Technology of Baked Goods - Sourdough Biotechnology - page 48

Gliadins and glutenins have different roles for the viscoelastic characteristics of
gluten [ 5- 7 ]. Gliadins can be easily deformed and stretched, thanks to their viscous
properties that are typical of fluids, because of their globular shape and interconnec-
tions via disulfide bonds. In contrast, glutenins form long chains by means of inter-
protein disulfide bonds, which resist deformation and form an elastic and tenacious
mass. Although durum wheat semolina is used for bread making, especially in the
Mediterranean regions, the common wheat ( T. aestivum ) flour may be the optimal
raw material because it gives the best leavening results [ 8 ] . Soluble proteins in
wheat flour are mostly proteins with enzymatic activity. Most of these enzymes are
hydrolases (amylases, proteases, lipases), which specifically act on the reserve mac-
romolecules. Also oxidative enzymes (lipoxygenases, peroxidases) are present. For
many wheat processes, the enzymatic activities have a central and strategic role
which has to be carefully monitored (see also Chap. 2 ) .

3.2.1 The Milling Process

About 70% of the worldwide wheat production is used for food [ 9 ] , mainly by bakery
industries. The essential preliminary step is the milling process. The objective of
milling is twice. On the one hand, it separates the starchy endosperm, the area con-
taining gluten proteins, from germ, pericarp and seed coats, that form bran and other
by-products. On the other hand, it reduces the particle size of the endosperm to
values lower than 150-200 mm for common wheat flour and 500 mm for durum
wheat semolina. The fine granulometry of the flour gives an optimal workability to
the resulting dough and contributes to its processing into a palatable and appetizing
food. The removal of bran improves both the hygienic properties of the flour, since
the peripheral parts of the grain are often contaminated by chemical residues and
biotic pollutants, and the technological characteristics of the flour. In fact, non-
starch polysaccharides and enzymes, which are abundant in the bran, worsen the
rheology properties of the dough [ 10 ]. The separation of the oil-rich germ prevents
rancidity, which compromises the storage of flour [ 11 ]. However, the elimination of
these two parts, which are rich in several functional components, decreases the
nutritional value of the flour.
Overall, milling is much more complex than mere grinding. Because of the par-
ticular morphological structure of wheat grain, which is characterized by the crease
or ventral furrow, an introflexion along the length of the kernel, which hides a dou-
ble layer of teguments, the milling has to promote the extraction of the flour from
grains. Therefore, the first step consists of opening the grain. Then, proceeding from
the inside towards the outside, the endosperm is recovered via repeated sequences
of size reduction and separation stages, excluding the more external areas (bran,
aleurone layer, etc.), which are known as tailing products. This procedure fully
justifies the definition of “flour extraction yield”, defined as the quantity of flour
produced from 100 parts of cleaned and conditioned wheat grains [ 12 ] . This is the
only solution for preventing the passage of the bran layers from the furrow, in which