Chemistry Reference
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bydecreasingextractablesucroseyieldandalteringsucrosecrystalmorphology.Itiswater-soluble
anddoesnotreduceFehling'ssolution.
Stachyoseistheonlytetrasaccharidefoundinnature,anditconsistsoftwoα-d-galactoseunits,
oneα-d-glucoseunit,andoneβ-d-fructoseunit(Figure2.20).Legumesandpulsesfrequentlycon-
tain valuable amounts of rafinose and stachyose, where galactose and glucose are linked with
α-(1→6)-linkages.Theseoligosaccharidesareacauseoflatulencewheningestedbecausehumans
donotproducetheα-galactosidaseneededtohydrolyzethegalactose-glucosebond(BeMillerand
Whistler1996).
2.5 pOLYSaCCharIDeS
Polysaccharidesareveryimportantforhumannutritionandwidelydistributedinnature.Itis
estimatedthatmorethan90%oftheconsiderablecarbohydratemassinnatureisintheformofpoly-
saccharides.Examplesincludestoragepolysaccharides,suchasstarchandglycogen,andstructural
polysaccharides,suchascelluloseandchitin.Polysaccharides(glycans)arehighmolecularweight
polymersofmonosaccharidesandarenamedaftertheircomponentmonosaccharide.Thedegreeof
polymerization(DP),whichisdeterminedbythenumberofmonosaccharideunitsinachain,varies
fromahundredtoafewhundredthousand.Polysaccharidescanbeeitherlinearorbranched.Based
onthenumberofdifferentmonomerspresent,polysaccharidescanbedividedintohomopolysac-
charides,consistingofonlyonekindofmonosaccharide(e.g.,celluloseandstarchamylose,which
arelinearandstarchamylopectin,whichisbranched),andheteropolysaccharides,consistingoftwo
ormorekindsofmonosaccharideunits(e.g.,hemicelluloseandpectins).Hydrolysisofglycosidic
bondsjoiningmonosaccharide(glycosyl)unitsinpolysaccharidescanbecatalyzedbyeitheracids
orenzymes(BeMillerandWhistler1996;Belitzetal.2009).
Althoughitseemsmacroscopicallyandmicroscopicallyamorphous,X-rayanalysishasrevealed
polysaccharides'microcrystallinestructure.Dependingonthestructure,polysaccharidesmayhave
distinctpropertiesfromtheirmonosaccharidebuildingblocks.Thus,theydonothavesweettaste,
they do not reduce Fehling's solution, and they differ in solubility—they may be easily soluble
(glycogen)orformcolloidalsolutionsinwater(starch)orareinsolubleinwarmwater(cellulose).
2.5.1 homopolysaccharides
Themajorhomopolysaccharidesarestarch,cellulose,andglycogen.
2.5.1.1 Starch
Among food carbohydrates, starch occupies a unique position. It is the predominant storage
carbohydrate in plants and provides 70%-80% of the calories consumed by humans worldwide
(BeMillerandWhistler1996;deMan1999).Starchandstarchhydrolysisproductsconstitutemost
ofthedigestiblecarbohydrateinthehumandiet.Starchisformedbyphotosynthesis,and,unlike
cellulose,itsglucosecanbemetabolizedwiththeaidofamylasesinordertocovertheenergyneeds
ofanorganism.Itoccursassmallgranuleswiththesizerangeandappearancecharacteristictoeach
plantspecies,thusallowingthemicroscopicidentiicationofthestarch.
Starchisapolymerofd-glucose.Moststarchgranulesarecomposedofamixtureoftwopoly-
merforms:alinearfraction(amylose)andahighlybranchedfraction(amylopectin).Theratioof
amyloseandamylopectininstarchsourcesvariesdependingonthestarchorigin,buttypicalvalues
are20%-30%foramyloseand70%-80%foramylopectin.Variationsinstructureandproperties
ofstarchmaybeassociatedwithdifferentspecies,growthconditions,environments,andgenetic
mutations of plants. Starch granules are birefringent, indicating a high degree of internal order.
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