Chemistry and Functional Properties of Carbohydrates and Sugars (Monosaccharides, Disaccharides, and Polysaccharides) - Sweeteners: Nutritional Aspects, Applications, and Production Technology - page 30

molecules,resultingtostarchgranuleswelling.Thestickingtogetherofgranulesisbelievedtobe
theresultofmoleculesfromadjacentgranulesbecomingattractedandenmeshedinoneanother.
Results of starch swelling can be monitored using a Brabender Visco/amylo/graph or a Rapid
Viscoanalyzer,whichrecordstheviscositycontinuouslyasthetemperatureisincreased,heldcon-
stantforatime,andthendecreased(Meyer1987;BeMillerandWhistler1996;Liu2005).
Gelformationoccursthroughtheformationofathree-dimensionalnetworkofstarchmolecules,
particularlythelongstraightchainamylosemolecules.Thedegreeofassociationmaybeaffected
by a number of factors, such as molecular weight, distribution of molecular weight and ratio of
amylosetoamylopectin.Otherfactorscouldbethelengthofouterbranchesintheamylopectinand
itsbranchingdegree,aswellasthepresenceofnaturallyoccurringimpuritiesofanoncarbohydrate
nature.Sinceamyloseaidsgelling,highamylosestarchesareusedwherearigidgelisneededand
forinstantgelatinization,whereashighamylopectinstarches(i.e.,waxystarches)donotcrystallize
readilyandgelonlyathighconcentrations(Lee1975;Meyer1987;GamanandSherrington1990).
Thesubjectionofanaqueoussuspensionofstarchtoheatabovethecriticaltemperaturecauses
furtherswellingofthegranulesandlossofcrystallinity,thusincreasingthepasteviscosity,paste
clarity,andstarchsolubility.Bystirringthestarchpaste,thesehighlyswollengranulesareeasily
brokenanddisintegrated,resultinginarapiddecreaseinviscosity.Oncooling,somestarchmol-
eculespartiallyreassociatetoformaprecipitateorgel.Thisprocessisreferredtoas“setback”or
retrogradationtoindicateapartialreturnofagranule-like,orderedstructurewithinthestarchgel
(Lee1975;BeMillerandWhistler1996;Smith1982;Owusu-Apenten2005).
Highsugarconcentrationretardsgelatinization,whilelipidsandsurfactantsalsoaffectthepast-
ingbehaviorofstarch.Thelatter,bycomplexingwithamylose,retardgelatinizationanddecrease
thereleaseofexudateduringpasting(PenieldandCampbell1990).Theprotein-starchinteractions
arealsoimportantandplayasigniicantroleinbakeryproductswheregluteninteractswithstarch.
Starchgelatinizationcanoccuralsowhenfoodstarchisdryheated(dextrinization).Thus,many
foodscontainingstarchalsocontainsmallamountsofdextrins.Onheating,dextrinspolymerizeto
formbrown-coloredcompounds,calledpyrodextrins.Pyrodextrinscontributetothebrowncolorof
manycookedfoodsincludingtoastandbreadcrust(GamanandSherrington1990).
Because gelatinization is of such great importance in food processing, a variety of analyti-
cal techniques have been employed to probe the phenomenon and to understand its mechanism.
Theseincludeviscometry,opticalmicroscopy,electronmicroscopy,differentialscanningcalorim-
etry(DSC),X-raydiffraction,nuclearmagneticresonance(NMR)spectroscopy,Fouriertransform
infrared(FTIR)spectroscopy,andsimultaneousX-rayscattering(Liu2005).
Starch retrogradation :Starchretrogradationisaprocessthatoccurswhenthemoleculescom-
prisinggelatinizedstarchbegintoreassociateinanorderedstructure.Initsinitialphases,twoor
morestarchchainsmayformasimplejuncturepoint,whichthenmaydevelopintomoreextensively
orderedregions.Ultimately,underfavorableconditions,acrystallineorderappears(Atwelletal.
1988).
Although amylose-containing starches gel best, they are less stable than high amylopectin
starches. The amylose molecules tend to unwind, and the gel becomes opaque and like a pulpy
sponge.Thus,starchgelsarereadilycutbyshearingforcesandreducedtoliquid,whichisoften
importantinfoodpreparation.Onstanding,thegelformsagain.Onaging,moststarchgelsshow
markedsyneresisorweepinginwhichthewatergraduallypassesoutoftheintersticesofthegel.
Thisphenomenon ismadeobviousaspartofthestarchaggregatesandformsmicrocrystalsthat
precipitatewithtime.Retrogradationoccursparticularlywhenfoodsarefrozenandthenthawed
(Meyer1987;GamanandSherrington1990).
Stalingofbreadisgenerallyattributedtostarchretrogradation.Retrogradationofamylosein
theintergranularspaceiscompletebythetimethefreshlybakedloafiscool,andfurtherchanges
recognizedasstalinginvolveamylopectinchainsthatassociatewithinthegranules.Storagetem-
peratureaffectstherateofstaling.Stalingisrapidatrefrigeratortemperaturesbutveryslowator