2009_Book_FoodChemistry

730 15 Cereals and Cereal Products
Fig. 15.44. (continued) 18–22: Transmission electron microscopy. 18: Slightly extended gluten protein strand
with rough surface, enlarged section. 19: Protein strand, more extended and with smoother surface compared with
18. 20: Platelet-like structures on a highly extended gluten film. 21: Protein threads on a gluten film in a) water,
b) triethylamine solution, c) dithioerythritol solution. 22: a) Highly extended protein filbrils with thickenings,
b,c) enlarged
mechanical stress and are stabilized in the form
of superimposed layers due to intermolecular
interactions.
These protein threads have a diameter of 10–
30 nm and all look similar, irrespective of the
type of preparation, e. g., in water (21a), in triethylamine
(21b) or in dithioerythritol solution
(21c). One-dimensional stretching causes individual
protein threads to be partly stretched into
fibrils, which, including the metal layers vapor
deposited for stabilization, have a diameter of
only 3 nm (22a, b, c).
Based on the microscope pictures, dough formation
can be summarized as follows. The
individual flour particles consist of a sponge-like
protein matrix in which starch is embeded. After
addition of water, the matrix protein becomes
sticky and causes the flour particles to form
a continuous structure on kneading. At the
same time, the protein matrix is extended and
protein films are formed at the branch points
of the strands. In an optimally kneaded dough,
the protein films are the predominant structural
element and should contribute to the gas-holding
capacity. Further kneading causes increased
perforation of the films with formation of short,
irregular protein strands, which are characteristic
of overkneaded dough.
15.4.2.5.2 Dough Strengthening
A wheat dough is kneaded to the optimum and
pressed, rolled or formed after a resting time of,
e. g., up to 3 minutes or longer. This dough is subjected
to a relatively weak shear compared with
kneading. In this case, the resistance to extension
is increased in tensile tests in the extensograph
(Fig. 15.45).
Microscopic studies show that an unmixing of
starch and gluten occurs. While starch and gluten
are homogeneously distributed in freshly kneaded
dough (Fig. 15.46a), the gluten relaxes in the sub-