Food Chemistry

4.2 Monosaccharides 249

tetroses) and ketoses (starting from 2-pentuloses)
undergo intramolecular cyclization with hemiac-
etal formation to form lactols (Formula 4.3). With
the exception of erythrose, monosaccharides are
crystallized in these cyclic forms and, even in
solution, there is an equilibrium between the
open chain carbonyl form and cyclic hemiacetals,
with the latter predominating. The tendency to
cyclize is, compared to hydroxyaldehydes, even
more pronounced in monosaccharides, as shown
by ΔG◦-values and equilibrium concentrations
in 75% aqueous ethanol (cf. Formula 4.3).

(4.2)

Lactols can be considered as tetrahydrofuran or
tetrahydropyran derivatives, hence, they are also
denoted as furanoses or pyranoses.

(4.3)

4.2.1.2 Configuration...........................................

Glyceraldehyde, a triose, has one chiral center,

so it exists as an enantiomer pair, i. e. inD-

andL-forms. According to the definition, the

secondary hydroxyl group is on the right in

D-glyceraldehyde and on the left inL-glycer-

aldehyde. Although this assignment was at first

arbitrarily made, it later proved to be correct. It is

possible by cyanhydrin synthesis to obtain from

each enantiomer a pair of diastereomeric tetroses

(Kiliani–Fischer synthesis):

(4.4)