A working methodology for the total water determination in polysaccharide by the volumetric Karl Fischer titration : the case of inulin, a dietary fibre.

(1)

A working methodology for the total water determination in polysaccharide

by the volumetrie Karl Fischer titration: the case of inulin, a dietary fibre

S

ébastien

N. Ronkart

l•2,

Michel

Paqu

e

t'

, C

hri

s

tian F

o

u

g

ni

esê,

C

l

a

ud

e

D

e

r

o

ann

e-,

J

ea

n

C

laud

e

Van H

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nd

C

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t

o

ph

e

Bl

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m

cosucra

Groupe Worcolng 1Gembloux Agricultural University.DcpartmentofFoodTechnclcgy(Head: Prof.C.Deroanne). URL:hnp:llwww.fsagx.ac.beII3

2 OemotouxAgriculturaJ University.Departmcnt of Industrial Biological Chcmistry (Head:Prof.M.PaqUOC).PassagedesDéportés.2.8-5030 Gembloux,Belgium.

3 Cosucra GroupeWarcoing SA.Ruedelasucrerie. 1.8-7740Warcoing, Bclgium

Introduction

lngeneral,water contentisdetermined byoven drying anddoes not correctlyrepresentthe water content of asample.Indeed, high temperaturescan induce theformation andthereleaseof

volatile substances (water andothers), while water included in the crystals might or might not becompletely detected. Moreover, due totherapid adsorption of water onanhydrous material

after oven drying, this technique presents poorreproducibility. The Karl Fischer (KF)titrationisawell-established methodfor the water contentdetennination. Thistechniqueallowsthe determinationofthetotalwaterofa sample andisnowthe mostimportant primarymethodfor determiningwatercontentin foodstuffs. T0determinethe watercontentofasarnple, itmustbe dissolved inthe reaction medium, usedas solvent. lnsorne cases,themedium doesnot completely dissolve thesample. lnorder toincrease solubility, sorneauthorsused aninternai homogeniser, worked at elevated temperatures or modified thepolarity of the workingmedium byaddingfurthersolventstoil.These methodologies usingtheKF titration presentsorne inconveniencesasnot ailtitrationdeviees areequipped withaninternai homogeniser or a heatable titrationvesselwhichlirnits theuseofthese techniquesforwider applications.Moreso,at hightemperature, possible evaporationofsulphur dioxide fromtheboilingworkingmediummakesarefluxcondensernecessary,

For thesereason,we developeda suitable sample preparation forthe total determination of watercontentand applyittoinulin, aselecteddietary fibre.Inulinsamples wereanalysedbyKF

titration usingtheusual methodof introducingthepowder directlyin the methanol baseworkingmedium, modified or not byformamide (1:3, v/v)inorderto increasesample solubility.ln addition,analternative methodology basedonthe introductionof an inulinsolution in purefonnamide wasdeveloped and compared tothe standard KFtitration. Thesemethodologieswere applied totwoinulin sample which presented either anamorphous or acrysralline state.

I

:

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Inulins were

Experimentation

stcrcd cver PPs for thrœ w

and re

ceks al 20

s

°

uIts

C to obtain a

:

~

dchydratedproduct. thenanalyzcd byWide AnglcXcray Scattcring(WAXS).

i

1" ?o .~ .§

,

_..

"

.

"

.

,

.

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.

,

.

Diffraction angle (29) Diffraction angle (20)

WAXS diffractogram ofamorphous inulin WAXS diffractogram ofsemi-crystalline inulin

Tbedetennination or thewater contentisbasedon thereaction dcscnbcd An iodine molecule isanractedtothenegativetycharged platinum

byR.W.Bunsen in 18.53: pin.ltthen ecquiresIwO electrons and tums10 iodide(21').Thetwc

negeuvely charged iodide ions are immediatelyattracted 10 the

12 +S02 +2HP - 2Hl +H1S04 positivelycharged platinum pin.wherethe)' ecretetheelectrons and

~'""' erdiscovercdthatthisrcectiorcouldbe-~-/

Iorrnaniodine molecule agam

~

When Ihercagentconsumpnonraiereachestheseme(or aslighuy

determination byatwo-srcp reaction:

1

higher) valueasnumcdiarelybcforcthe sian or thc anatysis.itls

R-QH+SOl+B-+BH' +ROS02' (Eq. 1) assumcd thar

ailthe waterorthe sampleisdctccted. and theanatysisis stoppcd.

