Handbook of Hygiene Control in the Food Industry

The balancebetweencleaning fromwaterand hydroxide was shownclearly
by Christian (2004); a pilot-scale PHEwas cleanedby hydroxide and waterin
rotation.Figure29.2(b) shows typicalresults for pressure dropas a function of
timewhen the flow is switched. Essentially, cleaning stops when sodium
hydroxide is replacedby water.
Two-stage cleaninginvolvesthe circulation of an alkalisolution(commonly
NaOH)and an acid solution (nitricor phosphoric), usuallyseparated by a water
rinsestep(Romney,1990).The orderor circulation dependson the typeof
deposit; a protein-richsoil is removedmoreeffectively by initialcirculationof
the alkalisolution,but this maybe reversedwherea deposit withhighmineral
contentis present. Minerals present within the proteinmatrixare removedwith
the protein during circulationof the alkalisolution.The acidstep that follows
removesminerals remaining on the surface. The overall process maybe time
consuming, useslarge amountsof waterand maynot givesufficientlyclean
results(Timperley and Smeulders,1987).Rinsingstepsmaybe omittedto save
timeand water, althoughthis resultsin ineffectiveusageof the chemicals.
In contrast, single-stage cleaning usesa complexblendof chemicals, con-
tainingformulateddetergents, surface-activeagents(including wettingagents)
and chelatingcompounds (Kaneand Middlemiss,1985;Romney, 1990).They
usuallycontain sodium hydroxideor othersodium salts.Single-stage cleaning

Table29.4 Majorclassesof componentsin alkalinedetergents,and theirfunction,with
examplesof commonsubstances(basedon Kaneand Middlemiss,1985,and Karlsson,
1999)

Component Function

Alkalis
Sodiumhydroxide
Sodiumsilicates
Trisodiumphosphate

Soil dissolution,proteinhydrolysisand

solubilisation,fat saponification

Sequesteringagents
Sodiumpolyphosphate
EDTA
NTA

Formationof complexeswithcalcium

and magnesiumions to avoid

precipitationof salts

Surfactants
Alkylaryl sulphonates(anionic)
Alkylsulphonates(anionic)
Alkylphenolethoxylates(non-ionic)

Wetting,soil removal,solubilisation,

emulsification

Oxidizers, e.g. hydrogenperoxideand
sodiumhypochlorite

For intensifyingthe cleaningeffect

Minorcomponents
Dispersingagents
Antifoamingagents
Anticorrosionagents
Stabilisers

Promotingdispersion,preventingfoam

formation,preventingalkalineerosion,

prolongingstorage

Improvingthe cleaning of heatexchangers 481