Biomedical Engineering Reference
In-Depth Information
a continuous structure with gluten during kneading. In freshly baked bread, the
main networks are the continuous gluten network which forms a matrix between the
swollen, gelatinized starch granules and the transient starch network, consisting of
entangled, gelatinized starch polymers [
4
]. During cooling the main changes occur
in the starch fraction, where amylose forms a partially crystalline amylose network,
with crystalline amylose and amylose-lipid complex. The amylose network together
with the gluten network formed during baking determines the resilience of the fresh
bread. Additionally pentosans (arabinoxylans) and transient gelatinized amylopec-
tin networks also contribute to the structure formation of freshly baked wheat bread.
During storage bread texture becomes harder largely because of physical changes
that occur in the starch-protein matrix of the bread crumb [
5
] . Retrogradation is the
process by which starch reverts to a more crystalline form after gelatinization.
Because amylose is already completely retrograded in fresh bread, the amylopectin
retrograding over time is primarily responsible for bread staling [
6
] . In addition,
water migrates within the crumb and from crumb to crust. This water loss is another
important factor contributing to the bread staling process [
4
] .
Compared to wheat flour, rye flour differs with regard to gluten composition,
starch gelatinization, and a -amylases activity [
7
] . Consequently, factors determin-
ing structure formation in rye dough and rye bread structure differ from wheat
dough and bread. Whereas sourdough was an essential ingredient for ensuring bak-
ing properties of dough containing more than 20% of rye flour, its addition to wheat
dough remains optional. The use of sourdough has gained increased interest in the
last decade as a means to improve the quality and flavor of wheat bread [
1
] . However,
there is much disparity in the results concerning the effect of sourdough on the final
wheat product and the use of sourdough has been shown to either decrease [
8-
10
]
or increase final bread volume [
10-
15
] .
8.3
Organic Acids
The production of organic acids and consequently the pH drop in sourdough have a
major effect on structure-forming components like starch, gluten, and arabinoxy-
lans (Table
8.1
). The primary effect of acids on the protein fraction is the increased
swelling and solubility of gluten proteins [
16,
17
]. This effect is explained by a posi-
tive net charge of the proteins in an acidic environment. Increased intramolecular
electrostatic repulsion causes gluten proteins to unfold and expose more hydropho-
bic groups. The presence of strong intermolecular electrostatic repulsive forces pre-
vents the formation of new bonds. This results in softer dough with less stability and
shorter mixing time [
18,
19
] . Furthermore, softness of the gluten promotes swelling
and increased water uptake [
20
]. Partial acid hydrolysis of starch also exerts posi-
tive effects on the starch granules leading to an increased water binding capacity [
8,
21
]. In addition to the impact of low pH on dough components, secondary effects of
acidification and fermentation time include changes in the activity of cereal enzymes.
Flour proteases have their pH optima in the acidic range and increased proteolysis
