Considering the importance of maize starch and the lack of knowledge about the effects of storage temperature on the isolated starch properties; maize grains were stored during 12 months at different temperatures (5, 15, 25 and 35 ◦C). The extraction yield and the physicochemical, thermal, pasting, crystallinity and morphological properties of starches were determined.
Trang 1j ourna 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
Maurício de Oliveiraa, Elessandra da Rosa Zavarezea, Ricardo Marques e Silvad,
Cristiano Dietrich Ferreiraa, Moacir Cardoso Eliasa
a Department of Agroindustrial Science and Technology, Federal University of Pelotas, 96010-900 Pelotas, RS, Brazil
b Processed Foods Research Unit, WRRC, ARS, United States Department of Agriculture, 800 Buchanan Street, Albany, CA 94710, United States
c Department of Food Science, University of Guelph, Ontario N1G 2W1, Canada
d Department of Electron Microscopy, Federal University of Pelotas, 96015-560 Pelotas, RS, Brazil
a r t i c l e i n f o
Article history:
Received 27 September 2013
Received in revised form 5 November 2013
Accepted 7 November 2013
Available online 20 November 2013
Keywords:
Maize
Storage temperature
Starch
Pasting properties
Crystallinity
a b s t r a c t
Consideringtheimportanceofmaizestarchandthelackofknowledgeabouttheeffectsofstorage temperatureontheisolatedstarchproperties;maizegrainswerestoredduring12monthsat differ-enttemperatures(5,15,25and35◦C).Theextractionyieldandthephysicochemical,thermal,pasting, crystallinityandmorphologicalpropertiesofstarchesweredetermined.Thestarchisolatedfromgrains storedat35◦Cwasyellowishandshoweda22.1%decreaseinstarchextractionyieldcomparedtofreshly harvestedmaizegrains.At35◦C,areductionincrystallinitywasobservedbytheendof12months, despiteaparallelrearrangementofthestarchchainswhichresultedinanincreaseinX-raypeak inten-sities,gelatinisationtemperaturesandenthalpy.Thestarchisolatedfrommaizegrainsstoredat35◦C appearstohavesmallergranules,whichpresentssomepointsintheirsurface,potentiallyattributedto theproteinmatrixcompressingthegranuleswithinmaizegrains
© 2013 Elsevier Ltd All rights reserved
1 Introduction
Starchiswidelyusedinthefoodindustries,especiallyinthe
preparationof soups,sauces, baked goods,dairy,confectionery,
snacks, pasta, coatings and products made with meat (Davies,
1995).Theabilityofstarchtoformaviscouspastewhenheatedin
waterfollowedbythecoolingpropertymakesstarchsuitablefor
varioususesinthefoodandnon-foodindustries(Nguyen,Jensen,
&Kristensen,1998).Themainbotanicalsourceusedforextraction
ofstarch ismaize, accountingfor about80%of theworld
mar-ket(Jobling,2004).Amongallkindsofstarches,maize starchis
animportantingredientintheproductionoffoodstuffs,andhas
beenwidelyusedasathickener,stabiliser,colloidalgellingagent,
waterretentionandasanadhesive(Singh,Singh,Kaur,Sodhi,&
Gill,2003).Starchisthemainconstituentofmaizekernels,about
72–73%ofthetotalweight(Sandhu,Singh,&Lim,2007)
Afterharvested,themaizegrainsaresubjectedtovarious
post-harveststeps,suchascleaning,dryingandstorage.Severalstudies
haveelucidatedtheeffectsofdryingtemperatureontheproperties
∗ Corresponding author Tel.: +55 53 32757258; fax: +55 53 32757258.
E-mail addresses: paraginskiricardo@yahoo.com.br , ricardo@labgraos.com.br
(R.T Paraginski).
