2009_Book_FoodChemistry

15.4 Baked Products 731
Fig. 15.45. Tensile tests with dough made of flour from
the wheat cultivar Soisson (according to Kieffer and
Stein, 1999). Dough after 45 min rest without shear (–
), dough with shear after 135 min (•–•), dough without
shear after 135 min (—)
sequent dough resting stage and contracts to form
islands which are only slightly connected to each
other (Fig. 15.46b). In shearing (Fig. 15.46c),
these islands aggregate to form a network made
of thicker gluten strands. The extent of the effect
depends on the variety of wheat. In cultivars with
weak gluten, the gluten is finely distributed even
after shearing and the net is only weakly formed.
The driving force behind the unmixing of starch
and gluten is the tendency of gluten proteins to
aggregate via intermolecular interactions, e. g.,
hydrogen bridges and hydrophobic interactions.
Rye contains fewer gluten proteins than wheat.
In dough development, its aggregation is additionally
hindered by pentosans so that no gluten
network can be formed.
Baking experiments have shown that the dough
stability during fermentation is better, the baking
form rounder and the baking volume larger if
a clear unmixing of starch and gluten occurs due
to the shearing of dough.
15.4.3 Baking Process
15.4.3.1 Conditions
The oven temperature and time of baking for
some baked products are summarized in Table
15.50. Conditions for baking of rye and
rye mix bread sometimes deviate from these
values. They are prebaked at higher temperatures,
for instance at 400 ◦ C for 1–3 min, and
then post-baked at 150 ◦ C (for the effect on
quality see Table 15.51). In a continuous process,
tunnel-type ovens with circulation heaters are
used. Gratings frequently serve as the conveyor
band.
In an oven with the temperatures given in
Table 15.50, since heat transfer occurs slowly
in dough, there is a steep temperature gradient,
200 → 120 ◦ C, inward from the crust of the
dough piece. By the end of baking, a temperature
of 96 ◦ C is attained within the product. Higher
temperatures up to 106 ◦ C are found when the
crust is able to resist the rise in inner steam
pressure. The water evaporates only in the crust
region during dough baking. Water diffusion
towards the center of the bread can give the fresh
crumb a higher moisture content than the dough.
The steam concentration in the oven also affects
the baking results. A steam header is provided in
most oven designs to regulate oven moisture.
A baking weight loss is experienced as a result
of water evaporation during crust formation. The
extent of the loss is related to the form and size of
the baked bread and is 8–14% of the fresh dough
weight.
15.4.3.2 Chemical and Physical Changes –
Formation of Crumb
The foamy texture of dough is changed into the
spongy texture of crumb by baking. The following
processes are involved in this conversion.
Up to ca. 50 ◦ C, yeast produces CO 2 and ethanol
at a rate that initially increases. At the same
time, water and ethanol evaporate and, together
with the liberated CO 2 , expand the exisiting gas
bubbles, further increasing the volume of the
baked product. Parallel to this, the viscosity of
the dough falls rapidly in the lower temperature
range, reaches a minimum at ca. 60 ◦ C, and
then increases rapidly (Fig. 15.47). The increase
is caused, on the one hand, by the swelling of
starch and the accompanying release of amylose
and, on the other hand, by protein denaturation.
These processes result in a tremendous increase
in the tensile stress of the dough and in the
pressure in the gas bubbles at temperatures