Sourdough: A Tool to Improve Bread Structure - Sourdough Biotechnology - page 215

Table 8.1 Effects of sourdough metabolites on bread structure
Metabolites Effect on flour components Effect on dough/bread structure

Organic acids Increased swelling and solubility of the

gluten

Shorter mixing time, less
stability of the dough

Increased water uptake of gluten and
starch

Increase of elasticity and
softness of the dough

Increased proteolysis of gluten proteins
through flour endogenous proteinases

Both increase and reduction
of bread volume

Organic acids Increased solubility of pentosans

through acid hydrolysis and endog-
enous pentosanases

Improved volume and crumb
structure of rye and wheat
bread

Organic acids Inhibition of endogenous a-amylases Bakeability of rye bread

Enzymes Proteolysis Weakening of the gluten

structure, dough softening

Glutathione-reductase
Exopolysaccharides Increased water absorption

Increased softness of dough and
bread texture, increased volume

Interaction with gluten-starch network Increased shelf life (anti-

staling)

Inhibiting retrogradation of starch

CO 2 Expansion of gas cells Leavening of dough and bread

occurs in doughs at pH 4 in comparison to nonacidified systems [ 22 ] . In comparison
to straight dough processes, the increased activity of cereal proteases is also attrib-
utable to longer fermentation times. The rheological consequence of gluten degra-
dation is a major reduction of elasticity and firmness of the sourdough and subsequent
bread dough [ 10, 23, 24 ]. Whether this has positive or negative effects on bread
volume and staling depends on the acidity profile and gluten network.
Physicochemical changes in the protein network resulting from sourdough fermen-
tation enhance gas retention and allow greater expansion due to softer and more
extensible doughs [ 11, 13, 25 ]. Confocal laser-scanning microscopy visualized the
effect of incorporation of sourdough on the dough microstructure: Relative to the
fine well-oriented network of the control, the gluten in the dough with added sour-
dough had a more amorphous nature and there were greater areas of aggregated
material composed of thicker protein strands [ 23 ]. The presence of thicker strands
could also account for a greater increase in loaf volume [ 26 ] . Increased volume cor-
relates with the increased softness of the crumb, and is associated with a reduced rate
of staling [ 13, 27 ] . However, if the acidity of sourdough is further increased, bread
volume decreases [ 8- 10 ]. Quantitative analysis of gluten in sourdough and chemi-
cally acidified doughs showed that gliadins and glutenins are hydrolyzed [ 28, 29 ] .
Especially high molecular glutenins are completely degraded, which leads to a
strong gluten softening. Weaker gluten increases the expansion of dough, but also
decreases gas retention. Accordingly, the acidity level of sourdough and subsequent
bread dough must be carefully controlled to attain increased volume [ 30 ] .
The drop in pH during fermentation not only affects cereal proteases but modulates
amylase activity. Acid conditions partially inactivate amylases. This is an important
aspect in rye baking since excessive a-amylase activity results in a sticky crumb, a
very open grain, and a reduction in loaf volume [ 7, 21 ] . In wheat fl our, a -amylases are