Chemistry and Functional Properties of Carbohydrates and Sugars (Monosaccharides, Disaccharides, and Polysaccharides) - Sweeteners: Nutritional Aspects, Applications, and Production Technology - page 18

O

H OH

HO H

H OH

H OH

D -Glucose

OH

+ Alkali

OH O

OH

O HO

HO

H
H

OH

HO H HO H

H OH

H OH H OH

H OH

H OH H OH

OH

OH OH

D -Fructose

Enol form

D -Mannose

Figure 2.9 Enol forms of glucose.

CH OH CHOH CHO ( ) → CH OH CHOH CH OH

sor

( )

2 4 2 4 2

(2.2)

- glucose - bitol

D D

Reactions with acids : Although monosaccharides do not react with weak acids even at mild
temperatures,orreactwithstronginorganicacidssugarswithslowrates,thesereactionsareaccel-
eratedbyheat.Monosaccharidesheatedwithhydrochloricorsulfuricacidformdisaccharidesand
highersaccharidesbyintermoleculardehydration,whereasbyfurtherendomoleculardehydration,
theyformfurfuralandhydroxymethylfurfuralfrompentosesandhexoses,respectively.Thesecom-
poundsarecondensedwithα-naphthol(Molischreaction)formingcoloredproductscharacteristic
ofthesugars'presence(Multon1997;Stick2001;StickandWilliams2009).
Reactions with bases :Sugarsinthepresenceofbasesaresubjectedtoisomerization,knownas
epimerization.Thereactiontakesplacebyketo-enolequilibrium.Thetransformationinvolvesthe
formationofintermediateenolforms(Figure2.9)leadingtoachangeofpositionofahydroxylgroup
inthecarbonatomandtoformationoftwoepimers(aldose-ketoseisomerization).Epimerization
ofglucoseleadstoenediol(thesamemaybeformedfrommannoseaswell),whichcanisomerize
tofructose.Ontheinluenceofstrongalkalies,particularlyathightemperatures,sugarmolecules
aresubjectedtomoreseveretransformations(Multon1997;Stick2001;StickandWilliams2009).

2.3.3 Basics on Monosaccharides

Pentoses :Pentosesarerarelyfoundfreeinnature;however,theyusuallyoccurboundinvari-
ouscomponentsofplantsourcesasinpolysaccharides,knownaspentosans.Pentoseslikehydroxy
aldehydes exhibit the typical properties of sugars; they rotate polarized light, present reducing
action,andcannotbefermentedbybaker'syeasts,whiletheyarefermentedbypentosanases.
Themostsigniicantpentosesare:d-xylose,l-arabinose,andd-ribose(Figure2.10).d-xylosecanbe
obtainedbyhydrolysisofplant/woodcomponents,l-arabinoseisacomponentofhemicelluloseandpec-
tin,whiled-riboseisabasicconstituentofnucleicacids(BeMillerandWhistler1996;Izydorczyk2005).