A working methodology for the total water determination
in inulin bythe volumetrie Karl Fischer titration
Sébastien N.Ronkartl.2, Christophe Blecker2, Christian Fougnies-, Claude Deroanne! and MichelPaquet' )Gembloux Agricultural University. Departmentof food Techoology. Passage desDéportés. 2.8·5030 Gembloux,Belgium.
2Gembloux Agricultural University. Departmcnt of Industrial Biological Cbernistry.Passage desDéportés. 2.8·5030 Oembtoux.Belgium.
3 CosucraGroupe WarcoingSA,Ruede la sucrerie. 1.8·7740 warcoing, Belgium.
cosucra Groupe Warcoing
Introduction
lngeneral,water content is determined by oven dryingand doesnot correctlyrepresentthe water content of asample.Indeed,high temperatures caninducetheformation and the releaseof
volatilesubstances(warerand others), whilewater included inthe crystals mightormight not be completelydetected.Moreover,duetothe rapid adsorption ofwateron anhydrousmaterial
afteroven drying,this technique presentspoor reproducibility. TheKarl Fischer (KF)titrationisawell-established method for the watercontent determination. This technique allowsthe determinationof thetotal water of asampleand isnowthe mostimportant primarymethod for detennining water content in foodstuffs.Todeterminethewater contentof asample,itrnust be
dissolved in thereaction medium, used as solvent. lnsorne cases, the medium doesnotcompletely dissolve thesample. lnorder toincrease solubiliry, sorne authorsused an internai homogeniser, worked at elevatedtemperaturesor modified thepolarity of the workingmediumby addingfurthersolventstoit.Thesernethodologies usingthe KFtitrationpresentsorne inconveniences as notailtitration devieesare equippedwithan internaI homogeniser or aheatabletitrationvesselwhichlimitsthe use ofthese techniquesforwiderapplications. More50,at
highternperature,possibleevaporationofsulphurdioxidefromtheboiling workingmediumrnekesareflux condenser necessary.
For thesereason,wedevelopeda suitablesarnple preparation for thetotaldeterminationof water content andapplied ittoinulin,aselected dietaryfibre.Inulinsamples were analysed byKF
titration usingtheusualrnethod of introducingthe powder directlyin the methanolbase working medium,modified ornot byfonnamide (1:3,v/v)inorderto increasesamplesolubility.ln addition,an alternative methodologybased onthe introductionof an inutinsolution inpure fonnamide wasdevelopedandcompared tothestandard KF titration.Thesemethodologieswere
apptiedtotwoinulinsample whichpresented eitheranamorphousor a crystaJlinestate,
Experimentation and resuIts Semi-crystalline inulin
Amorphous inulin
Inulins were srcred over P20j for Ihrcc wceks al 200e10obreina
dehydrated product. then3031)7.00 byPowdcr X-raydiffraction.
~ ~
.
_-
~
---.~I---
---Diffraction angle (29)
X-raydiffractogramofsemi-crystalllne inu/in
Diffraction angle (29)
X-raydiffractogram of amorphousinu/in
An iodine molecule is attractcd tothenegaüvely charged platinum pin.IIthen ecquiresIwO electrons and tums to iodide(21').TheIwO
negatively chargcd iodide ions are immcdiately enracted 10the positivelychargcd ptatinumpin. where theydonate theelectrons and fonnan iodinemolecule again.
When the reagemccnsumpdon rate reaches the same (ora slightly highcr) value asirnmcdierely bcfore thestartof theanalysis.il is
assurncd thatail thewater ofthesampleisdetected.andtheanalysisis stoppcd.
Ifthe sample isnOIcomplctclydissolved.waterdiffusion islow,50
theanatysisisdclaycd and underestimated.
1Re:detcrmination of thewatercontentisbasedon thereactiondescribcd byR.W.Bunsen in 1853:
Kart Fischer discovered that thisreaction could beuscdforwater/
determination bya two-srepreaction:
R-OH+SOl+B....• BH"+ROSû1' (Eq. 1) BW+ROS02' +Il+H20+2B....•3BH~+ROSOJ' +21' (Eq.2)
R-OH+S02+12+H10+3B....• BH"+ROSO)' +21' (Eq.1+Eq.2)
Anatysis of powdered inulin by Karl Fischertitration Anülysisof powdered inulinbyKarlFischertitration
without {ormamidein the workin/! medium withformamide in the workin!:medium(J:3, v/v)
1
Analysis of inu/indissolved informamide (1.5:7.5,w/w)
b KarlFischertitration
The dissolution of seml-crystattrne
inulin wasIncreaseë.
lnulinsclutionin
l
~
"
~
-~.
- The influence of a possible change of the drift duringthe analysis wasrcduced.
- The total water content for both inulinwasechieved.
The dissolution of inulin was compteted in pure fonnamide for both semt-crystantne and amor
-Ilhous inulin.
