the amount of ATPon a surface and henceits cleanliness. The level of ATP
withincellsvariesdepending upon the typeof cell,e.g. animal,yeast, bacteria,
and its phaseof growth, but the ATPpoolin livingcellsis normally kept
consistent by regulatorymechanisms(Davidson et al., 1997).Theenzyme-
substrate complexluciferin±luciferaseconvertsthe chemical energyassociated
withthe ATPintolightin a stoichiometricreactionwith1 photon of light
produced by the hydrolysisof 1 molecule of ATP.The lightemitted is normally
measured in relativelight units(RLUs), calibrated for eachmakeof instrument
and set of reagents.Therefore the readingsobtainedfromassessingfoodplant
cleaning needto be compared withbaseline datarepresentingacceptable clean
values(see Section36.5.2).
A range of luminometersand testsare availableand majornew developments
in assaysand equipment occurapproximatelyevery4 years.Originallylumino-
meterswerelargeand only suitablefor laboratoryuse. Thesehaveevolvedover
the yearsinto small hand-held models, whichcan be usedanywhere within a plant.
Manyluminometersuse a photomultipliertubein the lightdetectionsystem,
althoughas part of the driveto cheaper,smallerinstruments,somemanufacturers
havereplacedthesewithless sensitivephotodiode-basedsystems. Thismay,
dependingon the chemistryof the assaysystem,reduce overalltest sensitivity.
Luminometerscan incorporatea printer,althoughthis needhas beenobviatedin
mostof the newerinstrumentswithtrendanalysissoftware,which can storeand
then downloaddata to a PC. Thissoftware is veryuseful for comparingdata over
timeand fromdifferentsitesand plants.It indicatesareasfrequentlyimproperly
cleaned and surfaces that are movingtowards loss of control, and allows
comparisonbetweencleaningoperatives.One manufacturerhas addedthe ability
to performadditionalchecks,e.g. pH and temperaturemeasurement,by adding
additionaltest probesand facilitiesto the luminometer.This may be useful but can
be problematicif one part develops a faultand, at the end of the day, it is how well
the luminometerand the test designedfor it actuallyperformthat is the most
importantdeterminantof choice.Mostmanufacturersoffercalibrationand/or
positive/negativecontrolsto helpensure accuracyof readings.
Changesin the assays haveresulted in the abilityfor testingto be performed
by non-technicalstaff,withtubesand pipettesreplacedby simple,singleshot,
*pyrophosphateFig. 36.6 Schematicrepresentationof the ATPbioluminescencereaction.Improvingsurfacesamplingand detection of contamination 605