Chemistry Reference
In-Depth Information
Birefringenceistheabilitytorefractlightintwodirectionsandisevidencedbydistinctivepolar-
izationpatternsinpolarizingmicroscope(PenieldandCampbell1990;Jane2009).Purestarchis
awhite,tasteless,andodorlesspowderthatisinsolubleincoldwaterorethanol.Starchisdispersed
incoldwater;thesize,shape,andgeneralintegrityofthestarchgranulesremainunchangedincold
water(Owusu-Apenten2005).Theviscosity-building(thickening)powerofstarchisrealizedonly
whenaslurryofgranulesisthermallyprocessed(starchpaste).
Amyloseisessentiallyalinearmacromoleculeconsistingofα-d-glucoseunitslinkedtogether
by(1→4)bonds(Figure2.21).TheDPisusuallyintherangeof250to350(Meyer1987),butcan
bemanythousands.Ithasamolecularweightof150,000-1,000,000,dependingonitsorigin(Hood
1982).Amyloseisfoundintheinnerportionofastarchgranule,itformsacolloidaldispersionin
hotwaterwithoutswelling,anditsinteractionwithiodineresultsinformationofcomplexeswiththe
characteristicbluecolor.Amyloseconstitutes20%-30%ofthetotalstarchinthenonwaxycereal
starchesandinpotatostarch.Waxystarchessuchasthosefromwaxymaizehaveessentiallyno
amylose.Somesourcesofrelativelyhigh-amylosestarcharethewrinkledpeastarchthatcontains
about66%amyloseandsomecornvarieties(amylomaize)thatcontainupto75%amylose(Penield
andCampbell1990).
Amylopectin is a highly branched polysaccharide. Its structure consists ofα-d-glucose units
linked mainly by (1→4)-linkages (as in amylose) but with a greater proportion of nonrandom
α-(1→6)-linkages, providing a highly branched structure (Figure 2.22). The DP ranges from 10 4
α-(1 4)-linkage
CH 2 OH
CH 2 OH
CH 2 OH
CH 2 OH
O
O
O
O
OH
OH
OH
OH
O
O
O
O
OH
OH
OH
OH
DP
Figure 2.21
Glucose unit linkage in amylose.
CH 2 OH
OH
O
HO
O
CH 2 OH
OH
α-(1 4)-linkage
O
HO
α-(1 6)-linkage
O
CH 2 OH
O
CH 2 OH
CH 2
CH 2 OH
O
O
O
OH
OH
OH
OH
O
O
O
O
OH
OH
OH
OH
DP
Figure 2.22
Glucose unit linkage in amylopectin.
 
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