Chemistry of Cereal Grains - Sourdough Biotechnology - page 15

2.2.1.3 Changes in Starch Structure During Processing

In many cereal manufacturing processes flour and also starch is usually dispersed in
water and finally heated. In particular heating induces a series of structural changes.
This process has been termed gelatinization [ 22 ]. Depending on water content, water
distribution, and intensity of heat treatment the molecular order of the starch granules
can be completely transformed from the semicrystalline to an amorphous state.
The mixing of starch and excess water at room temperature leads to a starch sus-
pension. During mixing starch absorbs water up to 50% of its dry weight (1) because
of physical immobilization of water in the void space between the granules, and (2)
because of water uptake due to swelling. The latter process increases with tempera-
ture. If the temperature is below the gelatinization temperature, the described changes
are reversible. As the temperature increases, more water permeates into the starch
granules and initiates hydration reactions. Firstly, the amorphous regions are hydrated
thereby increasing molecular mobility. This also affects the crystalline regions, in
which amylopectin double helices dissociate and the crystallites melt [ 23, 24 ] . These
reactions are endothermic and irreversible. They are accompanied by the loss of
birefringence, which can be observed under the polarization microscope. Endothermic
melting of crystallites can also be followed by differential scanning calorimetry
(DSC). Viscosity measurements, for example in an amylograph or a rapid visco ana-
lyzer, also allow one to monitor the gelatinization process. Characteristic points are
the onset temperature ( T o; ca. 45 °C), which reflects the initiation of the process, as
well as the peak ( T p; ca. 60 °C) and conclusion ( T c; ca. 75 °C) temperatures. These
temperatures are subject to change depending on the botanical source of the starch
and the water content of the suspension. The loss of molecular order and crystallinity
during gelatinization is accompanied by further granule swelling due to increased
water uptake and a limited starch solubilization. Mainly amylose is dissolved in
water, which strongly increases the viscosity of the starch suspension. This phenom-
enon has been termed “amylose leaching,” and it is caused by a phase separation
between amylose and amylopectin, which are immiscible [ 25 ] . During further heating
beyond the conclusion temperature of gelatinization swelling and leaching continue
and a starch paste consisting of solubilized amylose and swollen, amorphous starch
granules is formed. The shapes of the starch granules can still be observed unless
shear force or higher temperatures are applied [ 23, 26 ] .
Upon cooling with mixing the viscosity of a starch paste increases, whereas a
starch gel is formed on cooling without mixing at concentrations above 6%. The
second process is relevant in cereal baked goods. The changes that occur during
cooling and storage of a starch paste have been summarized as “retrogradation” [ 22 ] .
Generally, the amorphous system reassociates to a more ordered, crystalline state.
Retrogradation processes can be divided into two subprocesses. The first is related
to amylose and occurs in a time range of minutes to hours, the second is caused by
amylopectin and takes place within hours or days. Therefore, amylose retrograda-
tion is responsible for the initial hardness of a starch gel or bread, whereas amylo-
pectin retrogradation determines the long-term gel structure, crystallinity, and
hardness of a starch-containing food [ 27 ] .