2009_Book_FoodChemistry

15.4 Baked Products 727
Fig. 15.43. The reducing sugar content in wheat bread
crumb as affected by water content of dough (according
to Wassermann and Doerfner, 1971).
(flour + water) × 100
Water flour ratio (WFR) =
flour
RS: Reducing sugar expressed as maltose
oriented in the direction of stretching (1b, 1c)
and partially adhere to the glass at one end (2).
If circular movements are made with the cover
glass, the protein strands are two-dimensionally
stressed and most of the starch is released (3 and
4). As a result of the stickiness of the protein,
the strands can be easily aggregated to a ball
by further rotary movements. Another way of
representing the protein structure is to spread
flour particles on the water surface (5). The protein
strands which radially grow out of the flour
particle during hydration are linked by protein
films and, thus, bent. After appropriate fixation
of these structures, the protein films can be selectively
removed with 60% ethanol and the strands
lose their taut structure (8). The ethanol-soluble
gliadins and the strand-shaped insoluble glutenins
possibly exist separately even in the grain. Under
a scanning electron microscope at higher magnification,
a flour particle, after the removal of
starch with amylase, looks like a protein sponge
(6) in which starch granules were inserted.
One-dimensional stretching gives strands (7).
Similar gluten structures are detected in dough as
in flour particles, but the proteins form differently
arranged aggregates, which are more resistant to
tear, because of the strong mechanical treatment.
In ripe, dry grain, the gluten proteins are stored
as particles in the endosperm cells. The diameter
of these particles is 1–10 µm, depending on the
wheat cultivar. In addition, these particles can
still fuse together in the cell to form aggregates
with a diameter of up to 50 µm. At the start of the
kneading process, the particles and the aggregates
are hydrated and they form net-like structures
(10) as a result of their cohesive properties. The
exceptional cohesiveness of the gluten proteins is
due to their high glutamine content, which allows
the formation of innumerable hydrogen bridges.
Due to the mechanical processing in the kneader,
the proteins are increasingly brought into close
contact so that they aggregate to larger networks
(12). Strong shear forces are present in the dough
because of the low amount of free water. Thus,
like in a ball mill, the proteins are mixed with
other flour constituents and can react with them.
With increasing kneading time, the interactions
between the gluten proteins become stronger and
stronger, making the structures denser and denser
(14) until the kneading resistance reaches a maximum,
which can be measured in a farinograph.
As a result of the high content of starch (70% of
the dough), which is homogenously distributed
in the dough, the net-like structures are still very
thin (Fig. 15.46a). These structures are partially
broken down (56) again by overkneading,
weakening the support function of the gluten.
If the gluten is extended two-dimensionally to
a thin membrane, it starts to perforate (15) and
with increasing relaxation forms strands, which
are as round as possible. The energetic state of
these strands is lower than that of the membranes
because of the low surface area.
The connected gluten framework formed in this
way is responsible for the gas retention capacity
of wheat dough. In fact, stands which are as thick
as possible but easily extensible under the pressure
of the fermentation gases are of advantage
for the stability of the dough.
With the help of transmission electron microscopy,
it can be shown at still higher
magnification that the surface of unstretched
protein strands has an irregular globular structure
(18). As a result of the washing out of gliadin
with a large excess of water, these strands should
essentially consist of glutenin. On twodimensional
stretching, the globular surface is flattened
(19) and platelet-like forms appear (20) which
are arranged parallel to the plane of stretching
and are less than 10 nm in thickness. The globular
surface structures are probably highly tangled,
strand-shaped proteins which are unfolded due to