Biomedical Engineering Reference
In-Depth Information
A further criterion for differentiating baked goods is the evaluation of their lightness
and softness, and so of the specific volume (correlated with lightness) and humidity
(which determines the softness). These two structural characteristics are the result
of complex phenomena that occur during each stage of the technological process.
Indeed, the final characteristics of the product not only depend on leavening but also
on mixing and baking (see Sect.
3.4
). A light and soft baked good usually has a
specific volume higher than 2.5-3.0 mL/g and humidity higher than 15-18%. A dry
and friable baked good has a specific volume between 1.3 and 2.5 mL/g and humid-
ity values lower than 5-10% [
2
]. On the basis of these two criteria, four categories
of baked products can be identified. Moreover, each category can be further subdi-
vided according to the leavening method used (see Sect.
3.3.3
).
3.2
Wheat, a Preferred Raw Material for Bread
and Baked Goods
The choice of ingredients is of fundamental importance for the production of leavened
baked goods that satisfy consumer tastes. Although barley and rye flours provide a
workable dough, bakers realized a long time ago that the best results in terms of vol-
ume development were always obtained with wheat flour. The superior quality of this
cereal, especially its most evolved species
Triticum durum
and
T. aestivum
, is of a
purely technological nature. Only wheat flour provides a cohesive and homogenous
dough, where the single and original particles of flour are no longer recognizable.
The property that makes wheat dough unique is its viscoelasticity. This rheologi-
cal property enables the mass to be stretched and deformed without rupturing. At
the same time, the dough is elastic and tenacious, capable of maintaining its shape
even when subjected to physical stress. After baking, gluten proteins denature and
loose viscoelasticity, ensuring the maintenance of the final shape of baked goods.
The reason for this versatile behaviour does not depend on differences in the quan-
tity of components. Indeed, the protein content of the numerous varieties of wheat
extends over quite a wide interval, from 9 to 16% of the weight of the grain [
3
] . This
variability coincides with that of other cereals. The technological superiority of
wheat is related to complex qualitative differences at the level of protein fractions.
Wheat storage proteins are generally classified based on molecular weight (MW),
solubility and conformation. Storage proteins, gliadins and glutenins, account for
ca. 80% of the entire protein fraction and have a particular amino acid composition.
Gluten proteins have a high percentage of glutamine, about one third of all amino
acids, and proline, and low levels of lysine (Chap.
2
,
[
4
] ). This composition is
responsible for the low nutritional value of wheat protein. However, it also accounts
for the protein-protein interactions that lead to the formation of gluten, the three-
dimensional network which is continuous and homogenous throughout the mass.
Both non-covalent bonds, such as hydrophobic interactions and hydrogen bonds
(guaranteed by the large amounts of glutamine), and covalent bonds are involved,
the most significant of which are the disulfide bonds between cysteine residues.
