Chemistry Reference
In-Depth Information
reducetheFehling'ssolution—theyarenonreducingsugarsanddonotshowmutarotation.Sucrose
isthemainrepresentativeofthisclass.
2. Maltosetype,inwhichthehemiacetalhydroxyloftheonemonosaccharideisjoinedthroughether
bond with one nonacetal hydroxyl of the other monosaccharide, allowing a free hemiacetal unit.
Thus,thesedisaccharidescanreducetheFehling'ssolution—theyarereducingsugarsandpresent
mutarotation.Maltose,lactose,andcellobiosebelongtothiscategory(Jackson1999;deMan1999).
Sucrose (saccharose)
:Sucrose(commonlyknownassugarortablesugar)occursinallplants,but
itiscommerciallyobtainedfromsugarcane(
Saccharum oficinarum
)andsugarbeet(
Beta vulgaris
ssp.
vulgaris
var.
altissima
).Itiscomposedofanα-d-glucopyranosylunitandaβ-d-fructofuranosyl
unitlinkedwithanα-d-glycosidicbond;thus,itisanonreducingsugar(Figure2.13).Itrotatesthe
plane-polarizedlighttotheright(+)andhasaspeciicopticalrotation
[
20
= +
.Sucroseis
readilysolubleinwater,anditssolubilityincreaseswithtemperature.Bycoolingorevaporatinga
saturatedsucrosesolution,asupersaturatedsolutionisobtained.Sucroseislightlysolubleinethanol
andinsolubleinethylether(Huberlant1993;BeMillerandWhistler1996;Izydorczyk2005;Belitz
etal.2009).
Sucroseisnotdirectlyfermentable;itishydrolyzedintod-glucoseandd-fructosebytheenzyme
sucrase,whichispresentinthehumanintestinaltract,andthereforecanbeutilizedbyhumansfor
energy.Themonosaccharidesd-glucoseandd-fructose,beingsigniicantinourdiets,donotneed
toundergodigestionbeforeadsorption(BeMillerandWhistler1996;Izydorczyk2005).
Becauseoftheuniquecarbonyl-to-carbonyllinkage,sucroseishighlylabileinacidmedium,
andacidhydrolysisismorerapidthanwithotheroligosaccharides.Sucrosecanbeenzymaticor
acidhydrolyzed.Theplantenzymeinvertaseisabletohydrolyzesucroseintoitstwoconstituent
sugars in equimolar mixture of d-glucose and d-fructose (inversion reaction). Since the speciic
rotation is +66.5° for sucrose, +52.2° for d-glucose, and -93° for d-fructose, the mixture has a
differentvalueofspeciicrotation(-20.4°)andistermedinvertsugarduetotheinversionofthe
directionofrotation(deMan1999;Izydorczyk2005).Invertsugarisusedintheproductionofjams,
boiled sweets, and some other sugar confections. Sucrose is not a digestible carbohydrate, but it
becomesoneafterhydrolysis.
Caramelizationofsucroserequiresatemperatureofabout200°C.Whensucroseisheatedto
210°C,partialdecompositiontakesplaceandcaramelisformed.Althoughcaramelizationoccurs
most readily in the absence of water, sugar solutions (syrups) will caramelize if heated strongly
enough.Caramel,producedbyheatingsucrosesolutioninammoniumbisulite,isusedincolasand
othersoftdrinks,bakedgoods,candies,andotherfoodproductsasacolorantandlavorcompound
(Gaman and Sherrington 1990; Huberlant 1993; deMan 1999; Izydorczyk 2005; Owusu-Apenten
2005).
Sucroseisthetraditionalsweeteningagentthatisderivedfromsugarcaneandsugarbeet,and
ithasapurelysweettaste.Thecomparativesweetnessofdifferentsugarsiscommonlygivenusing
apointscaleinwhichthesucrosesweetnessistakenas100(Table2.1).Sweetnessisasensorial
characteristic,anditsperceptiondependsonthehydrogenbondingbetweenglycolgroupsonthe
sugar molecules and receptor sites on the tongue. This property is dificult to measure reliably,
beingaffectedbythesugarconcentration,thetemperatureofthesolutions,andtheviscosityofthe
carriermedium.Regardlessoftheconditions,fructoseusuallyisrankedasthesweetestsugarand
α
D
]
66 5
.
°
CH
2
OH
O
O
H
H
H
HOH
2
C
H
H
HO
OH
H
O
CH
2
OH
HO
α
β
H
OH
OH
H
Figure 2.13
Structure of sucrose (Haworth type).
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