pressuredropneeded to pumpthe fluidthroughthe heatexchanger willbe
increased and this changein pressuredropcan be usedas a real-timemonitor.
This methodhas similarlimitationsto heattransfermeasurement in that the
pressure dropis the differencebetweenthe product inletand outletpressure in
the heatexchanger.The actualdifferenceand the accuracyof the probeswill
therefore determinethe accuracy of the measured values. The pressure dropcan
be usedto monitorbothfoulingand cleaning and will tend to haveits maximum
sensitivitywhenthe surface is heavily fouledand the pressure dropis highest. It
is therefore possible to combineheat transfer and pressuredropmeasurements to
provide an enhancedlevel of informationon the conditionswithinthe heat
exchanger(Hasting 2002).35.3.5 Microbiologicaland ATPmethods
Untilthe 1980s,monitoringthe conditionof a foodcontactsurface was routinely
carried out by conventionalmicrobiologyculture methodsbasedon agarplate
counts. These methodscan detect specificindicatororganismsas well as provide
informationon the numbers of organismspresent on a surface. The limitations
of suchmethodsare the timetakento obtaina result evenwithmodernrapid
analysistechniques and the skilledresourcerequired to ensure the required
levels of accuracy.
During the 1980sa bioluminescence assaywas developedto detectadenosine
triphosphate (ATP), which is alwayspresentin viable microbes,as a means of
monitoringproductcontact surfaces. The test uses the enzyme luciferase, which
emits light in the presenceof ATP.The lightis measuredin an instrumentcalled
a luminometerand results,usuallyexpressed as RelativeLightUnits(RLU), are
availablein a few seconds. The degree of lightemitted is related to the levelof
ATP.Morerecent developments enablethe technique to be used for analysing
rinse watersamples for the assessmentof closedplantCIP systems.The limita-
tion for this typeof application is that evena consistently low ATPlevel in the
final rinseis no guaranteethat thereis no residual soil withinthe plant. ATP
techniquesare coveredmorefullyin Chapter36.35.3.6 Conductivity
The measurementof conductivity,eitherin-line or off-line,can be usedto
provide a measure of the concentrationof the detergentat the pointof measure-
ment, with conductivitybeing a directfunction of concentration.Thiscan then
be usedto controlthe concentrationof detergent,for example in a CIP recovery
system, by dosingadditionalproduct into the recoverytank.While this maybe
acceptablefor simple detergent applications,it maynot alwaysbe appropriate
where morecomplexformulationsare used.Different componentswithinthe
formulationwill havespecificfunctions and may not all be depletedat the same
rate and a single measure cannotmonitorthis. An example wouldbe a detergent
containingsodium hydroxide for organic soil removaland the sequestrantedta
580 Handbookof hygiene controlin the foodindustry