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BIOLOGICAL AEROSOL MEASUREMENT *N DAIRY PLANTS

Figurę 1. The AU Glass lmpinger-30 (AGI-30).

Impoaion methocls. Impaciion usually involvcs ihe collec-lion of microbial aerosols on an agar surface, bul dry or coaied surfaces may bc used for spccial purposes such as particie size deiemiinalion. An impacior consists of an air jel Ihal is directcd over ihe impaciion plale so thai par-ticles colłide wiih and stick onto che surface. Therc are iwo lypes of impaciors, slil samplers (e.g. Casclla slil sampler) and sieve samplers (e.g. Andersen muliisiage sieve sampler. Fig. 2).

This is also a Class B meihod in ihc 15ih edilion of Standard Mcihods for the Examinaiion of Dairy Producis (14).

Slil sampler. The slil sampler usually has a tapered slil which produces a jcl slrcam when ihc air is samplcd by vacuum. The slil sampler may havc a lum labie for ro-lating ihc agar place so ihal aerosol paniclcs are disirib-uicd evcnly on ihe agar surface. Some slil samplers have a timing dcvice on ihe lum-table which nliows cominuous monitoring of airbomc viablc panicles couni (c.g. New Brunswick STA sampler). These samplers will collect parlicles which are grealer ihiin 0.5 |im in size (44).

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Figurę 2. A. Andersen Muliisiage Sieve Sampler. 8. Schemotic cross-section of ihe Andersen sampler. (Courtesty of Andersen Samplers Inc.).

Sieve sampler. Sieve samplers are opcraicd by drawing air ihrough a large number of smali, evenly spaced holes drillcd in a metal piąte (sieve). The suspendcd panicles are impacted on an agar surface located a few mitlimctcrs below the perforalcd piąte. There are single stage (e.g. Ross Micro-ban sampler) and mullistage sievc samplers (e.g. Andersen sampler). A muliisiage sieve sampler consists of a senes (2, 6 or 8) of stacked sicves and plates each with succes-sively smallcr holes. This causes inereased particie velociiy as air flows ihrough the apparatus. Large panicles impact ai ihe iniiial stage and smali panicles follow the air flow until acceleraied sufficicntly to impact a( a later stage (Fig. 2.B).

When the conccntration of viable panicles in an aerosol is high, one sieve hole may allow morę ihan one viable particie to pass through rcsulting in the formation of single colony from two or morę viable panicles. 'ITiis inaccuracy can bc correcied by reducing samplinp, limę or by using eilher Ihe microscopic method or a “positive hole” method for enumeration. The microscopic method involves count-ing panicles ihrough a dissecling lype microscopc bcforc cołonies merge. The ”posiiive hole” mcihods, designed for ihc Andersen 2-stage and 6-stage sampler, arc csscntially a couni of ihe jcis which dclivered viablc paniclcs 10 ihc Peiri plates. This couni is convencd lo a viablc particie couni by ihe use of ihc “posilivc hole” convcrsion tablcs (4,4/).

The muliisiage sicve sampler provides particie size distribuiion informalion. The usefulness of this information in piani sanilalion programs has yel lo bc dcicrmincd.

Umiiaftons. Usually. impaciion mcihods givc higher particie recovery than olher mcihods (27.69.70). Impaciion resulis in Iow sarnpling stresses and aficr collcciion samplc manipulalion is nol requircd. Muliisiage sievc samplers are cumbcrsome lo handle and arc cxpcnsivc. The exacl volume of agar musi bc poured inio all plates aseptically so ihal ihe gap belween ihc sievc and agar surf;icc mccis ihc manufaciurer's spccificaiion. The insidc of iłie snrn* plcr and cvcn ihc oulside of pre-poured agar plates should

JOURNAL Ob FOOD PROTFCTION. VQL. 52. JULY IW#