Chemistry and Functional Properties of Carbohydrates and Sugars (Monosaccharides, Disaccharides, and Polysaccharides) - Sweeteners: Nutritional Aspects, Applications, and Production Technology - page 28

to10 5 ,makingamylopectinoneofthelargestnaturallyoccurringmacromolecules.Itslargesize
andhighlybranchedstructureareresponsibleforthehighviscosityofamylopectindispersions.On
average,amylopectinhasonebranchpointevery20to30(typically25)residues;thebranchesin
amylopectinmoleculesarefarshorterthanthoseinamylosemoleculesandconsistof10-25(typi-
cally20)glucoseunits.Thebranchpointsarearrangedinclustersandarenotrandomlylocated.
Themolecularweight(Mw)ofamylopectinrangesfrom10 6 to10 9 g×mol -1 .Becauseofitsvery
highMw,largepolydispersity,andsusceptibilitytosheardegradation,amylopectinMwdetermina-
tionisdificult,andresultsvarywiththeanalyticaltechniqueandmethodusedtodispersethesam-
ple.Itisfoundintheouterportionofstarchgranule,itswellsinwater,and,whenheated,itresults
instarchpaste(Manners1985;PenieldandCampbell1990;ZobelandStephen1995;Thompson
2000;Wrolstadetal.2005;Jane2009).
Starch hydrolysis : Hydrolysis of starch may be brought about by the action of an acid or an
enzyme.Ifstarchisheatedwithanacid,itisbrokendownintosuccessivelysmallermolecules,with
theinalproductbeingglucose.Therearevariousstagesinthisreaction.Thelargestarchmolecules
are irst broken down into shorter chains of glucose units known as dextrins. The hydrolysis for
a short time produces amylodextrin (gives a blue color with iodine); further hydrolysis produces
erythrodextrin(givesaredcolorwithiodine)and,later,achrodextrin(notcolorablebyiodine).The
dextrinsproducedfromstarchhydrolysisconsistofamixtureoftheabove-mentionedtypesofdex-
trins,inproportionsthatvaryaccordingtothehydrolysisconditions.Thedextrinsarefurtherbro-
kendownintomaltose,andinally,maltoseisbrokendownintoglucose(GamanandSherrington
1990;PotterandHotchkiss1995).Duringenzymatichydrolysis,similarproductsareformed.These
polysaccharidefragmentsarecalledlimitdextrinsbecausetheenzymehasreachedthelimitofits
abilitytohydrolyze.Dextrinsaresolubleinwaterandinsolubleinethanol,theydonothaveasweet
taste,andtheyarenotreadilyfermentablebyyeasts(Lee1975;BeMillerandWhistler1996).
Themostcommonenzymesforstarchhydrolysisincludeα-amylase,β-amylase,glucoamylase,
oligosaccharide hydrolases, and phosphorylases. Such enzymes isolated from fungi, yeasts, bac-
teria,andplantkingdomsareofparticularinterestforthefoodindustrybecausetheyareusedto
modifystarch . α - Amylasescanhydrolyzetheα-(1→4)glucosidicbondsofstarchinarandomway
atany(1→4)-linkagewithinthestarchchaintorapidlyreducethemolecularsizeofstarchandthe
viscosity of the starch solution.α-Amylases result in a mixture of linear and branched oligosac-
charides,andeventuallymaltotriose,maltose,glucose,andarangeofbranchedα-limitdextrins.
β-Amylases also hydrolyzeα-(1→4) bonds from the nonreducing ends of the outer chains of
starchmoleculeandthusconvertamylosealmostcompletelytomaltotriose,maltose,andglucose.
Amylopectinishydrolyzedinasimilarway,startingfromthenonreducingendsoftheouterchains.
β-Amylasesfreemaltosefromthebranchesofamylopectinuntilbranchpointsarereached,result-
ingtoalimitdextrin(inadditiontomaltose).Sinceβ-amylasesareunabletobypassorhydrolyze
anα-d-(1→6)bond,theresultinglimitdextrincontainsalloftheα-d-(1→6)bondsandhasahigh
molecular weight. Because of the accumulation of maltose, β-amylases are called saccharifying
enzymes.
A mixture of α-amylase and β-amylase is used in starch industry with the name “diastase.”
Diastaseisalsopresentinlourandgerminatinggrain,andthusitisimportantinbreadmakingand
brewing.Glucoamylasecatalyzesthehydrolysisofα-d-(1→4)andα-d-(1→6)-linkagestoconvert
starchtod-glucose.Otherenzymesalsousedareoligosaccharidehydrolasesandphosphorylases
(Lee1975;Meyer1987;GamanandSherrington1990;PenieldandCampbell1990;BeMillerand
Whistler1996).
Starch fractionation : Amylopectin and amylose can be fractionated using aqueous leaching,
dispersion, and precipitation processes. Starch granules are completely dispersed in hot water or
aqueous dimethyl sulfoxide, amylose is precipitated as a crystalline complex by the addition of
hydrophilicorganicsolvents,whileamylopectinisrecoveredfromthesupernatantbylyophilization
(Liu2005).