth-ratescan be
higher if conditionspermit.
Microbesmayalso penetrate throughverysmallleaks.Thereis considerable
evidencethat theycan passthroughmicroscopicopenings veryrapidly and that
pressuredifferences mayretard, but not prevent, passage,evenif the pressure
differenceis as highas 0.5 bar. The bacteriumSerratia marcescensmay moveat
a speedof 160 mm per hour(Schneider and Dietsch, 1974).Motilebacteria may
propel themselves against the flow of liquidthrough a leak. Whethermotile or
not, theymayalso penetrate by forminga biofilmon the surface. Studies on the
migration of microorganismsthroughmicroscopic channels showthat passage
can occurthroughholesa few micrometresin diameterin a metalplateof
0.1 mm thickness(Bre¬notet al., 1995).
Whenattractedto a surface,microbesare deposited,attach and initiate
growth. As they grow and multiply,the newlyformedcellsattachto eachother,
as wellas to the surface, forming a growing colony. Whenthis massof cells
becomes largeenoughto entrapdebris,nutrients and other microorganisms,a
microbial biofilm is established(IFT,1994).Biofilms formin two stages.First,
an electrostatic attraction occursbetweenthe surfaceand the microbe.The
process is reversible at this stage. The nextphaseoccurswhen the organism
formsan extracellularpolysaccharide, whichfirmlyattaches the cell to the
surface.The cell thenmultiplies, formingmicro-coloniesand,ultimately, the
biofilm (Notermanset al., 1991).Thesefilmsare verydifficult to remove during
cleaningoperations(Firstenberget al., 1979).Microorganisms that appear to be
more difficult to removebecause of biofilm formation includethe pathogens
Staphylococcus aureusandListeriamonocytogenes(Notermans,1979).Current
informationsuggests that heat treatment is more effective than the applicationof
chemical sanitisers, and Teflonappearsto be easierto clearof biofilm than
stainlesssteel(Marriott, 1999).
Biofilm developmentmay take placeon any type of surfaceand is difficult to
prevent, if conditionssustain microbial growth. Manyorganisms,includinga
number of pathogens (Listeria monocytogenes, Salmonella Typhimurium,
Yersinia enterocolitica,Klebsiella pneumoniae,Legionella pneumophilaand
Staphylococcus aureus) formbiofilms, evenunderhostileconditions,suchas
the presenceof disinfectants. Adverse conditionsevenstimulatemicroorganisms
16 Handbookof hygiene controlin the foodindustry