ofisolatedstarches(Altay&Gunasekaran,2006;Eckhoff&Watson, 2009; Haros,Tolaba, &Suarez, 2003;Lasseran, 1973; Malumba
etal.,2010; Malumba,Massaux,Deroanne,Masimango,&Béra, 2009;Setiawan,Widjaja,Rakphongphairoj,&Jane,2010) Accord-ingtoMalumbaetal.(2009),dryingtemperaturesofmaizegrains
upto100◦Ccausechangesinthepastingandtexturepropertiesof thestarchgel,andreducetheextractionyieldaswellaspurityof starch
Thestorageresultsinreducedsolubilityanddigestibilityofgrain proteins(Rehman,Habib,&Zafar,2002),increasedfreefattyacids (Park,Kim,Park,&Kim,2012),andthesemayformcomplexeswith amyloseoramylopectinshortchains,alteringthenutritional prop-ertiesandthephysicalcharacteristicsofthefinalproducts(Hasjim
etal.,2010;Salman&Les,2007).Longperiodsofstoragereducethe yieldofcassavastarchextractionduringwet-milling,asaresultof starchdegradationandtheinteractionsbetweenstarchandother constituents(Abera&Sudip,2004).Setiawanetal.(2010)stored maizegrainsat27◦Candaround85–90%ofrelativehumidityfor
6months,reportingchangesinthepasting,thermal, morphologi-calandcrystallinitypropertiesofstarch;withoutconsideringthe effectsofstoragetemperatureonthepropertiesofisolatedstarch
Yousifetal.(2003)reportedanincreaseingelatinisation tempera-tureofadzukibean(Vignaangularis)starchwithincreasingstorage temperature.Rupolloetal.(2011)evaluatedtheeffectsofstorage
0144-8617/$ – see front matter © 2013 Elsevier Ltd All rights reserved.
Trang 2phys-icochemical,pasting,crystallinityandmorphologicalpropertiesof
isolatedstarch,observingchangesinthethermalpropertiesand
crystallinityofstarchisolatedfromgrainsstoredat25◦Cduring
360days
Consideringtheimportanceofmaizestarchintheworldmarket
andthelackofknowledgeabouttheeffectsoftemperatureduring
maizegrainsstorageontheisolatedstarchproperties,theaimof
thisstudywastoevaluatethephysicochemical,pasting,thermal,
morphologicalandcrystallinitypropertiesofstarchesisolatedfrom
maizegrainsstoredfor12monthsatdifferenttemperatures
2 Materials and methods
2.1 Storageofgrains
Maizegrainsproducedin the2012growingseasonatSanto
Augusto (27◦5318S, 53◦4720W, 489m) in the State of Rio
Grande do Sul, Brazil, were used.The grains were placed into
raffia bagsafter harvested and immediately transported tothe
Postharvest,IndustrialisationandQualityofGrainsLaboratoryof
DCTA-FAEM-UFPel,wheretheexperimentswerecarriedout.The
grainswereharvestedmechanically,subjectedtoartificialdrying
withairtemperatureof35◦Cuntil14%ofmoisturewasachieved,
andsubsequentlypurgedusingaluminiumphosphidetoprevent
theinterferenceof insects intheexperiment Themaize grains
werestoredinpolyethylenebagsof0.2mmthickplasticfilmwith
acapacityof0.9kgattemperaturesof5,15,25and35◦Cfor12
months,intriplicate.Thegrainsweremaintainedcoveredfromthe
lightbyanaluminiumfoil
2.2 Starchisolation
Theisolationwasperformedaccordingtothemethoddescribed
by Sandhu, Singh, and Malhi (2005), with some modifications
Maizegrains(200g)wereaddedto500-mlof0.1%sodiumbisulfite
(NaHSO3)indistilledwater,andmaintainedfor20hat50◦C.After
thisperiod,thewaterwasdrainedandthegrainswerecrushed
ina grinder(ElectronicFilter600W,Britânia,SãoPaulo,Brazil)
untilthe smallestpossiblefraction(wet milling) wasachieved
Thecrushedsamplesweredoublefilteredthrough100and
270-meshsieves.Theprotein–starchfiltratesweredecantedfor4h.The
supernatantwasremovedandthesedimentedprotein–starchlayer
wasresuspendedindistilledwatertobecentrifugedat5000×g
for20min.Theresultingproteinrichsupernatant wasremoved
andtheremainingstarchslurrywasresuspendedonceagainin
distilledwaterbeforefurthercentrifugationtocompletelyremove
anyremainingproteincontent.Thecollectedstarchwasdriedat
40◦Cfor12hinanovenuntil11%ofmoisturewasachieved.Once
