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

596 Part 5: Fruits, Vegetables, and Cereals

Amylases The primary function of starch-hydrolyzing en-

zymes is to mobilize the storage polysaccharides to read-

ily metabolized carbohydrates when the grain germinates

(Hoseney 1998).

α-Amylase hydrolyzesα-1,4 glycosidic bonds at random in

the starch molecule chain but is unable to attack theα-1,6 link-

ages at the branching points on the amylopectin molecule. The

activity ofα-amylase causes a rapid reduction in size of the large

starch molecule, and the viscosity of a heated solution or slurry

of starch is greatly decreased. It is most active on gelatinized

starch, but granular starch is also slowly degraded.

β-Amylase splits off two glucose units (maltose) at a time

from the nonreducing end of the starch chain, thus providing a

large amount of fermentable carbohydrate.β-Amylase is also

called a saccharifying enzyme since its action causes a marked

increase in sweetness of the hydrated cereal. Neither the hydrol-

ysis of amylopectin nor of amylose is completed byβ-amylase,

since the enzyme is not able to move past the branching points.

The presence of bothα-andβ-amylases, however, leads to

a much more comprehensive hydrolysis, sinceα-amylase pro-

duces several new reducing ends in each starch molecule.

The level ofα-amylase is very low in intact grain but increases

markedly on germination, whereasβ-amylase levels in intact and

germinated grain are similar.

Flour containing too muchα-amylase absorbs less water and

therefore results in heavy bread. In addition, the dough is sticky

and hard to handle, and the texture of the loaf is usually faulty,

having large open holes and a sticky crumb texture. However,

some activity is required, and bakers may add amylase either

as an enzyme preparation or as wheat or barley malt in order

to slightly increase loaf volume and improve crumb texture.

The thermal stability of amylases from different sources dictates

their activity during the baking process. Microbial amylases with

greater thermal stability have been used in bread to decrease

firming (retrogradation) upon storage since these enzymes are

not fully denatured during baking.

Proteases Proteinases and peptidases are found in cereals, and

their primary function is to make small amino nitrogen com-

pounds available for the developing seed embryo during germi-

nation, when the levels of these enzymes also increase. However,

whether these enzymes have a role in bread baking is not certain.

Peptidases may furnish the yeast with soluble nitrogen during

fermentation, and a proteinase in wheat that is active at low pH

may be important in acidic fermentations such as sourdough

bread.

Lipases Lipases are present in all grains, but oats and pearl

millet have a relatively high activity of lipase compared with

wheat or barley (Linko et al. 1997). In flour of the former grain

types, hydrolytic rancidity of the grain lipids and added baking

fat may be a problem.

Lipids

Lipids are present in grains as a large number of different com-

pounds, and they vary from species to species and also within

each cereal grain. Most lipids are found within the germ. Wheat

flour contains about 2.5% lipids, of which about 1% are po-

lar lipids (tri- and diglycerides, free fatty acids, and sterol es-

ters) and 1.5% are nonpolar lipids (phospholipids and galactosyl

glycerides). During dough mixing, much of the lipid forms hy-

drophobic bonds to the gluten protein (Hoseney 1998).

BREAD

Many different types of bread are produced in the world. Bread

formulations and technologies differ both within and between

countries due to both traditional and technological factors in-

cluding: (1) which cereals are traditionally grown in a country

and their suitability for bread baking, (2) the status of bread in

the traditional diet, (3) changes in lifestyle and living standards,

(4) globalization of eating habits, and (5) economic possibilities

for investing in new types of bread-making equipment.

The basic production of most bread involves the addition of

water to wheat flour, yeast, and salt. Other cereal flours may

be blended into the mixture, and other optional ingredients in-

clude sugar, fat, malt flour, milk and milk products, emulsifiers,

and gluten (for further ingredients and their roles in bread, see

Table 31.1). The mixture is worked into an elastic dough that

is then leavened by the yeast to a soft and spongy dough that

retains its shape and porosity when baked. An exception to this

is the production of bread containing 20–100% rye flour, where

the application of sourdough and low pH are required.

Bread Formulation

The formulation of bread is determined by several factors. In

a simple bread, the baking properties of the flour are of vital

importance in determining the characteristics of the loaf using

a given technology. In addition, the bread obtained from using

poor bread flour or suboptimum technology may be improved

by using certain additives (Table 31.1).

The major methods used to prepare bread are summarized in

Figure 31.2. In the straight dough method, all the ingredients are

added together at the start of the process, which includes two

fermentation steps and then two proofing steps. In the sponge

and dough method, only part of the dry ingredients are added

to the water, and this soft dough undergoes a fermentation of

about 5 hours before the remainder of the ingredients are added

and the dough is kneaded to develop the structure. Although

these processes are time consuming, their advantages are that

they develop a good flavor in the bread and that the timing and

technology of the processes are less critical (Hoseney 1998).

Mechanical dough development processes, such as the Chorley-

wood bread process developed in the United Kingdom in the

1960s, radically cut down the total bread-making time. The fer-

mentation step is virtually eliminated, and dough formation is

achieved by intense mechanical mixing and by various additives

that hasten the process (Kent 1983). The resulting loaf has a high

volume and a thin crust but lacks flavor and aroma. The trend

is now away from this kind of process due to customer demand

for more flavorful bread and reduced use of additives.