The high-, medium-, and low-amylose rice starches were isolated by the alkaline method and acetylated by using acetic anhydride for 10, 30, and 90 min of reaction. The degree of substitution (DS), the Fourier-transformed infrared spectroscopy (FTIR), the X-ray diffractograms, the thermal, morphological, and pasting properties, and the swelling power and solubility of native and acetylated starches were evaluated.
Trang 1j ou rn a l h o m e pa g e : w w w e l s e v i e r c o m / l o c a t e / c a r b p o l
a Departamento de Ciência e Tecnologia Agroindustrial, Universidade Federal de Pelotas, 96010-900 Pelotas, RS, Brazil
b Department of Food Science, University of Guelph, Ontario N1G 2W1, Canada
c Processed Foods Research Unit, WRRC, ARS, United States Department of Agriculture, 800 Buchanan Street, Albany, CA 94710, United States
d Laboratório de Microscopia Eletrônica, Curso de Engenharia de Materiais, Universidade Federal de Pelotas, 96015-560 Pelotas, RS, Brazil
a r t i c l e i n f o
Article history:
Received 28 September 2013
Received in revised form
25 November 2013
Accepted 23 December 2013
Available online 2 January 2014
Keywords:
Rice starch
Amylose
Acetylation
Degree of substitution
Acetyl groups
a b s t r a c t
Thehigh-,medium-,andlow-amylosericestarcheswereisolatedbythealkalinemethodand acety-latedbyusingaceticanhydridefor10,30,and90minofreaction.Thedegreeofsubstitution(DS),the Fourier-transformedinfraredspectroscopy(FTIR),theX-raydiffractograms,thethermal, morpholog-ical,andpastingproperties,andtheswellingpowerandsolubilityofnativeandacetylatedstarches wereevaluated.TheDSofthelow-amylosericestarchwashigherthantheDSofthemedium-andthe high-amylosericestarches.TheintroductionofacetylgroupswasconfirmedbyFTIRspectroscopy.The acetylationtreatmentreducedthecrystallinity,theviscosity,theswellingpower,andthesolubilityof ricestarch;however,therewasanincreaseinthethermalstabilityofricestarchmodifiedbyacetylation
© 2014 Elsevier Ltd All rights reserved
1 Introduction
Starchiscomposedofamyloseandamylopectinmoleculesand
theratiobetweenbothmoleculesvariesaccordingtothebotanical
originofstarch.Starchisthemajorconstituentofricegrainsand
isconsideredanimportantingredientthathasbeenusedinfood
preparation(Bao,Kong,Xie,&Xu,2004;Blazek&Gilbert,2011)
Duetothewiderangeofamyloselevels,ricestarchhasbeenused
asaningredientinvariousfoodandindustrialproducts,suchas
desserts,bakeryproducts,andalternativestofats(Puchongkavarin,
Varavinit,&Bergthaller,2005)
Nativestarchesdonotalwayshavethedesiredpropertiesfor
certaintypesofprocessing.Inordertoachievesuitable
functional-itiesforvariousindustrialapplications,starchhasbeenmodified
bydifferent methods Basically,there are fourkinds of
modifi-cations:chemical,physical,genetic,andenzymatic(Kaur,Ariffin,
Bhat,&Karim,2012).Chemicalmodificationscanpromote
struc-turalchangesandintroducenewfunctionalgroupsthataffectthe
∗ Corresponding author Tel.: +55 53 32757258; fax: +55 53 32757258.
E-mail address: rosana colussi@yahoo.com.br (R Colussi).
physicalandchemicalpropertiesofstarches(Sandhu,Kaur,Singh,
&Lim,2008)
Acetylation converts the hydroxyl groups of the glucose monomers into acetyl groups (Graaf, Broekroelofs, Janssen, & Beenackers,1995).Theacetylatedstarchesareclassifiedintolow, intermediate, or high degrees of substitution (DS) Acetylated starcheswithalowDS(0.01–0.2)mayfunctionasfilm-forming, binding,adhesion,thickening,stabilizing,andtexturingagents,and arewidelyusedinalargevarietyoffoodsincludingbakedgoods, cannedpiefillings,sauces,retortedsoups,frozenfoods,babyfoods, saladdressings,andsnackfoods.Acetylatedstarcheswith inter-mediateDS(0.2–1.5)andhighDS(1.5–3)havehighsolubilityin acetoneandchloroformand,thus,havebeenreportedasa ther-moplasticmaterial(Luo&Shi,2012)
Acetylationmaybeperformedtoimprovethephysical, chemi-cal,andfunctionalpropertiesofthestarch(Xu,Miladinov,&Hanna,
2004)andhasbeenwidelystudiedbyseveralresearchers( Bello-Pérez, Agama-Acevedo, Zamudio-Flores, Mendez-Montealvo, & Rodriguez-Ambriz,2010;Diop,Li,Xie,&Shi,2011;Garg&Jana, 2011;Huang,Schols,Jin,Sulmann,&Voragen,2007;Mbougueng, Tenin,Scher,&Tchiégang,2012).Thechangesintroducedby acety-lationdependonthebotanicalsource,thedegreeofsubstitution, theratiobetweenamyloseandamylopectin, andthemolecular 0144-8617/$ – see front matter © 2014 Elsevier Ltd All rights reserved.
