The cleaning of the heatexchangers in the dairyis more problematic as the
fouling on the heatexchanger platesdiffers fromthe soilingin the coldmilk
area. The fouling of the heating plantsin the dairyis tightlystructured,it adheres
firmly to the solidsurfaces,and contains (apart frominorganicsubstances,
whichare not enzymaticallydegradable) primarilydenatured proteins.These
denatured proteins, are not recognized by mostof the specifically acting
proteases,limiting the numberto onlya few knownproteases that can be used
for the cleaningof milkheatexchangers.
Although the cleaningtrials of the milkheatexchangerperformedin the
laboratory,as wellas the fieldtrials in practiceachievedexcellent results,they
havenot yet beenusedin practice.Onepossible reasonmaybe that in most
dairies the cleaningvia a centralCIP stationoccurs with`universal chemicals' ±
oneacid andonealkalinecleaning agentfor all the applications,at bestin
differentconcentrations. A prerequisite for the introductionof the enzymatic
cleaningis the installationof decentralized cleaning stationswithchemicals
specificallyadapted to the objectto be cleaned.For the cleaning of the cold milk
area, otherprocessesand chemicals thanthose appliedin the cleaningof heat
exchangershaveto be used.The cleaning of UHTplants has still to be under-
taken conventionally as correspondinglaboratorytrialswerenot successful.
Conventional cleaning occursautomaticallyvia computer-assistedprograms.
This assumesbeforehand that the correspondingparameters (periods, tempera-
tures, volumes, flowvelocities, concentrations,but alsovalve functions)are
availableas measurablevalues or control factors. Aboveall, the mixing phases
haveto be safely recorded.In conventional cleaningthe phase separation
between product,waterand differentcleaningsolutionsoccursvia measurement
of cleaning solutionelectricalconductivity.In the conventionalmethodthis
functionswellas the measuring systems are reliableand largelymaintenance-
free, and as the conductivityof water,product and cleaningsolution differ
significantlyfromeachother(tap water0.50 mS/cm; milk30±60mS/cm; 0.5%
NaOHapprox.25 mS/cm; 3% NaOHapprox. 130 mS/cm;0.5%HNO 3 30 mS/
cm, 2% HNO 3 115 mS/cm). In the 1% solution of a ready-madeSavinaserecipe
in tap water,the conductivity is only0.80 mS/cm. Thus,it is extraordinarily
difficultwithconductivitymeasuring systemsset up for measuringareas up to
approximately 200 mS/cm, to differentiate in the zonenearto zerobetween tap
waterwith0.50 mS/cmand enzymatic cleaningsolution with0.80 mS/cm,and
to guarantee a safeseparationof the phases. However, this problem can be
solved by the installation of a pH measuring system. pH has its largestspread
angle in the neutralor low alkalinearea,whereasthe electricalconductivityhas
a roughlylinearcurve.A cleardistinction can be madebetween pH 7 (tap water)
and pH 9 (enzymatic cleaning solution). Contraryto the conductometric
measurement, pH measuring systemsneedregularcontrol, cleaning and calibra-
tion.Nevertheless, robust process measuring systemsare available on the
market. Theyare equippedwithautomaticallyperformedmaintenance func-
tions,so that theyare comparablewithmoreor less maintenance-free systems
for electricalconductivitymeasuring.
536 Handbookof hygiene controlin the foodindustry