624 Part VI: Fermented Foods
decreases, ethanol and water evaporate, and the gases
increase in volume. This results in a marked increase
in the volume of the dough, called “oven spring.”
As the temperature in the loaf continues to rise,
several other changes take place. The yeast is increas-
ingly inhibited, and its enzymes are inactivated at
about 65°C. The amylases in the dough are active
until about 65–70°C is reached and a rapidincrease
in the amount of soluble carbohydrate takes place.
Gelatinization of starch occurs at 55–65°C, and the
water that this requires is taken from the gluten pro-
tein network, which then becomes more rigid, vis-
cous, and elastic until a temperature is reached 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 gelati-
nized starch. These changes first occur near the crust
and gradually move into the crumb as the heat is
transferred inwards.
Towards the end of baking, the temperature at the
crust is much higher than 100°C. The crust becomes
brown, and aroma compounds, predominantly alde-
hydes and ketones, are formed, mainly from Mail-
lard reactions. The formation of flavor compounds is
two staged. First, compounds are formed from the
fermentation itself, and then during baking some of
these compounds may react with each other or with
the bread components to form other flavor com-
pounds. Some other flavor compounds formed dur-
ing the fermentation may be lost due to the high
temperature, but those that remain gradually diffuse
into the crumb after cooling. On further storage, the
levels of flavor compounds decrease due to vola-
tilization. Over 200 different flavor and aroma com-
pounds have been identified in bread (Stear 1990).
STALING
The 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 retrogra-
dation 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 grad-
ually reverses, and the starch molecules form in-
termolecular bonds and crystallize, expelling water
molecules and resulting in the firming of crumb tex-
ture. Starch retrogradation is a time- and temperature-
dependent process and proceeds fastest at low tem-
peratures, 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., hydrocol-
loids and proteins). The staling rate can also be
slowed by the incorporation of surfactants, shorten-
ing, or heat-stable -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 amy-
lose 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 man-
ner 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 produc-
tion will be started. This mixture will regularly de-
velop fermentation with lactic acid bacteria (LAB)
and yeasts. This forms the basis of sourdough pro-
duction, 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. In Africa, 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 prototype 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 sev-
eral places must have shared this experience inde-
pendently. It may be surmised that the experience
with gruels and porridge preceded the idea of mak-
ing bread.
Common bread fermented only with yeast ap-
peared 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