Trang 2intothestarchmoleculeduringacetylationandtheefficiencyof
thereactiondependonthetypeofreagent,reagentconcentration,
pHofreaction,presenceofcatalyst,reactiontime,botanicalorigin,
andsizeandstructurecharacteristicsofthestarchgranules(Huang
etal.,2007;Huber&BeMiller,2000)
Severalresearchershavereportedtheeffectsofacetylationon
potato,corn,andpeastarchproperties(Chen,Li,Li,&Guo,2007;
Elomaa,2004;Graafetal.,1995;Xu&Hanna,2005;Huangetal.,
2007).ArecentstudiedperformedbyLuoandShi(2012)showed
effectsofacetylationonwaxy,normal,andhigh-amylose maize
starchproperties.Therearefewstudiesabouttheeffectsof
acety-lationofstarcheswithawiderangeofamylosecontents.Sodhiand
Singh(2005)studiedthecharacteristicsofacetylatedstarchesfrom
differentricecultivarswithanamylosecontentbetween7.83%and
18.86%;however,thisstudydidnotconsidertheeffectsof
acety-lationreactiontime onstarch properties.Theaimofthis study
wastoevaluatetheeffectsofacetylationwithdifferentDSonFTIR
spectroscopy,X-raydiffraction,thermal,morphological,and
past-ingproperties,swellingpowerandsolubilityofhigh-,medium-,
andlow-amylosericestarches
2 Materials and methods
2.1 Material
Rice grains of cultivars IRGA 417 (high-amylose), IRGA 416
(medium-amylose),andMotti(low-amylose),withamylose
con-tentsof32%,20%,and8%,andpurityof99.4%,99.5%and99.1%,
respectively,wereused.Ricesamplesweredehulled,polished,and
groundinordertoobtainriceflour.Ricestarchwasisolatedwith
0.1%NaOH asdescribed by Wangand Wang (2004).Rice flour
wassoakedin0.18%NaOHata1:2(w/v)ratiofor18h.Thenit
wasblended,passedthrougha63mscreen,andcentrifugedat
1200×gfor5min.Thesofttoplayerwascarefullyremoved,and
theunderlyingstarchlayerwasre-slurried.Thestarchlayerwas
thenwashedtwicewith0.18%NaOHandcentrifuged.Thestarch
layerwaswashedwithdistilledwaterandcentrifuged.Thestarch
wasthenre-slurriedandneutralizedwith1.0MHCltoapHof6.5
andcentrifuged.Theneutralizedstarchwaswashedwithdistilled
waterthreetimesanddriedat40◦Cuntil7%moisturecontentwas
achieved
2.2 Starchacetylation
Thehigh-,medium-,andlow-amylosericestarcheswere
acety-latedaccordingtothemethoddescribedbyMarkandMehltretter
(1972),withsomemodifications.Starch(200g)wasdispersedin
600mlaceticanhydrideinaclosedreactorusing2000rpmfor5min
(RW20,IKA,Germany).Afterwards,20gof50%NaOHinwaterwere
addedtotheslurryandthetemperaturewasadjustedto90◦Cfor
15min.Thereactionwasperformedforthreedifferenttimes:10,
30,and90min.Whenthetimeofreactionfromeachtreatment
wasachieved,thetemperaturewasreducedto25◦Cand300mL
of92.6◦Glethanolwasaddedtotheslurryinordertoprecipitate
starch.Thematerialwascentrifugedat3000×gfor10min,
sus-pendedinalcoholforfourtimes,andfinallydriedinanovenat
40◦Cfor16h
2.3 Determinationofacetylpercentage(Ac%)anddegreeof
substitution(DS)
Thepercentageofacetylgroups(Ac%)andthedegreeof
sub-stitution(DS)oftheacetylatedstarchesweredeterminedbythe
titrationmethoddescribedbyWurzburg(1964).Acetylatedstarch
(1g)wasmixedwith50mlof75%ethanolindistilledwater.The
