∑ Swelling± alkali solution contacts the depositand causesswelling,forming a
protein matrixof highvoidfraction.
∑ Erosion± uniformremovalof depositby shearstressforcesand diffusion;
theremaybe aplateauregionof constant cleaningrate,but this depends on
the balance between swelling and removal.
∑ Decay± the swollendepositis thin and no longer uniform,removalof
isolated islandsoccurs by shearstressand mass transport.
This behaviour has beenseenby many workers,suchas Bird(1992),Gillham
(1997), Grasshoff (1997)and Tuladhar (2001), using visual methods, heat
transfer monitoringand fluiddynamic gauging.SEManalysis showed that a
more openstructure is formedwhen whey proteinscome in contactwithalkali
solution (Bird,1992;Gillham, 1997).At higherconcentrations(e.g.above 2
wt%NaOH) the structurecan becomeless susceptible to shearforces and more
difficultto remove.Proteindepositsare oftenremovedin largechunks, and
hencemayremoveminerals fromdifferentlayers withthem.Figure 29.1(b)
shows the proposed dependencyon the cleaningprocessparameters,which
suggests that the plateau regionis a functionof temperatureand the decayregion
is dependent on the flowvelocity.
Christian (2004), Birdand Fryer(1991)and Grasshoff (1997)carried out
mechanisticstudiesto determine the effectof processparameters(temperature,
flow rate and chemical concentration)on eachstageof the cleaning process,
discussedbelow. Theyconcludethat diffusionand reaction will be importantin
the initialstagesas the deposit swells;however, theirinvestigation also showed
that a key step is removalof swollen depositin lumpsby shear.
Monitoringcleaning
It is firstimportant to assesswhatis meantas clean'.Whatis sufficient for some cases maynot be for others;for example in the pharmaceutical industry, where a single microbe is potentially catastrophic, systems must be microbiologically clean.In foodprocessing,clean'is oftenconsideredas
returning the systemto the condition it was in beforefouling occurred;after
cleaning,the foulingrate is the sameas in previous runs± althoughthis might
misssmallamounts of fouling. It is importantto monitorfoulingand cleaning.
Types of monitors include(Karlsson, 1999):
∑ continuous± allows monitoringof soil removal,ordiscontinuous± enables
the amount of soil remaining aftercleaningto be determined;
∑ in-situ± measurementscarried-out in the experimentalequipment, orex-situ
± requires the removalof a samplefromthe set-up;
∑ direct± measurements at the surface, orindirect± monitors cleaningby the
amount removed.
Ideally monitors shouldtell bothwhena processrun should stopand when a
plant is clean.Foulingincreasespressure drop(P) and decreasesheat transfer
coefficient(HTC); thesevalues can be monitored,together with(for example)
478 Handbookof hygiene controlin the foodindustry