BH"+ROS02' +12+H20+2B-+3BH" +ROSOj' +21' (Eq.2) _I_fthe sample isnotcompletetydissctved.waterdiffusion is low.50 R-QH+ S02+ 12 +H10+3B•.• BH' +ROSOj' +21' (Eq.1+Eq.2 theanalysisisdelayedand underesI:imated.

Analysis of powdered inu/in hyKarlFischer titration Analysis cfpowdered inu/inbyKarlFischer titration Analysis of inu/in dissolved informamide (2.5:7.5,.",Iv) withoutformamide inthe workinemedium with formamide in the workingmedium (1:3,vlv) bvKarl Fischertitration

The dissolution of inulin WIiS nor The dissolution ofseml-crystatune

'.'

Y

I

Th. dissoluncn • mulin ••

r'

complete for both scml-crystaülne

r'

inulin was incrcased.

f'

pureformemide comntetcd tn pureIormamidefor

andamorphous inulin.

..

~

- An increascor thc watercontera

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amor

--ewatcr diffusion was low and

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value.

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delayed. _-_Th_e _w_ar_e_r content was - The analystswas stopped aûer1-2

-sIcdinc was conunuousty undercstimated. mm

consumcd. _Th_e _l_o_l_a_l _di_s_so_lul_io_n ₀₁ _a_m_o_r_- - TIlC Influence of a possible

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-eTbe analysis wasstopped bytbe pbcus inulin was achic\·ed.

change of the drift dunng the

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.

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imposed maximum utrericnlime or 10 -cThe analysiswasSlopped after4-.5 analysiswasrcduœd

minand notbythedriftcriterion. min. - The totalwatercontent Iorboth

j

,. ... _-₁₁_l_c

watcr content was

/

-11le total watcr content was -vi inulinwasachicvcd.

undcrcstimated.

achieved.

Methanol base-workingmedium Melhaoolbase-wœkinmgedium Methaool basc-wori;ing medium

modified byronn.midc (1:3.'1/'11)

~

1 ~

Conventional KFmethod Conventional KFmethod Proposed KYmethod

without Iormamide with formamide

1.3 ± 0.3(>IOmin) 2.1 ± 0.1(4-5mio) 2.1±0.2 (1-2mio)

1.1± 0.3 (>IOmio) 1.8 ± 0.3 (>IOmio) 2.4± 0.1(1-2mio)

Resutts ofthe VolumetrieKarl Fischer titration(gwater/100ganhydrous sample) [Amorphous inulin, Semi-crystajllne inulin)

Conclusions

The totalwatercontentof inulinhasbeendetermined by volumetrie Karl Fischertitration, using acomplete pre-dissolution of thesample in pure formamide, andwascompared tothe usual

introduction of thesample directly in the methanol-based working medium, moditied or not by addition of fonnamide (1:3,v/v) inorder to increase sample solubility. The described method allowedthedeterminationofwater includedinthecrystals,as shown bycomparing the resultsfor amorphous and semi-crystalline inulin samples.

Thetitration time for the proposed methodwas 1 -2min,while complete water detection wasnotachievedafter 10min bythestandard method. Dissolvingthe sample beforehand and transferring analiquotintothetitration cellistimeconsuming,but this iscompensated bythe shorter titration time.Thisadvantage is even moreimportant,when routineanalyses arecarriedout andseveral samples can be dissolvedsimultaneously. Moreover, theproposedmethodhasthe advantage ofworkingatambient temperature and can be applied toevery volumetrieKF titrator.

Thismethodologycanbe applied for thewater content determinationof other polysaccharidespresenting a solubilityproblemin the working medium.

Acknowledgment

s

Financial support was provided forthis study bythe Walloon Region of Belgium and Warcoing Research spr!.