dry,thestarchwasplacedinalaboratorymill(Perten3100,Perten
Instruments,Huddinge,Sweden)with70-meshsieveforattaining
uniformparticlesizedistribution.Atotal of 100gkernelswere
usedtodeterminethepercentageextractionyieldbyweighingthe
starchobtainedafterdrying.Thestarchwasisolatedfromfreshly
harvestedmaizegrains,beforestorage,andconsideredasthe
ini-tialtreatment.Then,thestarchwasisolatedfrommaizegrainsand
storedundertime-temperatureconditionsmentionedabove
2.3 Colourparameters
Thecolouroftheisolatedstarcheswasdeterminedusinga
col-orimeter(Minolta,CR-310,Osaka,Japan).Thecolourparameters
usedwereL*(100=whiteand0=black)andb*(positive=yellow
andnegative=blue)
2.4 Proteinandfatcontents ThenitrogencontentwasdeterminedusingtheAACCmethod 46-13(AACC,1995),andtheproteincontentwasobtainedusing
aconversionfactorofnitrogentoproteinof6.25.Thefatcontent wasdeterminedinaccordancewiththeAACCmethod30-20(AACC,
1995)
2.5 Swellingpowerandsolubility Theswellingpowerandsolubilityofthestarcheswere deter-mined as described by Leach, McCowen, and Schoch (1959) Samples(1.0g)weremixedwith50mlofdistilledwaterin cen-trifugetubes.Thesuspensionswereheatedat90◦Cfor30min.The gelatinisedsampleswerethencooledtoroomtemperatureand centrifugedat1000×gfor20min.Thesupernatantsweredriedat
110◦Cuntilaconstantweightwasachievedsothatthesoluble frac-tioncouldbequantified.Solubilitywasexpressedasthepercentage
ofthedriedsolidweightbasedonthedrysampleweight.Swelling powerwasrepresentedastheratioofwetsedimentweighttoinitial drysampleweight(deductingtheamountofsolublestarch) 2.6 Pastingproperties
Thepasting propertiesof themaize starches(3.0g,14% wet basis) were determined with a Rapid Visco Analyser (RVA-4; NewportScientific,Warriewood,Australia)andprofileStandard Analysis1.Theviscositywasexpressedinrapidviscounits(RVU) Thesampleswereheldat50◦Cfor1min,heatedto95◦Cat3.5min and heldat95◦C for2.5min.Thesampleswerethen cooledto
50◦Cin4minandheldat50◦Cfor2min.Therotatingspeedwas heldat960rpmfor10sandthenmaintainedat160rpmduringthe process.Parametersincludingpastingtemperature,peakviscosity, breakdown,finalviscosityandsetbackwererecorded
2.7 Differentialscanningcalorimetry(DSC) Gelatinisation characteristics of the maize starches were determinedusingdifferentialscanningcalorimetry(TA-60WS, Shi-madzu,Kyoto,Japan).Starchsamples(approximately2.5mgona drybasis)wereweigheddirectlyinanaluminiumpan(Mettler, ME-27331),anddistilledwaterwasaddedtoobtainanaqueous suspensioncontaining75%water.Thepanwashermeticallysealed andallowedtoequilibratefor1hbeforeanalysis.Anemptypan wasusedasareference.Thesamplepanswerethenheatedfrom
40to140◦Cattherateof10◦Cmin−1.Theonsettemperatureof gelatinisation(To),peaktemperature(Tp),conclusiontemperature (Tc)andgelatinisationenthalpy(H)weredetermined.Therange
ofgelatinisationwascalculatedbysubtractingTofromTc
2.8 Crystallinity The crystallinity of starches was determined withan X-ray diffractometer(XRD-6000,Shimadzu,Brazil).Thescanningregion
ofthediffractionrangedfrom5◦ to30◦ withatargetvoltageof
30kV,currentof30mAandscanspeedof1◦min−1.Therelative crystallinity(RC)ofthestarchgranuleswascalculatedasdescribed
byRabek(1980)usingfollowingtheequation:
RC(%)=AcAc+Aa×100 whereAcisthecrystallinearea;andAaistheamorphousareaon theX-raydiffractograms
Trang 3Table 1
Extraction yield, colour parameters and chemical composition of starch isolated from freshly harvested (initial treatment) and stored maize grains at different temperatures for 12 months.
a Results are the means of three repetitions ± standard deviation Values followed by different letter in the same column are significantly different (p ≤ 0.05).
b L* (100 = white; and 0 = black), and b* (positive = yellow; and negative = blue).
