2009_Book_FoodChemistry

264 4 Carbohydrates
Dehydrating Reactions. The heating of monosaccharides
under acidic conditions, e. g., during
pasteurization of fruit juices and baking of rye
bread, gives rise to a large number of furan and
pyran compounds (examples in Formula 4.35).
The formation of these compounds can be
explained by enolizations and dehydrating
reactions of the carbohydrates. It is noticeable
that in some compounds, the aldehyde group of
an aldose is formally retained at C-1 (furfural,
5-hydroxymethyl furfural, 5-methylfurfural) and
in other components, the aldehyde group is reduced
to a methyl group. As explained later, this
indicates the course of formation in each case.
(4.34)
The reaction pathway in acid starts slowly with
enolization to important intermediates called enediols.
Glucose gives rise to 1,2-enediol, and fructose
to 2,3-enediol as well (Formula 4.36). Starting
with the enediols, the further course of the reaction
is shown below.
The steps in the formation of 5-hydroxymethyl
furfural (HMF) from 1,2-enediol is shown in
Formula 4.37. HMF is also used as an indicator
for the heating of carbohydrate containing food,
e. g., honey. The (retro-Michael addition) water
elimination at C-3 and subsequently at C-4 leads
to a 1,2-diulose (3,4-dideoxyosone), which after
cyclization to a hemiacetal, a dihy-drofuran,
releases another molecule of water, producing
HMF. In the same way, e. g., furfural can be made
from pentoses and 5-methylfurfural from the 6-
methylpentose rhamnose. 2-Hydroxyacetylfuran,
which is preferentially formed from fructose,
can be obtained starting from the corresponding
2,3-enediol by water elimination at C-4, followed
by C-5 (Formula 4.38).
(4.35)