alone, for temperaturesbetween 35 and 90 ÎC. Nisbetand Langdon(1997)
reported that milkfilms heatedto 80 ÎC becamemoreresistantto desorption.
Above the optimum temperature heat denaturationeffects may causethe deposit
to be more tenaciouslyheld and hencenot removed.Experiments on the removal
of wheyprotein depositsfromstainlesssteel pipesshowa strongdependenceon
temperature(Gillhamet al., 1999).The initial swelling phasedoesnot seemto
be a strongfunction of temperature; however, the uniformphase(even removal
of the deposit) and decayphase(randomremovalof largechunksof deposit,
Section 29.2.3)showa strongsensitivityto temperature.The lengthof the decay
phasesignificantly decreased whenthe deposit±liquidinterfacetemperature
exceeds 50 ÎC, abovethis temperaturelittlefurthereffecton the length of the
decayphaseis seen.
29.3.2 Chemicaleffect
Although wateralonefailsto removemilk and other dairydeposits, it generally
constitutes95%of typicalcleaning fluids(Grasshoff,1997).Alkalisolutions
remove the highlyproteinaceous deposit whereas acidicsolutions are more
efficientin removingdeposits of highmineral content.Commercial cleaning
chemicalsnowoften consistof an alkalinedetergent-basedcleaningsolution
withotherchemicalsaddedto improve the removalof the deposit(Table 29.4).
Increasing cleaningsolutionconcentrationdecreasesthe timeto clean(for
example, Gallot-Lavalleeet al., 1984).Plett(1985)reporteda near linearincrease
in cleaningrate withincreasingdetergent concentration,but that a maximum
cleaningrate occurs.Someauthorshavereportedoptimalchemicalconcentrations
which minimisecleaningtime(includingDe Goederenet al., 1989; Jeurninkand
Brinkman,1994;Fryer and Bird,1994). This actionof the cleaningsolutionon the
deposit structurehas beenstudiedusing SEM(Tissierand Lalande,1986;Belmar-
Beinyand Fryer,1993).Afterexposureto a 2 wt%NaOHsolution,imagesshowed
a less openstructurethan at 0.5 wt%,that may not be as susceptibleto fluidshear,
and is therefore more difficultto removethan moreopenstructures.Otherauthors
have explained this effect by: (i) the swelling of the deposit inhibiting
transportationat higherconcentrations(Plett,1985),(ii) formationof a `glassy'
deposit (Bird andFryer, 1991) or (iii)a rubber-likedeposit(Jeurnink and
Brinkman,1994).Figure29.2(a),fromChristian (2004)showscleaningdata for a
set of differentflows and sodiumhydroxideconcentration.
Optimareported are difficult to compareas deposits, cleaningsolutionsand
equipmentvary. Bird(1992)reports an optimum concentrationof 0.5 wt%
NaOHfor removalof bothmilk and wheyprotein deposits frombench-scale
equipment.Tuladhar (2001)quotes an optimum at 0.5±1.0wt%NaOH for
bench-scaleremovalof WPCdeposit, at temperaturesof 50 ÎC and below.
Changani(2000),studyingremovalof similardeposits froma pilot-scale PHE,
did not observe an optimumin the rangeof 0.1 and 1.5 wt%NaOH. However, a
value of 0.3 to 0.7 wt%NaOHwas suggested for most efficientdepositremoval,
abovethis value thereis littlebenefitof increasing concentration.
480 Handbookof hygiene controlin the foodindustry