250mlflaskcontainingtheslurrywascoveredwithaluminumfoil andplacedinawaterbathat50◦Cfor30min.Thesampleswere thencooledand40mlof0.5NKOHwereadded.Theslurrywaskept underconstantstirringat200rpmfor72h.Afterthisperiod,the alkaliexcesswastitratedwith0.05NHCl,usingphenolphthalein
asindicator.Thesolutionwaslefttostandfor2handthenany addi-tionalalkali,whichmayhaveleachedfromthesample,wastitrated
Ablank,usingtheoriginalunmodifiedstarch,wasalsoused
Ac%= [blank−sample]×molarityofHCl+0.043×100
BlankandsampletitrationvolumeswereexpressedinmL, sam-pleweightwasexpresseding.DSisdefinedastheaveragenumber
ofsitesperglucoseunitthatpossessa(Whistler&Daniel,1995)
DS= 162×acetyl%
2.4 Fouriertransforminfrared(FTIR)spectroscopy Theinfraredspectraofthenativeandacetylatedstarcheswere obtainedusinga Fouriertransforminfrared(FTIR)spectrometer Prestige-21, Shimadzu, in theregion of 4000–400cm−1.Pellets werecreatedbymixingthesamplewithKBrataratioof1:100 (sample:KBr).Tenreadingswerecollectedataresolutionof4cm−1 2.5 X-raydiffraction
X-raydiffractogramsofthenativeandacetylatedstarcheswere obtainedwithanXRD-6000(Shimadzu,Kyoto,Japan) diffractome-ter.Thescanningregionofthediffractionrangedfrom5to40◦, withatargetvoltageof30kV,acurrentof30mA,andascanspeed
of1◦min−1.Therelativecrystallinity(RC)ofthestarchgranules wascalculatedasdescribedbyRabek(1980)usingtheequation
RC(%)=(Ac/(Ac+Aa))*100,whereAcandAaarethecrystallineand amorphousareas,respectively
2.6 Thermalanalysis Thermalanalysis of the starch sampleswas performed in a TG–DTAapparatus(DTGmodel2010,TAInstruments,NewCastle, USA).Changeinsampleweightagainsttemperature (thermogravi-metricanalysis,TG)andheatreleasedorabsorbedinthesample becauseofexothermicorendothermicactivityinthesample (dif-ferentialthermalanalysis,DTA)weremeasured.Samples(4–8mg) wereheatedfrom30◦Cto600◦Cataheatingrateof10◦C/min Nitrogenwasusedaspurgegasataflowrateof50mL/min Thegelatinizationcharacteristicsofstarchesweredetermined usingdifferentialscanning calorimetry(DSC)(DSCmodel 2010,
TAInstruments,NewCastle,USA).Starchsamples(approximately 2.5mg,drybasis)wereweigheddirectlyinanaluminumpan,and distilled waterwasaddedtoobtaina starch–waterratioof1:3 (w/w).Thepanwashermeticallysealedandallowedtoequilibrate foronehourbeforeanalysis.Thesamplepanswerethenheated from30to120◦Catarateof10◦C/min.Anemptypanwasusedas
areference.Thetemperatureattheonsetofgelatinization(To),the temperatureatpeak(Tp),thetemperatureattheendof gelatiniza-tion(Tc)andtheenthalpy(H)ofgelatinizationweredetermined 2.7 Morphologyofthestarchgranules
Starchsampleswith7% moisturecontentwereinitially sus-pendedinacetonetoobtaina1%(w/v)suspension,andthesamples weremaintainedinanultrasoundfor15mintoeliminatethe pres-enceofairbubbles.Asmallquantityofeachsamplewasspread directlyontothesurfaceofthestubanddriedinanovenat32◦C
Trang 3Table 1
Percentage of acetyl groups (Ac%) and degree of substitution (DS) of high-,
medium-and low-amylose rice starches acetylated under different reaction times.
Low-amylose 10.34 a 0.43 a 11.60 a 0.49 a 20.47 a 0.96 a
Results are the means of three determinations Values accompanied by different
letters in the same column statistically differ (p < 0.05).
