2009_Book_FoodChemistry

4.2 Monosaccharides 289
Fig. 4.10. Increase of Amadori compounds in two stage
air drying of carrots as influenced by carrot moisture
content. — 10, 20, 30 min at 110 ◦ C; −−−60 ◦ C; sensory
assessment: 1) detection threshold 2) quality limit
(according to Eichner and Wolf,inWaller and Feather,
1983)
The lactol group of monosaccharides heated
in alcohol in the presence of an acid catalyst
is substituted by an alkoxy or aryloxy group,
denoted as an aglycone (Fischer synthesis), to
produce alkyl- and arylglycosides. It is assumed
that the initial reaction involves the open form.
With the majority of sugars, the furanosides are
formed in the first stage of reaction. They then
equilibrate with the pyranosides. The transition
from furanoside to pyranoside occurs most probably
through an open carboxonium ion, whereas
pyranoside isomerization is through a cyclic one
(cf. Reaction 4.102). Furanosides are obtainable
by stopping the reaction at a suitable time. The
equilibrium state in alcohol is, as in water, dependent
on conformational factors. The alcohol
as solvent and its R-moiety both increase the
anomeric effect and thus α-pyranoside becomes
a more favorable form than was α-pyranose in
aqueous free sugar solutions (Table 4.7). In the
system D-glucose/methanol in the presence of
1% HCl, 66% of the methylglucoside is present
as α-pyranoside, 32.5% as β-pyranoside, and
only 0.6% and 0.9% are in α- andβ-furanoside
forms. Under the same conditions, D-mannose
and D-galactose are 94% and 58% respectively in
α-pyranoside forms.
A highly stereospecific access to glycosides is
possible by C-1 bromination of acetylated sugars.
4.2.4.4.10 Inhibition of the Maillard Reaction
Measures to inhibit the Maillard reaction in cases
where it is undesirable involve lowering of the
pH value, maintenance of lowest possible temperatures
and avoidance of critical water contents
(cf. 0.3.2) during processing and storage, use of
nonreducing sugars, and addition of sulfite. Figure
4.10 demonstrates by the example of carrot
dehydration the advantages of running a twostage
process to curtail the Maillard reaction.
4.2.4.5 Reactions with Hydroxy Compounds
(O-Glycosides)
(4.102)
(4.103)
In the reaction of peracetylated sugar with HBr,
due to the strong anomeric effect, α-halogenide
is formed almost exclusively (cf. Formula 4.103).