4.2.1 Halogen-releasing agents(HRA)
Chlorine-basedcompoundsare the most frequentlyappliedHRAs. Theyinclude
sodiumhypochlorite,chlorinedioxide,and theN-chlorocompoundssuchas
sodiumdichloroisocyanurate(NaDCC).A verycheapand frequentlyapplied
formulation is an aqueoussolutionof sodium hypochloriteproducing hypo-
chlorousacid (HClO)(Krop, 1990;McDonnell and Russell, 1999)(Table4.3 on
page77). HClO is the activecomponentand resultsin the inactivation of all
typesof microorganismssuchas bacteria,virusesand spores (Sofos and Busta,
1999).Another applied formof chlorine is chlorine dioxide (ClO 2 ). It is
synthesised by the reactionof chlorine and sodiumhypochlorite. However,
chlorine dioxideis muchmoreunstablethana standardhypochlorous solution
and decomposes chlorine into gas at temperatureshigher than30 ÎC when
exposed to light(Beuchat,1998).Thiscan lead to dangerous situations as high
concentrations of chlorine gas are explosive (Speek, 2002;Codex, 2003).
However, when the solution is kept cool and protected from light the
disinfectantcan be kept stable at concentrations up to 10 g lˇ^1 (ErcoWorldwide,
2004).
Modeof actionof hypochlorousacid
Although the exactmodeof action is not known, the maindisinfecting effectof
chlorine is causedby oxidative activity.In particular, nucleicacidsand proteins
are destroyed, resultingin irreversible changes and disruptionof DNA-protein
synthesis (Krop,1990).The mechanismof killingof spores differsowingto
theirthick proteinaceous coat. Therefore higherconcentrationsare neededthan
for inactivation of vegetativecells.Youngand Setlow(2003)concludedthat
hypochloriteaffectssporegerminationpossiblybecause of the severedamage to
the spore'sinner membrane.For sporesuspensions, Youngand Setlow(2003)
showedthat a concentrationof 50 mg lˇ^1 during 10 min at roomtemperatureis
sufficient to achieve 4 decimal reductions of Bacillus subtilis spores. A
concentrationof 50 mg lˇ^1 resultedin 1 decimal reductionofB. cereusspores
after1.5 min (Wanget al., 1973).Theseresultsshowthat the minimalinhibitory
concentrationcan varyper species.
Modeof actionof chlorine dioxide
Chlorine dioxide(ClO 2 ), if applied properly, appearsto be 2.5 times more
oxidative thansodiumhypochlorite (Speek, 2002 ; Rodgerset al., 2004),and is
effectiveagainstbacteria,virusesand spores (Hoxeyand Thomas,1999).The
actionof chlorine dioxide involvesdisruption of the cell'sproteinsynthesisand
membrane permeabilitycontrolmechanism. It produces no harmful by-products
as trihalomethans,nor doesit react withammonia.After treatmentwithchlorine
dioxide, spores of Bacillus subtiliscan undergo the initial steps in spore
germinationbut the process stopsbecause of membrane damage(Youngand
Setlow, 2003).An aqueous chlorine dioxidetreatmentof alfalfaseedsinoculated
withE. colifor 10 min at a concentrationof 25 mg lˇ^1 resulted in approximately
1 log reductionof the microorganism (Singhet al., 2003). Compared with
Pathogen resistanceto sanitisers 71