Chemistry and Functional Properties of Carbohydrates and Sugars (Monosaccharides, Disaccharides, and Polysaccharides) - Sweeteners: Nutritional Aspects, Applications, and Production Technology - page 15

2.3.1 properties of Monosaccharides or Sugars

Physical properties :Sugarsaresolidsubstances,sometimesdificulttocrystallize,whichoften
formsupersaturatedsyrupsolutions.Manyofthemhaveasweettaste,whereasothersaretasteless
orevenbitter.Simplesugarsaresolubleinwater,lesssolubleinmethanol,ethanol,andpyridine,
andinsolubleinlipophilicsolventssuchasethylether,chloroform,benzene,andpetroleumether
(Horton2008;Belitzetal.2009).
Mutarotation and transformations :Insugars,andparticularlyinaldohexoses,theopen(linear)
chain form cannot suficiently outline the aldehyde character of these compounds, as previously
mentioned.Morespeciically,thefactthatcertainreactionsofaldehydescannotproceedleadsto
thehypothesisthatthesemoleculesshouldappearinacyclo-hemiacetalform.Usingthisassump-
tion,C 1 inaldosesandC 2 inketosesbecomeasymmetricandthusmayappearintwocyclicisomeric
forms(anomers)αandβaccordingtothepositionoftheOH-intheanomericcarbon.Theoccurring
formsforglucoseandfructosearepresentedinFigure2.4.
Thecyclicformsofsugarshavearingwith5(knownasfuranoses:witharingoffourcarbon
atomsandoneoxygenatom)or6(knownaspyranoses:witharingofivecarbonatomsandone
oxygen atom) atoms. The pyranose structure is the one that occurs the most in nature due to its
stability.Thefuranosesareoftenfoundinketosesjoinedwithothersugarmolecules,thatis,fruc-
tose(joinedwithglucose)inthesucrosemolecule.Thecyclicformsofsugarsinaqueoussolutions
undergo ring opening to the acyclic aldehyde form, which, in turn, is converted to a mixture of
cyclicforms,asdescribedinFigure2.5.
Intheequilibriummixture,theacyclicformisfoundinasmallproportion,whereastheratioof
thecyclicformsvariesfromsugartosugar.Thisphenomenonresultsinthetransitionof [ α 20 with
timeduringpolarizationofasugarsolution.Theabovearemoreclearlyexplainedbelowinthecase
ofglucoseandfructose.
d-glucoseisisolatedintwoformsthataremirrorisomertotheC 1 atom.Theformsthatcanbe
isolatedare1)α-d-(+)-glucose(meltingpoint=146°Cand [

20

= − inafreshaqueoussolution),
whichcanbecrystallizedfromanaqueoussolution,and2)β-d-(-)-glucose(meltingpoint=150°C
and [

α D 113 °

α D ]

20

= + 18 7 . °

inafreshaqueoussolution),whichcanbecrystallizedfromapyridinesolution.

O

HO H H OH

H OH

H OH H OH

HO H

H H

HO O HO O

H OH

H OH H OH

H OH

OH
β-Hemiacetal form

OH OH

α-Hemiacetal form Aldehyde form

D -glucose

OH OH

OH

HO O

HO

HO
H

H
OH

HO H

O HO H

H OH

H OH

O

H H OH

H

OH

OH OH

β- D -Fructofuranose Keto form α- D -Fructopyranose

D -fructose

Figure 2.4 Isomeric forms of d -glucose and d -fructose.