(Table 38.7).Inappropriatequenching of activityor neutraliser toxicity will
result in overestimating the antimicrobialactivity of a formulation.
If thereis no neutraliseravailablefor a particularproduct,membranefiltration
can be usedas an alternative.The mixtureof biocideand test microorganismis then
filteredthrougha membranefilterwhichis immediatelywashedwitha rinsing
liquid.The membraneis then transferredto a recoverymediumto enablesurvivors
to producecoloniesafterincubation.The advantageof membranefiltrationis that
neutralisersmay not be used,hencereducingthe potentialtoxicityto injuredcells.
However,the antimicrobialis assumedto be washedoff veryrapidly.Thismight
not be the case withsurfactantsor withcompoundswitha largewater±oilpartition
coefficient.Whereneutralisersare usedin the rinsingfluid,it is also difficultto
verifytheirneutralisationactivity.The use of membranefiltrationand notablythe
rinsingsolutionmightalso add furtherstressto the microorganisms.In addition,
the rinsingsolutionshouldbe compatiblewiththe membranefilterand the filter
usedshouldnot retain/reactwiththe disinfectant/sanitiser.
Overall,the neutralisingor removalof the antimicrobial is of paramount
importance to assess the antimicrobial efficiencyof a disinfectant/sanitiser.
Overlooking the validation of the quenchingstep mightintroduce distortionsin
the results and an overestimation of the killing potency of a biocidal
formulation.Standard antimicrobial testsshould detailthe standardisationof
the neutralisation±dilutionor membrane filtrationprotocols.Physicalparameters
Standard testing protocolsshouldclearlydescribe the experimentalconditions,
including, for example, the diluentto use, test temperatureand pH. Temperature
and pH (Section 38.2.2)can affectthe antimicrobial efficacyof a biocide. As an
example, Taylor and colleagues(1999)studiedthe effectof temperatureon the
lethalefficacy of 18 disinfectants againstPs. aeruginosaandE. coliO157.The
number of disinfectantsthatfail to meet the required lethality against Ps.
aeruginosaincreasedat 10 ÎC (i.e. 11/18insteadof 13/18at 20 ÎC). As for the
diluent, steriledistilledwater,one-quarterstrength Ringer'ssolution,0.9%w/v
saline,peptone waterand nutrient brothhavebeen used,and some might
increase damageto treatedmicroorganisms.Thereis no such`universal diluent'
and moststandardprotocols recommend the diluentto be used.
38.4 Tests for disinfectants and sanitisers
There are a numberof protocolsavailablefor testingthe antimicrobialefficacy
of a disinfectant/sanitiser.Thereis a profusion of information fromthe literature
on exploratory protocolsand on the specificadvantages and inconveniencesof
selected tests, but especially on the factors affecting the viability and
reproducibility of a standard antimicrobial test.Readers can referto some
excellentreviews andchapters by Ayliffe(1989), Cremieux and Fleurette
(1991), Mulberry(1995),Reybrouck (1999)and Lambert (2004).656 Handbookof hygiene controlin the foodindustry