representative sampling(Kang& Frank,1989a). They,however, demonstrate an
inherent selectivityfor largerparticles and, since largerparticles are morelikely
to includeviableparticles, thereis a tendencytowards highercountsthanwith
other types of air samplers (Ljungqvist & Reinmu»ller, 1998).
The Reutercentrifugal sampler(RCS)is a portablehand-held instrument,
much usedin the biotechnologyand foodindustry (Griffiths & DeCosemo,
1994).Air froma distance of at least40 cm is sucked into the samplerby means
of an impeller. The air enters the impeller drumconcentricallyfroma conical
samplingarea. The air leavesthe samplingdrumin a spiraloutside the coneof
enteringair (Kang& Frank,1989a). An agarstripis insertedinto the drum
around the impeller blades.Thesampler gives no indication of the sizeof
particles (Griffiths & DeCosemo,1994).In someinvestigations the cut-offsize
of the samplerhas beenfoundto be above 3 m, so singlecellspass throughthe
sampler (Clarket al., 1981;Macher & First, 1983).
37.5.4 Filtersystems
Several different collectingmechanisms are active in filtration(impaction,
interceptionand diffusion). Usually a singlefilteris usedand all particlesizesare
collected withno partitioninginto size fractions(Henningson& Ahlberg, 1994).
In general, the filters are inexpensiveand simpleto operate(Kang & Frank,
1989a). The air filtration apparatus consistsof cellulosefibre,sodium alginate,
fibre glass,gelatine membranefilteror synthetic membranefiltersmountedin an
appropriate holderand connected to a vacuumsource througha flowrate
controller(Kang& Frank,1989a).Microbescan be removedby washing off from
the filterto carryout totalnumber ratherthanviable numberenumeration
(Griffiths& DeCosemo, 1994). The suspensioncan also be assessedwith
appropriatemicrobiological techniques.Membrane filterscan alsobe directly
placed on an agarsurfacefor incubation.The filtermethods are good for
enumeratingmouldor bacterialspores(Kang& Frank,1989a). One of the main
disadvantagesof usingfilters in collecting microbesis that theydo not protect
cells whenlarge volumes of air pass over the filterscausingdesiccation(Griffiths
& DeCosemo, 1994).Shortening of the samplingperiodfor this methodmay
reduce the stress(Kang& Frank,1989a).The culturabilityof somefungalspores
at low and highRH values of 30 and 85%decreased during the first few minutes,
but remainedapproximatelythe samefor samplingtimesrangingfrom30 min to
8 h. The relative culturability ofBacillussubtilisendosporesincreased with
increasing RH as wellbut decreasedwithsampling time.InsteadPseudomonas
fluorescensandSerratia marcescensbacterialvegetativecellswereculturable
onlyif sampledfor 10 minutes or less (Wanget al., 2001).
The largenumberof porespresentin membranesallowsa largevolumeof air
to be sampledduringa shorttime(Kang & Frank,1989a). Prolongedstorageof
the filtersbeforeassay causes deathof sensitive vegetativebacteria(Parkset al.,
1996).Yeastand mesophilic bacterialconcentrationshavebeen observedto
decrease significantlyin the nucleopore filtrationand eluationsamples afterone
628 Handbookof hygiene controlin the foodindustry