2.9 Scanningelectronmicroscopy(SEM)
Themorphologyofthestarchgranuleswasexaminedusinga
scanningelectronmicroscope(Shimadzu,SSX-550).Starch
sam-ples were initially suspended in acetone to obtain a 1% (w/v)
suspension,andthesamplesweremaintainedinanultrasoundfor
15min.Asmallquantityofeachsamplewasspreaddirectlyonto
thesurfaceofthestubanddriedinanovenat32◦Cfor1h
Sub-sequently,allofthesamplesweresputtercoatedwithgoldand
examinedanaccelerationvoltageof15kVandmagnificationsof
1500×and3000×
2.10 Statisticalanalysis
Analyticaldeterminationsforthesampleswereperformedin
triplicate,andstandarddeviationswerereported,exceptforDSC
analysisandX-raydiffractograms,whichwereperformedin
dupli-cate.AcomparisonofthemeanswasascertainedbyTukey’stestto
a5%levelofsignificanceusingananalysisofthevariance(ANOVA)
3 Results and discussion
3.1 Extractionyield,colourparametersandchemical
composition
Theextractionyield,colourparameters,proteinandfatcontents
ofmaize starchisolatedfromgrainspoststoragetreatmentare
presentedinTable1.Thelowestextractionyieldwasobservedin
thestarchisolatedfromgrainsstoredatthehighesttemperature
(35◦C).Nostatisticaldifferencesoccurredinstarchisolatedfrom
grainsstoredat5,15and25◦Ccomparedtostarchisolatedfrom
freshlyharvestedgrains(initialtreatment).Theextractionyields
ofstarcharesimilartothosereportedbyMalumbaetal.(2009),
whoobtainedextractionyieldsbetween43.3% and64.4% when
evaluatingtheextractionofmaizegrainssubjectedtodryingair
temperaturesbetween80and130◦C
The greatest starch colour changeoccurred in maize grains
storedat 35◦C, which presented a 3.97%decreased in L* value
anda41.24%increaseinb*valuecomparedtothestarchisolated
fromgrainsbeforestorage(Table1).Insummary,starchisolated
fromgrainsstoredat35◦Cisyellowish,whiletheothersareclear
TheincreaseintheL*valueandthedecreaseinb*valuecanbe
attributedtotheirhigherresidualcontentofproteins.Theprotein
contentofstarchisolatedfromfreshlyharvested(initialtreatment)
maizegrainswas0.23%andreachedupto0.74%inthegrainsstored
at35◦Cfor12months.Thisincreaseisduetotheinteractionsof
thestarchchainswithproteins,resultingfromthestrengthening
ofdisulfidebondsduringstorage(Martin&Fitzgerald,2002;Park
etal.,2012;Zhou,Robards,Helliwell,&Blanchard,2003)which
hindertheseparationofstarchandproteinduringthewet-milling
process.Nodifferenceswereobservedin theproteincontentof
starchisolatedfromgrainsstoredat5,15and25◦Ccomparedto
starchisolatedfromfreshlyharvestedmaizegrains(initial treat-ment).Theresiduallevelsofproteinsareinagreementwiththose reportedbyMalumbaetal.(2009),whoobservedvalueslowerthan 1.5%instarchesisolatedfrommaizegrainsasafunctionofdrying temperature(between80and120◦C)
Thefatcontentofstarchisolatedfrommaizegrainsstoredat
35◦Cwaslowerthantheothertreatments(Table1).Therewasno differencebetweenthefatcontentofstarchisolatedfromfreshly harvestedgrains(initialtreatment)andgrainsstoredat5,15and
25◦C.AccordingtoDebetandGidley(2006),theresidualpresence
oflipidsandproteinsinthestarchgranulemaycauserestriction
oftheswellingpowerofthestarchduringthegelling.Harosetal (2003)andAltayandGunasekaran(2006)statedthattheproteins thatremaininthemaizestarchmaypossiblyreducetheentryof waterintothegranulesduringgelatinisation,limitingthe interac-tionsbetweenthewaterandcomponents,andcausinganincrease
instarchgelatinisationtemperatures
