conveniently by theQ 10 value(changein activityfollowing a rise of 10 ÎC). The
Q 10 value is given by the equation:Q 10 àTime to kill atTC
Timeto kill atÖTá 10 ÜC
Standard testingprotocolsrecommendtestingat a temperatureof 20 ÎC1 ÎC
(e.g. CEN1276, 1997a) or around ambient temperature (18±25 ÎC) (e.g.
CEN13697, 2001). However, this does not reflect product usage at low
temperature,althoughthe activityof a compoundat additionaltemperature can
be tested.
Theeffectof pH on antimicrobialactivityis complex and can affect the
microorganism as wellas the compound(Russell,2004).For somebiocides,
theiractivestateis the non-ionised form(e.g.phenols, acetic acid, benzoic acid)
and increase pH decreases their activity. Others (e.g. cationic biocides,
glutaraldehyde)showan enhanced activity at an alkalinecondition.However,
testing for antimicrobialefficacy at differentpHs is usuallynot recommended
since the pH is usuallyset for a givenantimicrobialformulationand cannotbe
altered easilywithoutaffectingthe stabilityof the formulation.
Biocides withinan antimicrobial formulation can be partially inactivatedby
the componentsof the formulation.Surface-activeagents, notablynon-ionic
agents, can affectthe activity of antimicrobial compounds.It is thereforenot
surprisingthat many non-ionic surfactants are usedas neutralising agentsand
this is considered in more detail below (Section 38.3.3). The potential
inactivation of antimicrobial activity by non-ionic agentsis particularly a
concern for the preservationof pharmaceutical and cosmetic productswith
biocides suchas parabensand QACs. The critical micelleconcentration(CMC)
of the non-ionic agentis particularlyimportant.Belowthe CMCit is generally
believed that the non-ionic agentincreases membrane permeability, but also
releases the biocide to producea highlyactive solution.Metalionscan also
affect the antimicrobialactivityof a biocidal formulation,notably whenthe
formulationcontainspermeabilisingagentsin the formof ion chelators(e.g.
ethylenediaminetetraacetic acid).
Likewise, partitioning of the antimicrobialagentmightlead to a decreasein
bioavailability. For example, phenolcan absorbinto rubber, hencereducing its
effectiveaqueous concentration(Allwood, 1978).
The surfaceto be disinfected is not usuallylisted as a factorinfluencingthe
activityof a biocideas such,but needsto be considered here.The antimicrobial
efficacyof disinfectants or sanitisers will dependto someextent on the surface
uponwhichtheyare used.Surfacescan varygreatly, particularlywhetherthey
are porousor non-porous.Poroussurfaces will havea tendencyto entrapand
protect microbial contaminants, whereas non-porous surfaces can reduce
bacterialadhesionand facilitate a cleaningor a disinfection process.
Mostbiocidal formulations are dilutedprior to use in hardwater. Hardwater
and notably the presenceof divalentcations(e.g.Mg2+, Ca2+) may interfere with
the activityof some disinfectantsby blockingadsorptionsiteson the bacterial
648 Handbookof hygiene controlin the foodindustry