for1h.Subsequently,allofthesampleswerecoatedwithgoldand
examinedinthescanningelectronmicroscopeunderan
accelera-tionvoltageof15kVandmagnificationof5000×
2.8 Pastingproperties
Thepastingpropertiesofthestarchsamplesweredetermined
usingaRapidViscoAnalyser(RVA–4,NewportScientific,Australia)
withaStandardAnalysis1profile.Theviscositywasexpressedin
rapidviscounits(RVU).Starch(3.0gof14g/100gwetbasis)was
weighteddirectlyintheRVAcanister,and25mlofdistilledwater
wasthenaddedtothecanister.Thesamplewasheldat50◦Cfor
1min,heatedto95◦Cin3.5min,andthenkeptat95◦Cfor2.5min
Thesamplewascooledto50◦Cin4minandthenkeptat50◦C
for1min.Therotatingspeedwasmaintainedat960rpmfor10s,
anditwasmaintainedat160rpmduringtheremainingprocess
Parametersincludingpastingtemperature,peakviscosity,holding
viscosity,breakdown,finalviscosity,andsetbackwererecorded
2.9 Swellingpowerandsolubility
Theswellingpowerandsolubilityofthestarcheswere
deter-mined as described by Leach, McCowen, and Schoch (1959)
Samples(1.0g)weremixedwith50mLofdistilledwaterin
cen-trifugaltubes.Thesuspensionswereheatedat90◦Cfor30min
Thegelatinizedsampleswerethencooledtoroomtemperature
andcentrifugedat1000×gfor20min.Thesupernatantwasdried
at110◦Ctoaconstantweighttoquantifythesolublefraction.The
solubilitywasexpressedasthepercentageofdriedsolidweight
basedontheweightofthedrysample.Theswellingpowerwas
representedastheratiooftheweightofthewetsedimenttothe
weightoftheinitialdrysample(deductingtheamountofsoluble
starch)
2.10 Statisticalanalysis
Analyticaldeterminationsforthesampleswereperformedin
triplicateandstandarddeviationswerereported,exceptforX-ray
diffractionandthermalanalysis,whichwereperformedtwice.A
comparisonofthemeanswasascertainedbyTukey’stesttoa5%
levelofsignificanceusinganalysisofvariance(ANOVA)
3 Results and discussion
3.1 Percentageofacetylgroups(Ac%)anddegreeofsubstitution
(DS)
TheacetylationofstarchyieldeddifferentricestarchDSvalues
dependingontheamylosecontentandtimeofreaction(Table1)
Low-amylosecontentandlongtimeofreactionresultedinthe
high-estAc%and,thus,thehighestDS.Thericestarchesacetylatedfor
90minofreactionshowedhigherDSthanthestarchesacetylated
for10and30minofreaction.Thelow-amylosericestarch exhib-itedgreaterabilityfortheinsertionofacetylgroupscomparedto medium-andhigh-amylosestarches.LuoandShi(2012)studied thecharacteristicsofacetylatedhigh-amylose,normal,andwaxy maizestarches,reportingsimilarresults.Theseauthorsjustifiedthe greatereaseofinsertionofacetylgroupsofthewaxystarch com-paredtothehigh-amylosemaizestarchasbeingduetothegreater extentofreactionsitesinthewaxystarch.SodhiandSingh(2005) acetylatedthestarchfromdifferentricecultivarsandreportedthat thevariation inDS amongdifferentricestarchesmaybedueto thedifferenceinintragranularpackaging.Theyreportedthatthe wayinwhichtheamylosechainispackedinamorphousregionsas wellasthearrangementofamyloseandamylopectionchainscould affectthechemicalsubstitution reactionintheglucoseunitsof starchmacromolecules.However,theeffectsofstarchacetylation
asrelatedtoamylosecontenthavenotbeenexplained
Theacylationofstarchtakesplacebyanaddition–elimination mechanism(Xuetal.,2004).Eachoneofthethreefreehydroxyl groupsofthestarchshowsdifferentreactivity(Garg&Jana,2011) TheprimaryC(6)OHismorereactiveandisacylatedmorereadily thanthesecondaryonesonC(2)andC(3)duetosterichindrance Thisfactcanjustifythehighestdegreeofsubstitutionofthestarch withlowamylosecontent.OfthetwosecondaryOHgroups,C(2)OH
ismorereactivethanC(3),mainlybecausetheformerisclosertothe hemi-acetalandmoreacidicthanthelatter(Fedorova&Rogovin,
1963).SinceC(6)isthemostreactive,ithasbeenthemainreactive siteforsubstitutionofthehydroxylgroupsbyacetylgroups 3.2 Fouriertransforminfrared(FTIR)spectroscopy
