BLBS102-c31 BLBS102-Simpson March 21, 2012 14:0 Trim: 276mm X 219mm Printer Name: Yet to Come
31 Bakery and Cereal Products 595
crude and dietary fiber present in the grain. The pericarp, testa,
and aleurone also contain over half of the total mineral matter.
Whole meal flour is derived, by definition, 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 deter-
mines not only the whiteness of the flour, but also its nutritive
value and baking properties. Decreasing extraction rate results
in a marked, and nutritionally important, decrease in fiber, fat,
vitamins, and minerals (Kent 1983). The protein content of dif-
ferent cereal grains varies between 7% and 20%, governed not
only by cereal genus, species, or variety (i.e., genetically regu-
lated), but also by plant growth conditions such as temperature,
availability of water during plant growth, 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 concentration 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 2–3μm to about 30μm according to grain species. Barley,
rye, and wheat have starch granules with a bimodal size distri-
bution, with large lenticular and small spherical grains. Almost
100% of the starch granule is composed of the polysaccharides
amylose and amylopectin, and the relative proportions of these
polymers vary not only according to species of cereal, but also
according to variety within a species. 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 byα-1,4 linkages and only
occasionalα-1,6 linkages. Amylopectin is a much larger (up to
106 glucose units) and more highly branched molecule with ap-
proximately 4% ofα-1,6 linkages that cause branching in the
α-1,4 glucosidic chain.
During milling of grains, some of the starch granules 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. Therefore, the degree
of damage to the starch grains dictates the functionality of the
flour in various baking processes. When water is added to starch
grains, they absorb water, and soluble starch leaks out of dam-
aged 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 dis-
rupted and water forms hydration layers around the separated
molecules. The gelatinization temperature of starch from differ-
ent cereals varies from 55◦Cto78◦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 sufficient water to
totally hydrate the starch in most bakery foods, the heat causes
irreversible changes to the starch. On cooling a heated cereal
product, some starch molecules reassociate, causing firming of
the product.
Nonstarch polysaccharides, the pentosans, which are prin-
cipally arabinoxylans, comprise approximately 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 fermentation.
Protein
Cereal proteins contribute to the nutritional value of the diet,
and therefore the composition and amount of protein present are
inherently important. However, the protein content in cereals
also has several important aspects in fermented bakery prod-
ucts. The amount and type of some of the proteins is important
for the formation of an elastic dough and for its gas-retaining
properties. Other proteins in cereals are enzymes with specific
functions, not only for the developing germ, but also for var-
ious changes that take place from the processing of flour to
bakery products.
Gluten Proteins
The unique storage proteins of wheat are also the functional
proteins in baking. The gliadins and glutenins, collectively called
gluten proteins, make up about 80% of the total protein in the
grain and are mostly found in the endosperm. These proteins
have very limited solubility in water or salt solutions, unlike
albumins and globulins (Fig. 31.1). A good bread flour (known as
“strong” flour) must contain adequate amounts of gluten proteins
to give the desired dough characteristics, and extra gluten may
be added to the bread formulation.
Enzyme Proteins
The albumin and globulin proteins are concentrated in the bran,
germ, and aleurone.
Figure 31.1.Wheat proteins.