284 4 Carbohydrates
4.2.4.4.8 Formation of Colored Compounds
As a result of the mostly brown colors (bread
crust, meat) formed by the heating of reducing
carbohydrates with amine components, theMail-
lardreaction is also called nonenzymatic brown-
ing. Clinical biochemical studies have recently
shown that these browning products partly ex-
hibit antimutagenic and anticarcinogenic proper-
ties. As a result of the complex course of the reac-
tion, however, it has only rarely been possible to
identify colored compounds till now. One of the
first colored compounds identified in model re-
actions of xylose/amines and furfural/norfuraneol
is compound I in Formula 4.90. It is under dis-
cussion that this compound is formed via con-
densation reactions of the CH-acidic compound
norfuraneol with the aldehyde group of furfural.
Similar condensation reactions of 3-deoxyosone
with furfural and of acetylformoin with furfural
in model reactions led to the formation of the yel-
low products II and III (Formula 4.91).
However, both compounds could be stabilized
only as the full acetal, e. g., in alcoholic solutions.
In general, it is assumed today that condensation
reactions between nucleophilic/electrophilic in-
termediates of theMaillardreaction result in the
formation of the colored components, which are
also calledmelanoidins.
(4.90)(4.91)(4.92)A red colored pyrroline dye (IV, Formula 4.92)
could be identified in a model reaction of furfural
and alanine. This dye is formed from 4 molecules
of furfural and 1 molecule of alanine. Labeling
experiments with^13 C showed that one open-
chain molecule of furfural is inserted into the
pyrrolinone structure. The proline/furfural reac-
tion system indicated further that ring opening
proceeds via a cyanine dye with the structure
illustrated in V, Formula 4.93. Other colored com-
pounds could be obtained by the condensation of
3,5-dihydroxy-2-methyl-5,6-dihydropyran-4-one
with furfural (VI, Formula 4.94) and of 3-hy-
droxy-4-methyl-3-cyclopenten-1,2-dione (met-
hylene reductic acid) with furfural (VII,
Formula 4.94). Both dyes were also obtained
by heating theAmadoriproduct of proline and
glucose in the presence of furfural. The orange
colored compound VIII and the red compound IX
were identified in a reaction system containing
xylose/alanine/furfural (Formula 4.94).(4.93)