266 4 Carbohydrates
With the 2,3-enediol, not only water elimination
at C-4, but also the elimination of the hydroxyl
group at C-1 is possible (Formula 4.39). This
reaction pathway gives, among other compounds,
3,5-dihydroxy-2-methyl-5,6-dihydropyran-4-one,
which is also used as an indicator for the heating
of food. The formation of two different enediols
is the reason for the wider product spectrum from
ketoses, like fructose, than from aldoses. In the
presence of amino compounds, all the reactions
mentioned here proceed very easily also in the
pH range 3–7. Since free amino acids are present
in many foods, the reactions shown here also
occur in connection with the pathways discussed
in 4.2.4.4.
4.2.4.3.2 Reactions in Strongly Alkaline Solution
Alkaline reaction conditions occur in food, e. g.,
in the isolation of sucrose from sugar beet and in
the production of alkali-baked goods. Apart from
enolization reactions, which can also occur un-
der acidic conditions but proceed much faster un-
der alkaline conditions, the degradation of the
carbohydrate skeleton is an important characteris-
tic of base-catalyzed degradation reactions. Glu-
cose, mannose and fructose are in equilibrium
through the common 1,2-enediol. Also present is
a small amount of psicose, which is derived from
fructose by 2,3-enolization:
(4.40)In this isomerization reaction, known as theLobry
de Bruyn–van Ekensteinrearrangement, one type
of sugar can be transformed to another sugar in
widely differing yields. The reaction plays a role
in transforming an aldose to a ketose. For ex-
ample, in the presence of sodium aluminate as
a catalyst, lactose (4-O-β-D-galactopyranosido-
D-glucopyranose, I) is rearranged into lactulose
(4-O-β-D-galacto-pyranosido-D-fructose):(4.41)In this disaccharide, fructose is present mainly as
the pyranose (IIa) and, to a small extent, as the
furanose (lIb).
Lactulose utilization in infant nutrition is under
consideration since it acts as a bifidus factor and
prevents obstipation.
Oxidation of the enediol occurs in the presence of
oxygen or other oxidizing agents, e. g., Cu^2 +,re-
sulting in carboxylic acids. In such a reaction with
glucose, the main products areD-arabinonic and