Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
natural, modified carbohydrates, such as aminated (chitin, chitosan), esterified with
inorganic acids such as sulfuric acid (heparin, carrageenans) and phosphoric acid
(potato amylopectin), oxidized (pectins, hyaluronic acid), alkylated (glycosides), and
reduced (deoxy sugars such as fucose, deoxyribose).
Several natural substances composed of carbohydrate and protein moieties (glycoproteins)
and lipid (glycolipids) constitute a group of so-called gangliosides.
Monomeric carbohydrates combine to form oligomers and polymers. Carbohydrates
are also called saccharides (a term originally common for sugars), because the firstisolated
compounds of this class were sweet. Research in this area has shown that
only selected saccharides are sweet, and a number of compounds have a sweet taste
but their structures do not even resemble those of saccharides.
1.2 CARBOHYDRATES AS A CLASS OF COMPOUNDS
Carbohydrates as a class of compounds received their name based on results of
elemental analysis of first-characterized compounds of this class. The analysis
showed that formally they are a combination of n carbon atoms and n molecules of
water always in a 1:1 proportion. As per this criterion, metanal (formaldehyde) can
be considered the first member of the series. It follows that subsequent members of
the series must be either hydroxyaldehydes [glyoxal (CHO–CH 2OH), glyceric aldehyde
(CHO–CHOH–CH 2OH) and so on], or hydroxyketones [hydroxyacetone
(CH 2OH–CO–CH 2OH) and so on]. To include hydroxyaldehydes and hydroxyketones
among saccharides, an additional condition — the presence of at least one
asymmetric carbon atom — has to be met.. Thus, the series of saccharides beginning
with hydroxyaldehydes (oses) is that containing a three-carbon glyceric aldehyde
called, therefore, triose, or more precisely, aldotriose. The series of saccharides
beginning with hydroxyketones (uloses) is 1,3,4-trihydroxy-2-butanon (erythrulose),
a tetrulose generally called a ketotetrose. Because of the asymmetric center localized
at the C-2 atom (starred in the structures shown), two possible configurations of the
hydrogen atom and hydroxyl group at this atom can be distinguished in a glyceric
aldehyde [(1.1) and (1.2), Figure 1.1].
Chirality is a natural consequence of asymmetry although, for various reasons,
not all compounds with asymmetric centers are chiral. Thus a glyceric aldehyde has
two optically active (chiral) isomers called enantiomers, each a mirror image of the
H
CHO
CHO
C* OH HO C* H
CH 2OH
mirror
FIGURE 1.1 Enantiomeric D- and L-glyceric aldehydes.
CH 2OH