only one chemical rinse and result in a cleaner surface (Timperley and
Smeulders,1987).Where two-stage cleanersare usedthe orderof circulationof
acid and alkalistepsis subject to the natureof the deposit and variesfor each
plant. It wouldbe advantageousto knowthe exactrequired length of eachstep in
orderto reduce the overall process time.Cleaning mechanism
An understanding of howthe deposit is removedfromthe heat transfersurface
and the controllingprocessesinvolvedwould aid in optimisingthe overall
cleaningprocess. Untilthe behaviourof deposit removal is understood underall
conditions,the plant maynot be cleanedin the shortest timeand wastefromthe
cleaningprocesswill not be minimised.
Water alonewillnot removemilk deposit; coldwater runsoff without
wetting the surface, fats withinthe depositprevent contact. Hot wateris able to
meltthe fats, allowing bettercontactwiththe deposit. A cleaningfluid must be
selectedin whichthe deposit is soluble.Proteins are waterinsoluble,alkali
soluble and slightly acid soluble. Reactionof NaOHwiththe protein deposit
involves hydrolysis of the peptidebonds, whichlinkthe aminoacids in the
protein structure. The productis morewater-solublethan the nativedeposit. The
water solubility of minerals is variable; solubility of calcium phosphate
decreases withincreasing temperature, hence the scale forms at increased
temperatures.Mostminerals are acidsoluble(Grasshoff,1997)and therefore
removed moreefficiently by acidthanalkali.
Protein deposits swellwhentheyadsorb water, alkali solutionscan reinforce
this swelling and alsodissolvethe protein (Jeurninkand Brinkman,1994);
cracks can thenform,which increase penetration of the cleaningsolution into
the depositlayer.Plett (1985)gavea detailed description of the possible steps
involved in the overall removalof milksoil:∑ Bulkreaction betweencomponentsof the cleaning chemical and the bulk
fluid,e.g. hardwaterions. Muchof the chemical may not evenbe involvedin
the cleaning process.
∑ Transportto the surfaceof the cleaningchemical. The transport process
through the boundarylayeris affected by temperature,concentrationand
flowconditions.
∑ Transport into the depositlayer: penetrationof the chemical components into
the depositis dependent on the deposit structure. Reactionzonesmayappear
at the deposit/fluid interfacein thickdeposits. Surface-activeagentscan
increase penetrationowingto theirwettingability.
∑ Reaction between deposit and cleaning chemical including melting,
mechanical break-up, wetting, swelling, desorption, emulsification,
hydrolysis,saponificationand dispersion.
∑ Transport to the interface: reactionproducts diffuseout of the deposit.
∑ Transport to the bulk:concentrationgradients and hydrodynamicconditions
allowthe transport of the reaction products into the bulk.
476 Handbookof hygiene controlin the foodindustry