2009_Book_FoodChemistry

318 4 Carbohydrates
Fig. 4.25. Model of a crystalline region in a starch granule
(according to Galliard, 1987). Amylopectin double
helix ; mixed double helix of amylose and
amylopectin ; V-helix of amylose and enclosed
lipid ; free lipid ; free amylose
Fig. 4.26. X-ray diffraction diagrams of starches:
A-type (cereals), B-type (legumes) and V-type (swollen
starch, V a : water free, V h : hydrated) (according to Galliard,
1987)
changes occur starting at a certain temperature,
which is characteristic of each type of starch (50–
70 ◦ C, cf. Table 4.24), called the gelatinization
temperature. The starch granules absorb 20–40 g
of water/g of starch, the viscosity of the suspension
rising steeply. At the same time, a part of
the amylose diffuses out of the granule and goes
into solution. Finally, the granule bursts. In the
first step of gelatinization, the starch crystallites
melt and form a polymer network. This network
breaks up at higher temperatures (ca. 100 ◦ C), resulting
in a solution of amylose and amylopectin.
In gelatinization, water first diffuses into the granule,
crystalline regions then melt with the help
of hydration, and, finally, swelling gives rise to
a solution through further diffusion of water. In
this process, hydrogen bridges between glucose
chains in the crystallites are primarily disrupted,
and perhaps some of those in the amorphous regions
as well. It is probable that the swelling of
the amorphous regions facilitates the dissolving
out of amylose from the crystallites, which are
thereby destabilized. As with heating in water, the
same effect occurs when starch is suspended in
other solvents, e. g., liquid ammonia or dimethyl
sulfoxide, or mechanically damaged, e. g., by dry
grinding.
The course of gelatinization depends not only on
the botanical origin of the starch and the temperature
used, but also on the water content of the
suspension (Fig. 4.27). Thus, dried starch with
1–3% of water undergoes only slight changes up
to a temperature of 180 ◦ C, whereas starch with
60% of water completely gelatinizes at temperaturesaslowas70
◦ C.
If an aqueous starch suspension is maintained for
some time at temperatures below the gelatinization
temperature, a process known as tempering,
the gelatinization temperature is increased, apparently
due to the reorganization of the structure of
the granule. Treatment of starch at low water contents
and higher temperatures results in the stabilization
of the crystallites and, consequently, a decrease
in the swelling capacity. Figure 4.28 shows
the resulting change in the X-ray diffraction spectrum
from type B to type A, using potato starch as
an example.
The changes in the physical properties caused by
treating processes of this type can, however, vary
considerably, depending on the botanical origin
of the starches. This is shown in Table 4.25 for
potato and wheat starch. On wet heating, the
swelling capacity of both starches decreases,
although to different extents. On the other
hand, there is a decrease in solubility only of