2009_Book_FoodChemistry

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, the Maillard
reaction is also called nonenzymatic browning.
Clinical biochemical studies have recently
shown that these browning products partly exhibit
antimutagenic and anticarcinogenic properties.
As a result of the complex course of the reaction,
however, it has only rarely been possible to
identify colored compounds till now. One of the
first colored compounds identified in model reactions
of xylose/amines and furfural/norfuraneol
is compound I in Formula 4.90. It is under discussion
that this compound is formed via condensation
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 yellow
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 intermediates
of the Maillard reaction result in the
formation of the colored components, which are
also called melanoidins.
(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 openchain
molecule of furfural is inserted into the
pyrrolinone structure. The proline/furfural reaction
system indicated further that ring opening
proceeds via a cyanine dye with the structure
illustrated in V, Formula 4.93. Other colored compounds
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-hydroxy-4-methyl-3-cyclopenten-1,2-dione
(methylene
reductic acid) with furfural (VII,
Formula 4.94). Both dyes were also obtained
by heating the Amadori product 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)