3.2 Swellingpowerandsolubility The swelling powerand solubilityof starches isolated from maizegrainsstoredunderdifferenttemperaturesarepresentedin
Fig.1aandb,respectively.Ingeneral,therewasanincreaseinthe swellingpowerandsolubilitywithincreasingtemperaturefrom60
to90◦C,asexpected.Thestarchisolatedfrommaizegrainsstored
at5◦Cshowedthehighestswellingpowerat90◦C(p<0.05).The resultsareconsistentwiththosedescribedbySandhuandSingh (2007),whichreportedswellingpowerat90◦Cbetween13.0and 20.7gofwaterpergramofdrystarchinninemaizevarietiesin theIowaState(USA).AccordingtoLeachetal.(1959)the inter-nalbondstrengthofstarchgranulesinfluencetheswellingpower, beingahighlycomplexedstarch,itshouldberelativelyresistant
toswelling,consequently,shouldhavelowerswellingpower.The starchsolubilityat80and90◦C(Fig.1b)increasedforall treat-mentsatthe endof 12 monthsofstorage compared tofreshly harvestedgrains(initialtreatment)(p<0.05).Majorchangesin sol-ubilitywereobservedat80and90◦C,asaresultofamyloseleaching fromthestarchgranuleanddiffusionduringtheswelling.The high-estsolubilitycanbeattributedtoalessrigidstructureofthestarch granulesobtainedfromstoredgrains,allowingtheleachingof amy-loseduringheating
3.3 Pastingproperties ThepastingpropertiesofmaizestarchesverifiedintheRVAare showninTable2.Thehighestpastingtemperaturewasverifiedin starchisolatedfromgrainsstoredat35◦C.Therewasnodifference betweentheinitialtreatmentandtheotherstoragetemperatures AccordingtoSandhuand Singh(2007),thepastingtemperature
isthetemperaturethatthestarchviscositystartstoincrease.The increaseinthepastingtemperaturefrom70.50to76.30◦Cafter12 monthsofstorageat35◦Ccanbeattributedtothegreaterpresence
Trang 4Temperature (°C)
95 90 85 80 75 70 65 60
55
-1 )
0
2
4
6
8
10
12
14
16
18
20
Initial 5°C 15°C 25°C 35°C
Temperature (°C)
95 90 85 80 75 70 65 60
55
0
2
4
6
8
10
12
14
16
18
Initial
5°C
15°C
25°C
35°C
a
b
Fig 1. Swelling power (a) and solubility (b) of maize starches isolated from freshly
harvested (initial) and stored maize grains at different temperatures for 12 months.
ofresidualproteinsinstarch,whichhinderstheswellingofthe
granulesduringthehydrationprocessandeventuallyelevatethe
temperaturetowhichstarchgelatinisationoccurs.Similarresults
werefoundbySetiawanetal.(2010)
Thepeakandfinalviscositiesincreasedinthestarchesisolated
fromgrainsstoredat5,15and25◦Ccomparedtostarchisolated
fromfreshlyharvestedmaize(initialtreatment).Ontheotherhand,
thelowestpeakandfinalviscositieswereobservedinstarch
iso-latedfromgrainsstoredat35◦C(Table2).AccordingtoSinghetal
(2003),thereductioninviscosityreflectsthelowerabilityofstarch
granulestofreelyswellbeforetheirphysicalcollapse.The
break-downwashigherinthestarchisolatedfrommaizegrainsstored
at5◦Ccomparedtostarchisolatedfromfreshlyharvestedgrains
(initialtreatment),whilethebreakdownofstarchesisolatedfrom
grainsstoredat15,25and35◦Cwaslowerthanthebreakdown
ofstarchisolatedfromfreshlyharvestedgrains(initialtreatment)
Thermal properties of maize starches.
T o ( ◦ C) T p ( ◦ C) T c ( ◦ C)
a T o = onset temperature, T p = peak temperature, T c = conclusion temperature,
T = gelatinisation temperature range, and H = gelatinisation enthalpy.
Fig 2.X-ray diffraction patterns of starches isolated from maize grains stored for
12 months at different temperatures.
