616 Part VI: Fermented Foods
procedures and microbiological content and activity
provides us with the vast range of fermented cereal
products that are prepared and consumed in the
world today.
CEREAL COMPOSITION
Carbohydrates are quantitatively the most important
constituents of cereal grains, contributing 77–87%
of the total dry matter. In wheat, the carbohydrate in
the endosperm is mainly starch, whereas the peri-
carp, testa, and aleurone contain most of the crude
and dietary fiber present in the grain. The pericarp,
testa, and aleurone also contain over half the total
mineral matter. Whole meal flour is derived, by def-
inition, from the whole grain and contains all its
nutrients. When wheat is milled into flour, the yield
of flour from the grain (extraction rate) reflects the
extent to which the bran and the germ are removed
and thereby determines not only the whiteness of the
flour, but also its nutritive value and baking proper-
ties. Decreasing extraction rate results in a marked,
and nutritionally important, decrease in fiber, fat, vi-
tamins, and minerals (Kent 1983). The protein con-
tent of different cereal grains varies between 7 and
20%, governed not only by cereal genus, species, or
variety (i.e., genetically regulated), but also by plant
growth conditions such as temperature, availability
during plant growth of water and also of nitrogen
and other minerals in the soil. There is an uneven
distribution of different protein types in the different
parts of the grain, so that although the protein con-
centration is not radically affected by milling, the
proteins present in different milling fractions will
vary.
STARCH
The starch in cereals is contained in granules that
vary in size, from 2–3 m to about 30 m according
to grain species. Barley, rye, and wheat have starch
granules with a bimodal size distribution, with large
lenticular and small spherical grains. Almost 100%
of the starch granule is composed of the polysaccha-
rides amylose and amylopectin, and the relative pro-
portions of these polymers vary according not only
to species of cereal, but also to variety within a spe-
cies. However, both wheat and maize contain about
28% amylose, but in wheat the ratio of amylose to
amylopectin does not vary (Fenema 1996; Hoseney
1998). The amylose molecule is essentially linear,
with up to 5000 glucose molecules polymerized by
-1,4 linkages and only occasional -1,6 linkages.
Amylopectin is a much larger (up to 10^6 glucose
units) and more highly branched molecule with
approximately 4% of -1,6 linkages which cause
branching in the -1,4 glucosidic chain.
During milling of grains, some of the starch gran-
ules become damaged, particularly in hard wheat.
The starch exposed in these broken granules is more
susceptible to attack by amylases and also absorbs
water much more readily. The degree of damage to
the starch grains therefore dictates the functionality
of the flour in various baking processes. When water
is added to starch grains, they absorb water, and sol-
uble starch leaks out of damaged granules. Heating
of this mixture results in an increase in viscosity and
a pasting of the starch, which on further heating leads
to gelatinization as the ordered crystalline structure
is disrupted and water forms hydration layers
around the separated molecules. The gelatinization
temperature of starch from different cereals varies
from 55 to 78°C, partly due to the ratio of amylose
to amylopectin. The gelatinization of wheat starts at
about 60°C. Despite the fact that there is not suffi-
cient water to totally hydrate the starch in most bak-
ery foods, the heat causes irreversible changes to the
starch. On cooling a heated cereal product, some starch
molecules reassociate, causing a firming of the
product.
Nonstarch polysaccharides, the pentosans, which
are principally arabinoxylans, comprise approxi-
mately 2–3% of the weight of flour. They are derived
from the grain cell walls and are polymers that may
contain both pentoses and hexoses. They are able to
absorb many times their own weight in water and
contribute in baking by increasing the viscosity of
the aqueous phase, but they may also compete with
the gluten proteins for available water. Cereals also
contain small amounts (1–3%) of mono-, di- and
oligosaccharides, and these are important as an
energy source for yeast at the start of dough fer-
mentation.PROTEINCereal proteins contribute to the nutritional value of
the diet, and therefore the composition and amount
of protein present are inherently important. How-
ever, the protein content in cereals also has several