2009_Book_FoodChemistry

4.2 Monosaccharides 267
formic acids in addition to formaldehyde and D-
arabinose (Formula 4.42). Depending on reaction
conditions, particularly the type of alkali present,
further hydroxyacids are also formed due to enolization
occurring along the molecule.
The nonstoichiometric sugar oxidation process in
the presence of alkali is used for both qualitative
and quantitative determination of reducing sugars
(Fehling’s reaction with alkaline cupric tartrate;
Nylander’s reaction with alkaline trivalent
bismuth tartrate; or using Benedict’s solution, in
which cupric ion complexes with citrate ion).
Hydroxyaldehydes and hydroxyketones are
formed by chain cleavage due to retroaldol reaction
under nonoxidative conditions using dilute
alkali at elevated temperatures or concentrated
alkali even in the cold.
(4.42)
For example, fructose can yield glyceraldehyde
and dihydroxyacetone (Formula 4.43), and the
latter easily undergoes water elimination to give
2-oxopropanal (methylglyoxal). Starting from
1-deoxy-2,3-hexodiulose, several degradation
pathways leading to short-chain compounds are
possible (Formula 4.44). Among other compounds,
2-oxopropanal, monohydroxyacetone,
acetic acid, glyceraldehyde or glyceric acid
can be formed by retro-aldol reactions (a),
α-dicarbonyl cleavages (b) and β-dicarbonyl
cleavages (c).
Since enolization is not restricted to any part of
the molecule and since water elimination and
redox reactions are not restricted in amount,
even the spectrum of primary cleavage products
is great. These primary products are highly
(4.43)