(Table2).Thedecreaseinbreakdownindicatesa higherrigidity
ofstarchgranulesafterbeingstoredatthosetemperatures, mak-ingthegranuleresistanttodisruptandcollapsewhileheatingand shearing.Thehighest setbackwasalsopresented bystarch iso-latedfromgrains stored at5◦C (Table2).AccordingtoHughes
etal.(2009),thegreaterbreakdownandsetbackreflectthehigher swelling powerofthestarch granulesand rapidaggregationof leachedamylosechains,respectively.Thisstatementisin accor-dancewiththeresultsofswellingpowerpresentedinFig.1a,where thehighestswellingpowerat90◦Cwasobservedinstarchisolated frommaizegrainsstoredat5◦C
3.4 Differentialscanningcalorimetry(DSC) Thegelatinisationtemperatures,thetemperaturerangeof gela-tinisation(Tc−To)andthegelatinisationenthalpy(H)ofstarches isolatedfrom maizegrains storedat differenttemperaturesare presentedinTable3.Therewasasmallincreaseintheonset tem-peratureofgelatinisation(To),peaktemperatureofgelatinisation (Tp)andconclusiontemperatureofgelatinisation(Tc)ofstarch iso-latedfromstoredmaizegrainscomparedtostarchisolatedfrom freshlyharvestedgrains(initialtreatment)(Table3)
Pasting properties of starches isolated from freshly harvested (initial treatment) and stored maize grains at different temperatures for 12 months.
a Results are the means of three repetitions ± standard deviation Values followed by different letter in the same column are significantly different (p ≤ 0.05).
Trang 5Fig 3. Scanning electron micrographs (SEM) of starches isolated from maize grains stored for 12 months at different temperatures: initial (a and b), 5 ◦ C (c and d), 15 ◦ C (e and f), 25◦C (g and h) and 35◦C (i and j) at low and high magnifications, respectively.
Trang 6Table 4
Intensity of the main peaks of the X-ray diffractograms and relative crystallinity of
maize starches.
crystallinity (%)
a Counts per second.
Thestorageresultedinanincreaseintheenthalpyof
gelatinisa-tion(H)of3.13to15.40Jg−1 abovethevalueobservedinthe
starch isolatedfromfreshly harvested (initial treatment) maize
grains.Thelargestincreaseswereobserved inthestarches
iso-latedfromgrainsstoredat5and35◦C.ThehighestHpresented
bystarchisolatedfromgrainsstoredat5◦Cindicatesahighlevel
of starchchain intramolecular bonds,since theprotein and fat
contents(Table1)weresimilarbetweeninitial,5◦C,15◦Cand25◦C
treatments.Thisphenomenonisprobablywhythestarchisolated
fromgrainsstoredat5◦Cpresentedthehighestswellingpowerat
90◦C(Fig.1)andpeakviscosity(Table2).Ontheotherhand,the
increaseintheHofstarchisolatedfromgrainsstoredat35◦C
maybeduetothelowerpurityofstarch,asreportedinTable1
AccordingtoChung,Liu,Pauls,Fan,andYada(2008)andPiecyk,
Dru ˙zy ´nska,Worobiej,Wołosiak,andOstrowska-Lig ˛eza(2013),the
increaseinHmaybeinfluencedbytheresiduallevelsofproteins
andlipids,impairingthestarchgelatinisation.Thisincreasecanalso
beattributedtotheincreasedrigidityofthegranulesattheendof
storage,whichincreasestheenergyrequiredtodisruptthe
struc-tureofthestarchgranules,duetothecomplexationthatoccursin
thegrainconstituents.Inastudyconductedtoevaluatethe
ther-malpropertiesofriceunderdifferentconditions,Zhou,Robards,
Helliwell,andBlanchard(2010)reportedthattheenthalpyof
gela-tinisationandthegelatinisationtemperaturesareaffectedbyboth
temperatureandstoragetime
3.5 Crystallinity
TheX-raydiffractogramsof starchisolatedfromfreshly
har-vested (initial treatment) maize grains and from maize grains
storedatdifferenttemperaturesispresentedinFig.2.Themaize
starchesshoweda typicalA-typediffractionpattern,withmain
2peaksat15◦,17◦,18◦,20◦and23◦(Zobel,1964).Therewasa
decreaseinstarchcrystallinityattheendof12monthsofstorage
(Table4).Higherstoragetemperaturesprovidedhigherdecreases
in starch crystallinity The largest reduction was observed at
35◦C of storage, where the crystallinity reduced from 30.54%
(initialtreatment)to26.26% Althoughtherewasa reductionin
crystallinity,therewasanincrease intheintensityofthepeaks
15◦,17◦,18◦and20◦ ofthestarchisolatedfromgrainsstoredat
