Technology of Baked Goods - Sourdough Biotechnology - page 65

lines in parallel (even with different types of dough) and high levels of hygiene and
cleaning.

3.4.3.2 Leavening

The production of any leavened baked goods concerns the transformation of the
semi-solid mass of the dough, a kind of emulsion with a continuous phase repre-
sented by hydrated gluten that surrounds the starch granules, and a dispersed phase,
consisting of microbubbles of air, into a foam, where the continuous phase retains
considerable volumes of gas (Table 3.2 ).
Leavening is the stage associated with the significant expansion of the original
volume of the mass. This is possible thanks to the viscoelastic properties of the
dough, and in some cases also to the presence of emulsifiers. The number of alveoli
retained in the mass upon completion of the mixing is estimated to be between 10 2
and 10 4 per mm 3 [ 50 ]. Volume expansion can be obtained using biological and
chemical leavening agents and also through the physical approach. In this latter
case, the inclusion of air follows intense mechanical actions during mixing. Mixed
leavening is also considered. Microalveoli incorporated during mixing (physical
leavening) are further expanded following the chemical leavening in the oven.
Danish pastry, used for making particular sweet products, is obtained from mixed
leavening: the dough is first biologically leavened, then formed with a lamination
process to distribute the fat in thin and alternate layers within the dough. Although
CO 2 is considered the major gas responsible for the development of dough volume,
other gases and low-boiling substances may interfere with the overall volume of the
dough. For instance, ethanol is solubilized in the aqueous stage of the mixture and
forms an azeotrope with a boiling point of 78°C and water vapour.
The most obvious phenomenon associated with leavening is the volume expan-
sion. The CO 2 produced is solubilized firstly in the aqueous stage of the dough.
Once saturation is reached, the gas settles in the bubbles entrapped in the dough
gradually dilating and expanding them, without any breakages. The pressure inside
the alveoli increases but the dough reacts by stretching thanks to gluten viscoelastic-
ity. The high diameter of the bubble makes it possible to balance the overpressure
that is created. The film (ca. 1 mm) created by surfactants, soluble proteins, polar
lipids, or pentosans, on the surface of the alveolus plays the principal role in this
phenomenon [ 45, 50, 51 ]. The leavened dough is therefore a foam consisting of a
semi-solid aqueous phase where gas bubbles are distributed. The coalescence of
these gas bubbles is delayed as long as the lipoprotein film is able to expand, reduc-
ing its thickness. Its breakage is associated with the merger with adjacent bubbles.
Acidification during sourdough fermentation also influences the rheological
properties of the dough. As shown by extensographic analyses, the acidification
determines the full maturation of the dough. The extensibility of the dough is
modified so that it can better support the dividing and final moulding stages. A fully
maturated dough will break clean and sharp with minimum resistance to pull [ 47 ] .
The reasons for this behaviour are numerous, complex, and only partly understood.