FTIRspectroscopyanalysiswasusedtomonitorchangesinthe structureofthestarchespromotedbyacetylationbyanalyzingthe frequencyandtheintensityofthepeaks.Fig.1presentstheFTIR spectraofnativeandacetylatedhigh-,medium-,andlow-amylose ricestarches.TherewasnodifferenceintheFTIRspectraofhigh-, medium-,andlow-amylosericestarches
Thenativeandacetylatedstarchesshowedpeaksat3450cm−1, which is assigned tothe vibrationof O H deformation,and at
2960cm−1,whichcanbeattributedtoC Hbondstretching(Diop
et al., 2011) The acetylated high-, medium-, and low-amylose starches,atallreactiontimes,showedtheintroductionofthe car-bonylgroup(C O)oftheesterifiedacetylgroups,beingverifiedby thebandat1750cm−1(Fig.1).Moreovertherewasadecreasein theintensityofthebandat1650cm−1intheacetylatedstarches comparedtotheirrespectivenativestarches.Thepeakofstarch
at 1650cm−1 was assignedas C O C stretching,which can be attributedtothewaterassociatedtostarchmolecules.The reduc-tionofthisbandinacetylatedstarchesistheresultofloweraffinity
towaterascomparedwithnativestarches.LuoandShi(2012)also reportedthatacetylatedstarcheshaveahydrophobiccharacterdue
totheinsertionofacetylgroupsinthestarchchains
3.3 X-raydiffraction TheX-raydiffractogramsofnativeandacetylatedricestarches are presentedin Fig.2.The nativeand acetylatedrice starches showed diffraction patterns typical of A-type crystalline struc-ture as defined by peaks at 2 of 15◦, 17◦, 17.8◦, 19◦, and
23◦.Thecrystallinity of thenativestarchesfollowed theorder: low-amylose>medium-amylose>high-amylose.Thehigher crys-tallinity of the low-amylose native starch is attributed to its higheramylopectincontent.Theacetylatedricestarchesshoweda decreaseintheintensitiesofthepeakscomparedtothenativeones, withtheexceptionoflow-amylosestarchacetylatedfor90minof reaction.Acetylationreducedthecrystallinityofricestarches,and thelowestvaluesofrelativecrystallinitywereseeninacetylated
Trang 4450 650 850 1050 1250 1450 1650 1850 2050 2250 2450 2650 2850
3050
3250
3450
3650
Native
30 min
10 min
90 min
(c)
450 650 850 1050 1250 1450 1650 1850 2050 2250 2450 2650 2850
3050
3250
3450
3650
Wavenu mbe r, cm -1
Native
30 min
10 min
90 min
(b)
450 650 850 1050 1250 1450 1650 1850 2050 2250 2450 2650 2850 3050
3250
3450
3650
Wavenu mbe r, cm -1
Native
30 min
10 min
(a)
90 min
Fig 1.FTIR spectra of native and acetylated rice starches High-amylose starch (a),
medium-amylose starch (b), and low-amylose starch (c).
starcheswiththehighestDS.Shaetal.(2012)reportedthat,with
theincreaseintheproportioninacetylcontentofthericestarch,
crystallinitybecame graduallyloweredand thediffractionpeak
alsoreducedinturn.Theydescribedthatthechangesinthe
diffrac-tionpatternsindicatedthattheintermolecularhydrogenbonding
interactionwasdamaged
AccordingtoLuoandShi(2012),acetylationreducesthe
forma-tionofintermolecularhydrogenbonds,resultinginalowordered
crystalline structure of starch granules These authors studied
theacetylationofmaizestarcheswithvaryingDS,between0.27
and1.29,andreportedthatadestructionofcrystallinestructure
occurredinhigh-amylosestarchwith120minofreaction.Xuetal
Diffraction angle (2θ)
Native, CR= 22.86
10 min, CR= 19.90
30 min, CR= 18.27
90 min, CR= 14.79 (a) High-amylose
Diffraction angle (2θ)
Native, CR= 27.26
10 min, CR= 23.90
30 min, CR= 23.15
90 min, CR= 20.14 (b) Medium-amylose
Diffraction angle (2θ)
Native, CR= 33.71
10 min, CR= 25.99
30 min, CR= 25.75
90 min, CR= 19.03 (c) Low-amylose
Fig 2. X-ray diffraction pattern of native and acetylated rice starches High-amylose starch (a), medium-amylose starch (b), and low-amylose starch (c).
