BLBS102-c31 BLBS102-Simpson March 21, 2012 14:0 Trim: 276mm X 219mm Printer Name: Yet to Come
31 Bakery and Cereal Products 601reached at which the protein begins to coagulate. At this stage,
the structure of the dough has changed to a more rigid structure
due to denatured protein and gelatinized starch. These changes
first occur near the crust and gradually move into the crumb as
the heat is transferred inward.
Toward the end of baking, the temperature at the crust is
much higher than 100◦C. The crust becomes brown, and aroma
compounds, predominantly aldehydes and ketones, are formed,
mainly from Maillard reactions. The formation of flavor com-
pounds is two staged. First, compounds are formed from the
fermentation itself, and then during baking some of these com-
pounds may react with each other or with the bread components
to form other flavor compounds. Some other flavor compounds
formed during the fermentation may be lost due to the high tem-
perature, but those that remain gradually diffuse into the crumb
after cooling. On further storage, the levels of flavor compounds
decrease due to volatilization. Over 200 different flavor and
aroma compounds have been identified in bread (Stear 1990).StalingThe two main components of staling (firming of the bread crumb)
are loss of moisture, mainly due to migration of moisture from
the crumb to the crust (which becomes soft and leathery), and the
retrogradation of starch. The major chemical change that occurs
during staling is starch retrogradation, but a redistribution of
water between the starch and the gluten also has been proposed.
The gelatinization of the starch that occurs during cooking or
baking gradually reverses, and the starch molecules form in-
termolecular bonds and crystallize, expelling water molecules
and resulting in the firming of crumb texture. Starch retrograda-
tion is a time- and temperature-dependent process and proceeds
fastest at low temperatures, just above freezing point. Since the
rearrangement of the starch molecules is facilitated by a high
water activity, staling is retarded by the addition of ingredients
that lower water activity (e.g., salt and sugar) or bind water (e.g.,
hydrocolloids and proteins). The staling rate can also be slowed
by the incorporation of surfactants, shortening, or heat-stable
α-amylase. Freezing of baked goods also retards staling since
the water activity is drastically lowered. Much of the firming of
the loaf during cooling is due to retrogradation of the amylose
whereas the slower reaction of staling is due, in addition, to
retrogradation of amylopectin (Hoseney 1998, Stear 1990).SOURDOUGH BREAD
When cereal flour does not contain gluten, it is not suitable for
production of leavened bread in the manner described above.
However, if rye flour, which is very low in gluten proteins, is
mixed with water and incubated at 25–30◦C for a day or two,
there is a good possibility that first step of sourdough production
will be started. This mixture will regularly develop fermentation
with lactic acid bacteria (LAB) and yeasts. This forms the ba-
sis of sourdough production, and this low-pH dough is able
to leaven.
The use of cereal flour and water as a basis for spontaneous or
directed fermentation products is common in many countries. InAfrica, fermented porridge and gruel as well as their diluted thirst
quenchers, are the main products of these natural fermentations,
whereas Europeans and Americans and their descendants enjoy
a variety of sourdough breads. In all these areas beers are also
produced.
This great variety of fermented products has an historic pro-
totype in the earliest reported leavened breads in Egypt about
1500 bc. Considering the simplicity of the process and the ease
with which it succeeds, it has been suggested that peoples in
several places must have shared this experience independently.
It may be surmised that the experience with gruels and porridge
preceded the idea of making bread.
Common bread fermented only with yeast appeared later in
our history, and it was a staple food in the Roman Empire. This
also indicates that the Romans had wheat with sufficient gluten
potential.
It is possible to make leavened bread without gluten using
sourdough, and this bread has become a favorite among many
peoples (Hammes and G ̈anzle 1998, Wood 2000).Advantages of Making Sourdough Bread
Sourdough bread does not have to contain high levels of
gluten for successful leavening.
Low pH inhibits amylase, and thereby, degradation of
starch is avoided.
Sourdough improves the water-binding capacity of starch
and the swelling and solubility of pentosans.
Sourdough bread has very good keeping quality and an
excellent safety potential.
Less costly cereal flours can be used.
A different variety of flavor and taste attributes can be
offered.
Sourdough bread can nutritionally compete with regular
bread.
Phytic acid is degraded by phytase in flour and from lactic
acid bacteria. This improves the availability of iron and
other minerals.
Bread volume is increased, crumb quality is improved, and
staling is delayed.Rye flour is very low in gluten proteins, and instead, starch
and pentosans make an important contribution to bread struc-
ture. The swelling and solubility of pentosans increase when
LAB fermentation lowers pH. Gelatinization of starch occurs at
about 55–58◦C. Considering that the flour amylase has a tem-
perature optimum around 50–52◦C, it is crucial that the amylase
is actually inactivated in the pH range that is obtained during
sourdough fermentation. When mixtures of wheat and rye flour
are used for bread making, a sourdough process is necessary if
the content of rye flour exceeds 20%.
Rye and wheat flour contain phytic acid that binds miner-
als, particularly iron, that then become nutritionally unavail-
able. However, these cereals also contain phytases with pH op-
tima around 5.0–5.5; thus, phytate degradation is very good
in fermented flour, where these phytate complexes are also
more soluble. Lactic acid bacteria also appear to have some
phytase activity.