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compoundsarecalledhemicelluloseandarenotwelldeined.Thecrudehemicelluloseiscomposed
byxylanchainandisseparatedintotwofractions:aneutralfraction(hemicelluloseA)thatcontains
side chains of arabinose parts and an acidic one (hemicellulose B) that contains 4-O-methyl-d-
glucuronicacidmolecules.
Itisgenerallyagreedthatthefollowingdifferencesexistbetweencelluloseandhemicellulose
molecules:(1)celluloseshaveahigherdegreeofpolymerization(moremonosaccharideunits)per
moleculethanhemicelluloses;(2)cellulosesarelesssolubleinalkaliandlesseasilyhydrolyzedby
diluteacidsthanhemicelluloses;(3)cellulosesareibrous,whereashemicellulosesarenonibrous;
(4)cellulosesyieldd-glucoseonhydrolysis,whereashemicellulosesyieldpredominantlyd-xylose
andothermonosaccharides;and(5)celluloseshaveahigherignitiontemperaturethanhemicellu-
loses(Meyer1987).
However,therearesomecommonpropertiesbetweencelluloseandhemicellulosemolecules:
(1) theyarebothabundantintheplantkingdomandactprimarilyassupportingstructuresinthe
planttissues;(2)theyareinsolubleincoldandhotwater;(3)theyarenotdigestedbyhumansand
therefore do not yield energy to their nutrition; (4) they can be broken down to glucose units by
certain enzymes and microorganisms; and (5) their long chains may be held together in bundles
formingibers(Potter1995).Thesecriteriaareonlygeneral,qualitativemeansofdifferentiation.
2.5.2.2 Pectic Substances
The pectic substances are derivatives of polysaccharides and constitute an important group
ofcarbohydratesforfoodsofplantorigin.Theyarelocatedbetweenthecells,perhapscellwalls,
servingasabindingandcellretentionagent.Theirstructureandmolecularweightarebeinginves-
tigatedandhavenotbeenyetclearlydeined.Theycomprisenotonlythosesubstancesinfruitsand
vegetables(pectins),whicharecapableofforminggelswithsugarandacids,butanumberofother
compoundsaswell.
The pectic substances are polysaccharides of galacturonic acid or of its methyl ester (Figure
2.25).Themainproductsofthehydrolysisofpecticcomponentsaregalacturonicacid,aderivative
ofgalactoseinwhichthe6-carbonisoxidizedtoacarboxylgroup,andmethylalcohol.Depending
ontheirorigin,theymaycontaind-glucose,l-arabinose,andl-rhamnose.
Thepecticsubstanceshavebeenstudiedformanyyears,andconcerningtheirnomenclature,the
followingdeinitionsexist(Kertesz1951):
1.
Pectic substances
:Complexcolloidalcarbohydratederivativesthatoccurin,orarepreparedfrom,
plantsandcontainalargeproportionofanhydrogalacturonicacidunitsthatarethoughttoexistina
chainlikecombination.Thecarboxylgroupsofpolygalacturonicacidsmaybepartlyesteriiedwith
methylgroupsandpartlyorcompletelyneutralizedbyoneormorebases.
2.
Protopectin
:Thetermisappliedtothewater-insolubleparentpecticsubstancethatoccursinplants,
andthat,uponrestrictedhydrolysis,yieldspectinicacids.
3.
Pectinic acids
: The term “pectinic acids” is used for colloidal polygalacturonic acids containing
morethananegligibleproportionofmethylestergroups.Pectinicacids,undersuitableconditions,
COOH
COOCH
3
COOCH
3
COOH
O
O
O
O
H
H
H
H
O
O
O
O
O
OH
H
OH
H
OH
H
OH
H
H
OH
H
OH
H
OH
H
OH
Figure 2.25
Structure of pectic substance.
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