(2004)alsoreportedthatthehigh-amylose maize starch,when acetylatedwithDSbetween1.11and2.23,showeddestructionin theorderedcrystallinestructures
3.4 Thermalanalysis Thermogravimetricanalysis(TGA)hasbeenusedinthe eval-uationofthethermalstabilityofmaterialsandisconsideredone
ofthemainmethodsforevaluatingthermalpropertiesof acety-latedstarches.TheTGAcurvesshowedtwo-stageweightlossfor thestudiedstarches,beingthefirststagearound40–125◦Candthe
Trang 5100 20 0 30 0 40 0 50 0 60 0
-0
20
40
60
80
100
%
TGA
(c) Low amylose
Native
90 min
30 min
10 min
(c) Low-Amylose
20
40
60
80
100
%
TGA
(b) Medium amylose
30 min Native
90 min
10 min
(b) Medium-Amylose
-0
20
40
60
80
100
%
TGA
(a) High amylos(a) High-Amilose e
Native
90 min
10 min
30 min
Temperature (ºC)
Temperature (ºC)
Temperature (ºC)
Fig 3.Thermogravimetric analysis (TGA) curves of native and acetylated rice
starches High-amylose starch (a), medium-amylose starch (b), and low-amylose
starch (c).
secondonearound250–400◦C.Thefirstweightlossisattributedto
thelossofwater(Fig.3a–c).Thenativemedium-andlow-amylose
ricestarches (Fig.3aand b)had higherinitialweightlossthan
theacetylatedstarches,withvalues around10%intherangeof
40–125◦C, while theacetylated starches showed lossesaround
6%ofweight inthesamerange Byincreasingthetemperature
from250to400◦C,themedium-andlow-amyloseacetylatedrice
starchesunderdifferenttimesofreactionshowedsimilar
behav-ior,losingapproximately70%ofweight.Thisshowsthatacetylation
Table 2
Thermal properties of native and acetylated high-, medium- and low-amylose rice starches.
High-amylose
Medium-amylose
Low-amylose
influencedthethermalbehaviorofstarches;however,theintensity
ofacetylationdidnotaffecttheweightlossbecausetherewasno differencebetweenthestudiedtimesofreaction
The native and the 90min-acetylated high-amylose rice starchesshowedlowerlossofdrymatter(5.5and3.0%, respec-tively)intherangeof40–250◦C,whiletheacetylatedhigh-amylose starchesafter10and30minofreactionloseabout9.0%ofdry mat-ter.Thelowerweightlossinstarchacetylatedfor90minofreaction indicatesthehigherstabilityofthismaterialupto250◦C.Inthe temperaturerangebetween250◦Cand400◦C,thehigh-amylose ricestarchacetylatedfor90minshowedabout85.0%ofdrymatter loss,whilethenativestarchesandstarchessubjectedtoacetylation for10and30minofreactionshowedabout70%ofdrymatterloss
Ontheotherhand,forthelow-amylosericestarch,thehighestdry matterlossintherangeof250–400◦Cwasregisteredforthenative starchandstarchtreatedfor30min
GargandJana(2011)studiedacetylatedstarchesunderdifferent degreesofsubstitutionandverifiedthatacetylatedstarchsamples werethermallymorestablethannativestarch.Theincreasein ther-malstability wasduetothelowamountofremaininghydroxyl groups in starch molecules after modification The increase in molecularweightandcovalentbondingduetotheacetylationof hydroxylgroupswerealsoresponsiblefortheincreasedthermal stability
ThethermalpropertiesmeasuredbyDSCofthehigh-,medium-, andlow-amylosericestarchesarepresentedinTable2.Thenative starchesshowedhighergelatinizationtemperatures.Therewasno differenceinthegelatinizationtemperaturesofnativericestarches
asafunctionoftheamylosecontent.AcetylationreducedtheTo,Tp
andTcvaluesofricestarches,anditwasverifiedadecreaseinthe gelatinizationtemperatureswithanincreaseinthereactiontime usedforstarchacetylation.Thestarchgelatinizationiscontrolled,
inpart,bytheamylopectinmolecularstructureand thegranule structure.Thedecreaseingelatinizationtransitiontemperatures
isinagreementwiththeearlyruptureoftheamylopectin dou-blehelicesandthemeltingofthecrystallinelamellaeinstarches inducedbytheacetylationreaction.LuoandShi(2012)andSingh, Chawla,andSingh(2004),acetylatingthecornandpotatostarches, respectively, also reported a significant decrease in gelatiniza-tiontemperaturesafteracetylation.WottonandBamunuarachchi (1979)suggestedthattheintroductionofacetylgroupsinto poly-merchainsresultedindestabilizationofstarchgranularstructure, leadingtoadecreaseingelatinizationtemperatures
WhencomparingtheHvaluesofnativelow-,medium-,and high-amyloserice starches,itcanbeobserveda highHvalue forthelow-amylosericestarch.Thisfactcanbeexplainedbythe differenceinrelativecrystallinity,sincethecrystallinitylamellae
ofstarchgranulesrequireshigherenergyforgelatinizationthan theamorphouslamellae.TheacetylationprovidedlowHvalues
Trang 6Fig 4. Scanning electron micrographs of rice starches: native high-amylose starch (a), native medium-amylose starch (b), native low-amylose starch (c), acetylated high-amylose starch (d), acetylated medium-amylose starch (e), acetylated low-amylose starch (f) Figures d–f represent starches acetylated for 90 min of reaction.