- Theanalysiswas stoppcd after1-2
min. The dissolution of inulin was not complete for both semt-crystaülne and amerphcusInuün. - water diffusion was low and dclayed.
Il
,
I
l
l
.
~
"
"
.
:: .
.
"
"~"::.~<:":": .,'.' ....:.... - AninCJ'C<ISCof thewater content value. - The warer content undcrcstimatcd. The total dissolution ofphousinulinwas achleved.
- Theanalysiswas stoppcd after 4-5
min.
- Iodine was consumcd.
conunuousty
- The anajysiswas sroppcd bythe
imposed maximum ütranon lime
of 10min and oœ by thedrift
criterion.
) '.'
Me1luII101buc.-wort.ing medium
- The total water content was achieved.
- The water content
unëeresumared. Methanol base-worl:ingmedium
modifiai byfonnamÎdc (1:3,vlv) Methanol buc.-worl;ing medium
l
Conventional KF method without formamide Conventional KFmethod withformamide Proposed KFmethod 1.3 ± 0.3(>lOmin) 2.1± 0.1(4-5min) 2.1± 0.2(1-2min)1.1± 0.3 (>lOmin) 1.8 ± 0.3 (>IO,"in) 2.4 ± 0.1(1-2min)
Results of the VolumetrieKarl Fischer titrution (gwater/100 g anhydrous sample) (Amorphous inulin, Semi-crystalline inulin)
Conclusions
The total water content ofinulin hasbeendetermined by volumetrieKarl Fischertitration,usinga complete pre-dissolution of thesamplein pure formarnide,andwas comparedtothe usual introductionofthesample directlyinthe methanol-based workingmedium,modifiedornotbyaddition of fonnamide (1:3,v/v) in order to increasesamplesolubitity.The described method allowed the determinationofwater included in thecrystals,as shown by comparingtheresultsforamorphousandserni-crystalline inulinsamples.
The titrationtime for theproposed method was1-2min,while complete water detection was not achieved after 10minbythestandard method. Dissolvingthesample beforehand and
transferringan aliquotintothetitrationcell istime consuming,butthis is compensated bytheshortertitrationtime.This advantage iseven moreimportant,whenroutine analysesare carried out
andseveralsamples canbedissolved simultaneously. Moreover,the proposed method hasthe advantage ofworking atambient temperature and can be appliedto every volumetrie KFtitrator.
Thismethodology canbe applied for the water content determinationofother polysaccharides presenting a solubitityproblem in the workingmedium.
Acknowledgments
Financialsupport wasprovided for this studybytheWalloon Regionof Belgium and Cosucra GroupeWarcoing SA.
Rcfercnq: S.N.Ronkart,M.Paquet.C.Fougnies. C.Dcroannc,J.-C.Van Herck, C.Blecker.(2006). Determination ortotalwatcr cornentininulinusingthevolumetrie KarlFischer titraüon.Talanta,70,1006-1010.
A working methodology for the total water determination in inulin by the volumetrie Karl Fischer titration
Sébastien N.Ronkartl,2, Christophe Blecker',
Christian Fougnies'', Claude Deroannc' and Michel Paquot'
1,2Gembloux Agricultural University, 'Department of Food Technology; 2Department of Industrial Biological Chemistry, Passage des Déportés, 2, B-5030 Gembloux,Belgium.
3 Cosucra Groupe Warcoing SA, Rue de la sucrerie, 1,B-7740 Warcoing,Belgium,
The aim of this research work was to quantify total water in inulin by Volumetrie Karl Fischer Titration.This polysaccharide presents a solubility problem in the methanol working medium (MWM), even when the polarity of the system is increased by addition of formamide. So, when using the conventional Karl Fischer method, water diffusion of the sample is low, and the measure is stopped by the length of the analysis and not by the stabilization of the drift. By extemally dissolving inulin in formamide (2.5:7.5, w/w) and injecting this solution into
the MWM, total water including crystallization water, is successfully extracted and
quantified. The developed method was compared to the usual Volumetrie Karl Fischer Titration method of introducing the sample directly in the MWM, modified or not by formamide (1/3, v/v).
Amorphous inulin analysed with the developed method lead to the same water content determination as the introduction of inulin powder in the titration cell with addition of formamide (2.07 g water/l00 g anhydrous inulin), but the analysis time is reduced by at least a factor 2. However, the water content of semi-crystalline inulin is higher than that analysed by the introduction of inulin powder in the titration cell (2.35 instead of 1.10 g water/100 g anhydrous inulin). This difference is probably due to the release of water entrapped in the crystals. ln addition, the analysis time is only 1-2 min, due to the immediate availability of the water. This shorter analysis time reduces the influence of a possible change of the drift during the analysis. So,it can be assumed that the proposed method fully extracts the water content of inulin samples and can be transposed to various food ingredients presenting dissolution
problems in the VKFT working medium.
