Chemistry Reference
In-Depth Information
2.1 INtrODUCtION
Carbohydratesarewidelydistributedinnatureandareconsideredimportantfoodcomponents,
astheyareasourceofenergyfortheorganism,structuralmaterials(starch,glycogen,cellulose),
and taste and lavor factors as well. Carbohydrates are formed in plants through photosynthesis
(Equation2.1)andcanbeusedasasavingmaterialintheformofstarch.Inanimaltissues/organ-
isms,carbohydratesarerespectivelyfoundintheformofglycogen.
h
γ
(2.1)
6
CO
+
6
H O
→
C H O
+
6
O
2
2
6
12
6
2
Theterm“carbohydrate”wasinitiallyused,astheirstknownclassofcompoundsfromthis
category followed the general formula C
n
(H
2
O)
n
; these compounds were derived from hydrated
carbonatomswithhydrogenandoxygencontainedinthesameproportionasinwater.Theterm
“carbohydrate”isstillusedeventhoughknowncompoundssuchasaceticacid(C
2
H
4
O
2
)orlactic
acid(C
3
H
6
O
3
)followthegeneralformulaofcarbohydrateswithouthavingthesamefeatures.
Carbohydratesfromthechemicalpointofviewarealdehydeorketonederivativesofpolyvalent
alcohols,namely,polyhydroxyaldehydesorpolyhydroxyketones,orcondensationproductsthereof.
Theterm“sugars”iscommonlyusedforthesimplemembersofthegroup,whicharenormallysol-
ubleinwaterandexhibitdifferentpropertiesfromtheseniormembers,forexample,complexcarbo-
hydratesarerelativelyinsolubleinwater.Carbohydratescanbeconvertedintosugarsbyhydrolysis
bymeansofacidsorenzymes(Southgate1991;Robyt1998;Izydorczyk2005).
2.2 CheMIStrY OF CarBOhYDrateS—CateGOrIeS aND prOpertIeS
Carbohydratescanbedividedintothefollowinggroups:
1. Monosaccharidesorsimplesugarsthatarethesimplestmembersofcarbohydratesandcannotbe
subjectedtohydrolysis.
2. Oligosaccharidesconsistingofasmallnumberofmonosaccharides(usuallytwoto10molecules),
withdisaccharidesbeingthemostinterestingforfoods.
3. Polysaccharidesthatconsistofalargenumberofmonosaccharides,forexample,amylosecontains
100-2000unitsofthemonosaccharideglucoseinitsmolecule.Themacromolecularpolysaccha-
ride compounds present variations in their physical and chemical properties compared with the
monosaccharidesfromwhichtheyderived.Themostimportantpolysaccharidesinnaturearestarch
(energystorageinplants),glycogen(energystorageinanimals),andcellulose(supportingmaterial
andstructuralcomponentinplants)(BeMillerandWhistler1996;BeMiller2010).
2.3 MONOSaCCharIDeS Or SUGarS
Monosaccharidesorsugarsareclassiiedaccordingeithertothenumberofcarbonatomsorthe
aldehydeorketogrouptheycontain.Thus
1. Moleculescontaining3,4,5,or6carbonatomsintheirmoleculecanbedistinguishedintotrioses,
tetroses,pentoses(fouranozes),andhexoses(pyranoses),respectively,ofwhichhexosespresentthe
greatestinterestinfoodsfollowedbypentoses.
2. Moleculescontaininganaldehydeorketogroupareclassiiedasaldosesorketoses,respectively
(Stick2001;Belitzetal.2009).
Numbering of carbon atoms in sugar molecules according to Emil Herman Fischer (1852 to
1919)startsfromtheclosesttothereducinggroupcarbonatom.Triosesarethemostsimplesugars,
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