2009_Book_FoodChemistry

4.2 Monosaccharides 261
As described in 8.8.1.1, the AH/B/X-system has
been extended to explain the large differences in
structure and sweetening strength which can exist
in compounds of different substance classes.
4.2.4 Chemical Reactions and Derivatives
4.2.4.1 Reduction to Sugar Alcohols
Fig. 4.9. Bitter taste threshold values of limonin (◦—◦)
and naringin (×10 −1 •—•) in aqueous saccharose solution
(according to Guadagni, 1973)
Monosaccharides can be reduced to the corresponding
alcohols by NaBH 4 , electrolytically or
via catalytic hydrogenation. Two new alcohols
are obtained from ketoses, e. g., fructose, since
a new chiral center is formed:
for β-D-glucopyranose and β-D-fructopyranose
respectively:
(4.23)
β-D-glucopyranose and β-L-glucopyranose are
sweet. Molecular models show that the AH/B/Xsystem
of both sugar components fits equally
well with the complementary receptor system
AH R /B R /X R (Formula 4.24):
(4.26)
(4.24)
With asparagine enantiomers, the D-form is
sweet, while the L-form is tasteless. Here, unlike
D- andL-glucose, only the D-form interacts with
the complementary AH R /B R /X R -system:
(4.25)
The alcohol name is derived from the sugar name
in each case by replacing the suffix -ose or -ulose
with the suffix -itol. The sugar alcohols of importance
in food processing are xylitol, the only one
of the four pentitols (mesoribitol, D,L-arabitol,
mesoxylitol) used, and only D-glucitol (sorbitol)
and D-mannitol of the ten stereoisomeric forms of
hexitols [meso-allitol, meso-galactitol (dulcitol),
D,L-glucitol (sorbitol), D,L-iditol, D,L-mannitol
and D,L-altritol]. They are used as sugar substitutes
in dietetic food formulations to decrease water
activity in “intermediate moisture foods”, as
softeners, as crystallization inhibitors and for improving
the rehydration characteristics of dehydrated
food. Sorbitol is found in nature in many
fruits, e. g., pears, apples and plums. Palatinitol
(a mixture of glucopyranosyl glucitol and glu-