Food Biochemistry and Food Processing (2 edition)

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.