forthehigh-,medium-,andlow-amylosericestarches(Table2)
Hprimarilyreflectsthelossofdouble-helicalorderratherthan
lossofcrystallineregisterwithinthegranule.ThedecreaseinH
valuesofstarchacetatessuggeststhatsomeofthedoublehelices
presentinsemi-crystallineregionsofthegranuleweredisrupted
duringacetylation.ThelowerHsuggestsalowerpercentageof
orderedcrystallitesoralowerstabilityofthecrystals.Thehigher
theDSofthestarch,thelargerthedecreaseinHvalues(Table2)
3.5 Morphologyofstarchgranules
The morphology of starch granules was investigated using
scanning electron microscopy (SEM) and the micrographs are
presentedinFig.4.Themicrographsofthericestarchesshowed
the presence of polyhedral granules The high-, medium-, and
low-amylose rice starches subjected to 90min of acetylation
(Fig.4d–f)hadhigherDSandwerecomparedwiththeirrespective
nativestarches (Fig.4a–c) Noeffectofacetylationonthe
mor-phologyofstarchgranuleswasfound.SodhiandSingh(2005)also
reportedthattheSEMrevealednosignificantdifferencesbetween
externalmorphologyofnativeandacetylatedstarches.However, theseauthorsreportedthattheacetylationbroughtaboutslight aggregationofgranules.Similarobservationshavebeenreported regardingthemorphologyofacetylatedcorn,potato(Singhetal.,
2004),andricestarches(Gonzalez&Perez,2002).Shaetal.(2012) showed that the granule surface of acetylated starch was less smooth thanin nativestarch,but thestarchgranules still kept
arelativelycomplete particlestructure Astheacetyl increased, the intermolecular hydrogen bonds were damaged and more starchgranulesweredisrupted.Theseauthorsalsosuggestedthat the crystalline regions were also involved in the reaction; the differencewasthatcrystallinegranulesdidnotcollapse
3.6 Pastingproperties Thepastingpropertiesofnativeandacetylatedhigh-, medium-,andlow-amylosestarchesanalyzedwithaRapidViscoAnalyser (RVA)areshowninTable3andtheRVAcurvesarepresentedin Fig.5.Acetylationreducedthepastingtemperatureofricestarches, exceptforthehigh-amylosericestarchwiththelowestDS(10min
Trang 7Table 3
Pasting properties, swelling power and solubility of native and acetylated high-, medium- and low-amylose rice starches.
Pasting temperature ( ◦ C)
Peak viscosity (RVU)
Breakdown (RVU)
Final viscosity (RVU)
Setback (RVU)
Swelling power (g/g)
Solubility (%)
a Results are the means of three determinations Values accompanied by lowercase letter in the same column and uppercase letters in the same row, for each property, statistically differ (p < 0.05).
b nd, non-detected.
ofreaction).AccordingtoSaartrat,Puttanlek,Rungsardthong,and
Uttapap(2005),thepastingtemperatureshowedlowervaluein
acetylatedstarchthaninnativestarch,anddecreasedastheacetyl
groupscontentincreased.Thischaracteristicisoneofthemany
advantagesachievedwithacetylation,becauseitallowssuggesting
theusetheacetylatedstarches inprocesseswhereathickening
agentmustgelatinizeatlowertemperatures,orsimplytoreduce
energycostsduringthemanufactureofproductsinwhichthese
starchesareused(Betancur,Chel,&Canizares,1997)
Thehigh-amylosericestarchacetylatedfor10minandthe
low-amylosericestarchesacetylatedfor10and30minofreactionhad
higherpeakviscositiesthantheirnativestarches.When90minof
reactionwereusedforthehigh-andlow-amylosericestarches,the
peakviscositydecreasedcomparedtotheirnativestarches.The
peakviscosityofmedium-amyloserice starchessubjectedtoall
DShadlowervaluesthanthenativemedium-amylosericestarch
Acetylationreduced thefinalviscosityof ricestarch, exceptfor
thehigh-amylosericestarchacetylatedfor10minandthe
low-amylosericestarchacetylatedfor10and30minofreaction,which
showedequalandincreasedfinalviscosity,respectively,compared
withtheirnativestarches.Saartratetal.(2005)alsofoundthatthe
viscositiesofacetylatedcannastarcheswerelowerthanthoseof
nativestarches
Themarkeddecreaseintheviscosityofthehigh-,medium-,
andlow-amylosericestarchesacetylatedfor90min(Fig.5)