35◦C comparedtostarchisolatedfromfreshly harvestedgrains
(initialtreatement)(Table4).Thehighestpeakintensityindicates
thateventhoughthereislesscrystallineareainthestarchgranule;
therewasarearrangementthatleftthecrystalsinamoreparallel
array.ThisfindingisinagreementwiththeresultsobservedinRVA
(Table2)andDSC(Table3)analyses.Our resultsofcrystallinity
differfromthosereportedbySetiawanetal.(2010),whofound
increased relative crystallinity of starch isolated from maize
grainsaftersixmonthsofstorage.AccordingtoChrastil(1990),
storagecanalter theactivityand propertiesof theendogenous
enzymespresentin thekernels,suchasamylase,protease, and
phosphatase.Dhaliwal,Sekhon,andNagi(1991)andAwazuhara
etal.(2000)attributedchangesinthechainlengthofthebranched
amylopectintoenzymatichydrolysis,wherethe␣-amylaseattacks theamorphousregionofamylopectin,particularlythelongchains, reducingthemolecularweightofamylopectin
3.6 Scanningelectronmicroscopy(SEM) Thescanningelectronmicrographsofmaizestarchgranulesare showninFig.3.Thestarchgranulespresentedshapesvaryingfrom sphericaltopolyhedral,whicharetypicalofmaizestarch(Fig.3a andb).Althoughnochangesinthegranulesshapewasperceivedas
afunctionofstoragetemperature,thestarchgranulesisolatedfrom maizegrainsstoredat35◦Cshowedagreaterappearanceof sub-micronparticulatesonthesurfaceofthegranulesincomparisonto othertreatments(Fig.3 indicatedbyarrows).Further examina-tionviaX-rayphotoelectronspectroscopy(XPS)andatomicforce microscopy(AFM)couldperhapsshedlightonthenatureofthese particulates.Thestarchgranulesisolatedfrommaizegrainsstored
at35◦C appeartohavelowersizethanthegranulesfromother treatments,whichhaspossiblyoccurreddue tothepresenceof
a strongproteinmatrix compressingthestarchgranules within maizegrains.Thestrengthoftheinteractionsofstarchwith pro-teinsandlipidsresultedinagreaterresidualcontentofprotein (Table1)andcanbeassociatedwiththeobservedreductioninpeak viscosity(Table2).Setiawanetal.(2010)evaluatedtheeffectsof 6-monthsofmaizegrainsstorageafterthegrainsbeingdriedunder differentconditionsandfoundanincreaseinthenumberof dam-agedstarchgranules.Thedamageofstarchgranuleswasattributed
tostarch-hydrolyzingenzymeactivities.Similarobservationwas notverifiedinourwork
4 Conclusions
This was thefirst study to evaluate effects of storage tem-perature onthe physicochemical, pasting, thermal, crystallinity andmorphologicalpropertiesofmaizestarchisolatedfrommaize grainsstoredfor12months.Thestorageofmaizegrainsat35◦C causedareductionof22.1%intheextractionyieldofstarchand pro-videsayellowishcolourtostarch,whichmakesitlessattractivefor applicationswherepasteclarityisimportant.Thestarchisolated fromgrainsstoredduring12monthsshowedlowercrystallinity thanstarchisolatedfromfreshlyharvestedgrains.However,this hasprobablyresultedinamoreorganisedrearrangementofthe starchchainswithinthegranuleandpromotedinteractionswith otherconstituents, mainlyin starchisolated frommaize grains storedat35◦C.Thisproducedhighergelatinisationtemperatures andhigherenthalpyofgelatinisationasobservedbyDSCanalysis, andlowerpeakandfinalviscositiesverifiedinRVA.TheSEMof starchisolatedfrommaizegrainsstoredat35◦Cshowedthe pres-enceofsomepointsinthegranulesurfaceandthegranulesalso appeartobesmallerthanthosefromothertreatments,probably
asafunctionofastrongerproteinmatrixaroundstarchgranules withinthegrains.Furtherstudiesshouldbeconductedinorderto evaluateeffectsofthegrainstoragetemperatureduringshort-time storageonthepropertiesofisolatedstarch.Studiesabouteffectsof themoisturecontentofthegrainsinshort-andlong-timestorage
onthepropertiesofisolatedstarcharealsonecessary
Acknowledgements
We would like to thank CAPES (Coordenac¸ão de Aperfeic¸oamento de Pessoal de Nível Superior), CNPq (Con-selho Nacional de Desenvolvimento Científico e Tecnológico), SCT-RS(SecretariadaCiênciaeTecnologiadoEstadodoRioGrande
doSul)andPolodeInovac¸ãoTecnológicaemAlimentosdaRegião Sul
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