Food Chemistry

4.2 Monosaccharides 289

Fig. 4.10.Increase ofAmadoricompounds in two stage
air drying of carrots as influenced by carrot moisture
content. — 10, 20, 30 min at 110◦C;−−− 60 ◦C; sen-
sory assessment: 1) detection threshold 2) quality limit
(according toEichnerandWolf,inWallerandFeather,
1983)

4.2.4.4.10 Inhibition of the Maillard Reaction

Measures to inhibit theMaillardreaction in cases
where it is undesirable involve lowering of the
pH value, maintenance of lowest possible tem-
peratures and avoidance of critical water contents
(cf. 0.3.2) during processing and storage, use of
nonreducing sugars, and addition of sulfite. Fig-
ure 4.10 demonstrates by the example of car-
rot dehydration the advantages of running a two-
stage process to curtail theMaillardreaction.

4.2.4.5 Reactions with Hydroxy Compounds (O-Glycosides)

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 (Fischersynthesis), 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 prob-
ably 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, de-

pendent on conformational factors. The alcohol

as solvent and its R-moiety both increase the

anomeric effect and thusα-pyranoside becomes

a more favorable form than wasα-pyranose in

aqueous free sugar solutions (Table 4.7). In the

systemD-glucose/methanol in the presence of

1% HCl, 66% of the methylglucoside is present

asα-pyranoside, 32.5% as β-pyranoside, and

only 0.6% and 0.9% are inα-andβ-furanoside

forms. Under the same conditions,D-mannose

andD-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.

(4.102)

(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).