can-notbeattributedtothepartialgelatinizationofstarchgranules,
sincetherewasnolossofgranularintegrityaccordingtotheSEM
(Fig 4d–f) The decreasein the viscosityof acetylated starches
comparedtonativestarchescanbeattributedtotheinsertionof
acetylgroupsthathindertheassociationbetweenstarchchains
anddecreasedtheabilityofstarchgranulestoabsorbwater.Thus,
itgivesstarchahydrophobiccharacter
Acetylationreducedthebreakdownofricestarches,increasing thethermalandmechanicalstabilityofacetylatedstarches,except forthehigh-andlow-amylosericestarchesacetylatedfor10and
30minofreaction.Thehigh-amylosericestarchesacetylatedatall
DSandthelow-amylosericestarchacetylatedfor90minofreaction hadalowersetbackcomparedtotheirnativestarches.Therewas
anincreaseinthesetbackoflow-amylosericestarchacetylatedfor
10and30mincomparedtothenativelow-amylosericestarch.The reductioninthesetbackisduetotheintroductionofacetylgroups
instarchchains,whichcanpreventcloseparallelalignmentof amy-losechainsandthuslowersetbackviscosities.HoweverSodhiand Singh(2005)foundthatacetylatedstarchesshowhighersetback viscositiesthantheirnativecounterparts.Sucheffectwasobserved formedium-amylosestarchesandforlow-amylosestarch acety-latedfor10and30minofreaction(Table3)
3.7 Swellingpowerandsolubility Acetylationreducedtheswellingpowerofricestarches,except forthelow-amylosericestarchwhenacetylatedfor10and30min, whichshowedswellingpowersimilartonativestarch.The high-estdecreaseinswellingpowerwasverifiedinstarchesacetylated for90minofreaction(Table3 whichexhibitedhighDS(Table1)
Instarchesmodifiedbyacetylation,theintroductionof hydropho-bicacetylgroupscanmakethewaterintakeintostarchgranules difficult,thusdecreasingtheswellingpower
Comparingthericestarcheswithdifferentamylosecontents, thelow-amylosestarchshowedthelowestsolubilitycomparedto thehigh-andmedium-amylosericestarches,whichisprobablydue
totheloweramountofamylosemoleculesthatareleachedduring hydrationandheating.Thedecreaseinstarchsolubilityisduetothe loweramyloseleachingandcanbearesultofthehigherinteraction
Trang 8Fig 5. RVA curves of native and acetylated rice starches High-amylose starch (a),
medium-amylose starch (b), and low-amylose starch (c).
betweenamyloseandamylopectinmolecules,preventingthe amy-losefromleachingfromthegranule.Theincreaseinthemolecular weightofstarchduetotheintroductionofacetylgroupsmainly
inC(6)maymaketheleachingofamylosefromthestarch gran-uledifficult.Thesolubilitycharacteristicoftheacetylatedstarchis dependentoftheDSandthepolymerizationofamyloseand amy-lopectinchains.Lawal(2004)alsofoundsimilartrendsofdecreased solubilityfromnewcocoyamstarchacetylatedwith60minof reac-tionwhencomparedwithnativestarch
4 Conclusions
Thepresentstudywasthefirstoneaboutacetylationofrice starchofdifferentamylosecontents.Thelow-amylosericestarch wasmoresusceptibletoacetylationcomparedtothemedium-and high-amylosericestarches.Theintroductionofacetylgroupswas confirmedbyFTIRspectroscopy.Acetylation,mainlyover90minof reaction,reducedricestarchcrystallinityand,ingeneral,its past-ingtemperature,breakdown,peakand finalviscosities,swelling power,andsolubility.Thedecrease inpasting temperatureand breakdownofrice starchesenablesobtainingproductssensitive
tohightemperaturesandmorestableproductswhilecooking.The continuityofthisworkshouldevaluatethesusceptibilityof acety-latedstarcheswithdifferentamylosecontentandDStoenzymatic hydrolysis,aswellastheproductionofbiodegradablefilmsusing acetylatedricestarch
Acknowledgements
We would like to thank FAPERGS (Fundac¸ão de amparo a pesquisadoestadodoRioGrandedoSul),CAPES(Coordenac¸ãode Aperfeic¸oamentodePessoaldeNívelSuperior),CNPq(Conselho Nacional de Desenvolvimento Científico eTecnológico), SCT-RS (SecretariadaCiênciaeTecnologiadoEstadodoRioGrandedoSul) andPólodeInovac¸ãoTecnológicaemAlimentosdaRegiãoSul
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