2009_Book_FoodChemistry

322 4 Carbohydrates
Fig. 4.33. Unit cells and arrangement of double helices (cross section) inA-amylose(left) and B-amylose (right)
(according to Galliard, 1987)
The double helix mentioned above and shown in
Fig. 4.34 can, depending on conditions, change
into other helical conformations.
In the presence of KOH, for instance, a more
extended helix results with 6 glucose residues
per helical turn (Fig. 4.34, b) while, in the
presence of KBr, the helix is even more stretched
to 4 residues per turn (Fig. 4.34, c). Inclusion
(clathrate) compounds are formed in the presence
of small molecules and stabilize the V starch
conformation (Fig. 4.34, d); it also has 6 glucose
residues per helical turn. Stabilization may be
achieved by H-bridges between O-2 and O-3 of
neighboring residues within the same chain and
between O-2 and O-6 of the residues i and i + 6
neighbored on the helix surface. Many molecules,
such as iodine, fatty acids, fatty acid esters of
hydroxycarboxylic acids (e. g., stearyllactate),
monoglycerides, phenols, arylhalogenides,
n-butanol, t-butanol, and cyclohexane are capable
of forming clathrate compounds with amylose
molecules. The helix diameter, to a certain
extent, conforms to the size of the enclosed guest
molecule; it varies from 13.7Åto16.2 Å. While
the iodine complex and that of n-butanol have
6 glucose residues per turn in a V conformation,
in a complex with t-butanol the helix turn is
enlarged to 7 glucose residues/turn (Fig. 4.34, e).
Itisshownbyanα-naphthol clathrate that up
to 8 residues are allowed (Fig. 4.34, f). Since
the helix is internally hydrophobic, the enclosed
“guest” has also to be lipophilic in nature. The
enclosed molecule contributes significantly to the
stability of a given conformation. For example, it
is observed that the V conformation, after “guest”
compound removal, slowly changes in a humid
atmosphere to a more extended B conformation.
Such a conformational transition also occurs
during staling of bread or other bakery products.
Freshly baked bread shows a V spectrum of
gelatinized starch, but aged bread typically has
the retrograded starch B spectrum. Figure 4.35
illustrates both conformations in the form of
cylinder projections. While in V amylose, as
already outlined, O-2 of residues i and O-6 of
residues i + 6 come into close contact through
H-bridges, in the B pattern the inserted water
molecules increase the double-strand distance
along the axis of progression (h) from 0.8nmfor
the V helix to 1 nm for the B helix.
Cereal starches are stabilized by the enclosed
lipid molecules, so their swelling power is low.
The swelling is improved in the presence of alcohols
(ethanol, amyl alcohol, tert-amyl alcohol).
Obviously, these alcohols are dislodging and
removing the “guest” lipids from the helices.