Chemistry Reference
In-Depth Information
Moisture absorption by starch
: When held at room temperature, starch equilibrates with the
moistureintheatmosphereinwhichitisheldandreversiblyabsorbswater.Undernormalcondi-
tions,thisamountstoabout10%to17%moisture.Thegranulespossessalimitedamountofelastic-
ity,whichpermitsthistotakeplace(Lee1975).Ithasbeensuggestedthatwaterinstarchmaybe
heldinthreeways,namely,waterofcrystallization,absorbedwater,orasinterstitialwater(Leach
1965).Thisisreversible.
Starch gelatinization
:Nativestarchisinsolubleincoldwater,butwhenheatedatadeinitetem-
perature(gelatinizationtemperature),whichisuniqueforeachkindofstarch,theswellingbegins
andtheviscosityofthemixtureincreases(Meyer1987;Owusu-Apenten2005).Starchgelatiniza-
tionisthecollapse(disruption)ofmolecularorderlinesswithinthestarchgranulealongwithcon-
comitantandirreversiblechangesinpropertiessuchasgranularswelling,crystallitemelting,loss
ofbirefringence,viscositydevelopment,andsolubilization(Liu2005).Itisbroughtaboutinstarch
suspendedinwater,byheatatacriticaltemperature,andbycertainchemicalsatroomtemperature.
Heatgelatinizationdoesnotoccurallatonceataspeciictemperature,butoverarangeofabout
~10°C.Thistemperaturerangevarieswithstarchesfromdifferentsources;forexample,forpotato
starch, the gelatinization temperature range is 56°C-69°C, and for corn starch, it is 62°C-80°C
(Pomeranz1991).Ingeneral,theapparenttemperatureofinitialgelatinizationandtherangeover
which gelatinization occurs depend on the starch/water ratio, granule type, and heterogeneities
withinthegranulepopulationunderobservation,whichchangesaccordingtothemethodofmea-
surement(Lee1975;BeMillerandWhistler1996).
Granuleswellingisthemostimportantphenomenoninthegelatinizationofstarchinaqueous
medium.Swellingduringgelatinizationisfavoredinhighamylopectinstarches,anditdependson
amylopectinstructureandamylopectindensityinthestarchgranules.Starchgelatinizationbegins
duringheatingofstarch,butitisinishedoncoolingwhenthestarchmoleculesformanetwork
withthewaterenclosedinitsmeshes,soproducingagel.Themaingelatinizationcharacteristicsof
starchesaretheswellingpowerthatdeterminesthecriticalconcentrationofstarchtoformapaste
(gstarch/100gwaterat95°C)andtheexacttemperatureatwhichgelatinizationbegins(Lee1975;
GamanandSherrington1990;PenieldandCampbell1990).
Starchgelatinizationprocessdependsonanumberoffactors.Thetemperatureatwhichgelati-
nizationstartsandtheexactchangesduringthecourseofgelatinizationarecharacteristicsof,irst,
thestarchvariety.Inaddition,thepHatwhichgelatinizationisattained,thetemperatureatwhich
observationsaremade,theextentofheating,thepresenceofsalts,sugars,andacids,andthesizeof
thegranules(thetemperatureofgelatinizationdecreasesasgranulesdecreaseinsize)areimportant
forgelatinization(Meyer1987;GamanandSherrington1990).
Gelatinizationofstarchgranulesincludesthreestages:1)Incoldwater,~25%-30%ofwater
is imbibed. This is apparently a reversible effect because the starch may be dried again with no
observablechangeinstructure.Theviscosityofthestarch-watermixturedoesnotchangeduring
thisphase.2)Itoccursat~65°Cformoststarches,whenthegranulesbegintoswellrapidlyandtake
upalargeamountofwater.Thegranuleschangeinappearanceduringthissecondphase,andsome
ofthemoresolublestarchmoleculesareleachedoutofthegranules.Thisstageisnotreversible.
3) Moreswellingismarked.Thegranulebecomesenormous,oftenavoidisformed,muchmore
starchisleachedout,andinally,thegranuleraptures,spillingmorestarchoutintothesurrounding
luid.Theviscosityoftheluidincreasesmarkedly,andthestarchgranulessticktogethersothat
theycannolongerbepickedapart(Meyer1987).
The swelling of starch, particularly amylose, which results in an increase in viscosity of a
starch-watermixtureandtheformation,underproperconditions,ofagelisnowbelievedtooccur
throughthebindingofwater.Inastarchgranule,amyloseandamylopectinmoleculesareloosely
boundtogetherbyhydrogenbondsofthehydroxyls,formingmicelles.Asthetemperatureofthe
water-starchmixturerises,hydrogenbondingdecreases,starchmoleculesareseparated,thewater
moleculesbegintofreelypenetratebetweenstarchmolecules,andtheyareimbibedbythestarch
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