Food Chemistry

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 tri-
ethylamine (21b) or in dithioerythritol solution
(21c). One-dimensional stretching causes indi-
vidual 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 for-
mation 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 sub-

jected 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-