2009_Book_FoodChemistry

296 4 Carbohydrates
contain shorter or longer segments with periodically
arranged residues separated by nonperiodic
segments (alginate, carrageenans, pectin), or the
sequence may be nonperiodic all along the chain
(as in the case of carbohydrate components in
glycoproteins).
Fig. 4.11. Schematic representation of the hollow cylinder
formed by β-cyclodextrin
The β-cyclodextrin molecule is a cylinder
(Fig. 4.11) which has a primary hydroxyl (C6)
rim on one side and a secondary hydroxyl (C2,
C3) rim on the other. The surfaces made of
pyranose rings are hydrophobic. Indeed, the
water of hydration is very easily displaced from
this hydrophobic cavity by sterically suitable
apolar compounds, which are masked in this way.
In food processing, β-cyclodextrin is therefore
a suitable agent for stabilizing lipophilic vitamins
and aroma substances and for neutralizing the
taste of bitter substances
4.4 Polysaccharides
4.4.1 Classification, Structure
Polysaccharides, like oligosaccharides, consist of
monosaccharides bound to each other by glycosidic
linkages. Their acidic hydrolysis yields
monosaccharides. Partial chemical and enzymatic
hydrolysis, in addition to total hydrolysis, are
of importance for structural elucidation. Enzymatic
hydrolysis provides oligosaccharides, the
analysis of which elucidates monosaccharide sequences
and the positions and types of linkages.
Polysaccharides (glycans) can consist of one
type of sugar structural unit (homoglycans) or
of several types of sugar units (heteroglycans).
The monosaccharides may be joined in a linear
pattern (as in cellulose and amylose) or in
a branched fashion (amylopectin, glycogen,
guaran). The frequency of branching sites and the
length of side chains can vary greatly (glycogen,
guaran). The monosaccharide residue sequence
may be periodic, one period containing one or
several alternating structural units (cellulose,
amylose or hyaluronic acid), the sequence may
4.4.2 Conformation
The monosaccharide structural unit conformation
and the positions and types of linkages
in the chain determine the chain conformation
of a polysaccharide. In addition to irregular
conformations, regular conformations are known
which reflect the presence of at least a partial
periodic sequence in the chain. Some typical
conformations will be explained in the following
discussion, with examples of glucans and some
other polysaccharides.
4.4.2.1 Extended or Stretched, Ribbon-Type
Conformation
This conformation is typical for 1,4-linked β-Dglucopyranosyl
residues (Fig. 4.12 a), as occur,
for instance, in cellulose fibers:
(4.125)
This formula shows that the stretched chain
conformation is due to a zigzag geometry of
monomer linkages involving oxygen bridging.
The chain may be somewhat shortened or compressed
to enable formation of H-bonds between
neighboring residues and thus contribute to
conformational stabilization. In the ribbon-type,
stretched conformation, with the number of
monomers in turn denoted as n and the pitch (advancement)
in the axial direction per monomer
unit as h, the range of n is from 2 to ±4, while
h is the length of a monomer unit. Thus, the
chain given in Fig. 4.12 a has n = −2.55 and
h = 5.13 Å.
A strongly pleated, ribbon-type conformation
might also occur, as shown by a segment of