Food Biochemistry and Food Processing (2 edition)

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666 Part 5: Fruits, Vegetables, and Cereals

2004, Meuser et al. 1994, Seibel and Brummer 1991). As starter, ̈

a commercial culture containing lactic acid bacteria or a portion

of previous sourdough, equally containing lactic acid bacteria,

is used (Seibel and Brummer 1991). In homofermentative lac- ̈

tic acid fermentation, the lactic acid bacteria ferment glucose

mainly into lactic acid; in heterofermentative lactic acid fermen-

tation, considerable levels of acetic acid are formed in addition

to lactic acid. The formation of lactic acid and acetic acid during

lactic acid fermentation can and must be controlled, as the ratio

of lactic acid to acetic acid is quite important for optimal bread

quality. Sourdoughs with a high lactic acid content are used

mainly for rye and mixed rye breads, while sourdoughs with

a high acetic acid content are ideally suited for mixed wheat

bread (Seibel and Br ̈ummer 1991). One-, two-, and three-stage

sourdough processes are used (Meuser et al. 1994, Seibel and

Brummer 1991). These processes require different fermentation ̈

times, dough temperatures, and water to flour ratios, and the

resulting doughs have different lactic acid to acetic acid ratios

(Seibel and Brummer 1991). Depending on the proportion of rye ̈

flour in the sourdough, various degrees of acidity are required

(Seibel and Brummer 1991). ̈

There are different reasons for using sourdough. Acid con-

ditions have a positive influence on the swelling of rye flour

constituents and control the enzyme activity in the dough,

thereby preventing early staling (Meuser et al. 1994, Seibel and

Brummer 1991). Rye flours with high enzyme activity require ̈

a higher degree of acidification than flours with less enzyme

activity (Meuser et al. 1994). Bread sensory properties, such as

taste and aroma, can be improved by optimal use of sourdough

(Heini ̈o et al. 2003a, Meuser et al. 1994, Seibel and Brummer ̈

1991). It contributes to aroma in the bread due to the production

of acids, alcohols, and other volatile compounds (Heini ̈oetal.

2003a, Poutanen 1997). Sourdough bread has good crumb char-

acteristics (Seibel and Brummer 1991) and nutritional proper- ̈

ties (Kariluoto et al. 2004, Meuser et al. 1994, Poutanen 1997).

Acidification of rye dough also protects against spoilage by mold

growth, thereby improving product shelf life (Meuser et al. 1994,

Poutanen 1997, Seibel and Brummer 1991). When sourdough is ̈

used in wheat bread making, its main function is to improve sen-

sory properties and to prolong shelf life (Br ̈ummer and Lorenz

1991).

Mixing and Kneading Rye flour, yeast, salt, and water are op-

timally mixed and kneaded with sourdough, lactic acid, or lactic

and acetic acid to form a viscoelastic dough. The level of water

is that yielding the required dough consistency. Rye doughs have

to be mixed slowly because of their high arabinoxylan content,

which leads to tough doughs when the energy input is exces-

sive. The optimal mixing time depends on the composition and

enzymic activities of the rye flour (Seibel and Weipert 2001a).

Important dough quality characteristics are dough yield, that

is, the amount of optimally developed dough obtained from

100 g flour, and dough structure.

Fermentation Mixing and kneading of the dough is followed

by a floor time. Thereafter, the dough is divided into pieces,

which are molded by hand into the desired shape. The doughs

are then proofed to their best potential. Fermentation takes place

at 30–36◦C and 70–85% relative humidity. Fermentation time

depends on the composition of the flour, the activity of the flour

enzymes, the dough formulation, and the fermentation temper-

atures.

During fermentation, carbon dioxide is produced, and the gas

is retained by the dough, resulting in a volume increase of the

dough. Although the rate of gas production in rye flour dough

is high, the gas-retaining capacity is low, resulting in dense,

compact loaves (He and Hoseney 1991).

Baking After fermentation, the dough is placed in an oven and

transformed into bread. Bread quality depends on baking tem-

perature, uniformity of temperature in the oven, and baking time.

Optimum baking temperature and time depend on the weight of

the dough pieces and the shape of the loaves, which determine

the rate of heat transfer. In rye bread making, at the beginning

of the baking stage, steam is often used in the oven to develop

good crust characteristics (Seibel and Weipert 2001a).

Important bread quality characteristics are specific volume,

crumb structure, crumb elasticity, crumb firmness during stor-

age; and sensory properties such as taste, flavor, and odor.

Impact of Rye Constituents on Dough and Bread

Quality Characteristics

Different rye varieties show highly different bread-making qual-

ity as influenced by harvest year and genotype (Hansen et al.

2004, Nilsson et al. 1997b, Rattunde et al. 1994). This can be at-

tributed to significant differences in the content and structure of

the major rye constituents, which are influenced by both factors

(Hansen et al. 2003).

Starch Starch gelatinization andα-amylase activity are very

important parameters for the bread-making quality of rye flour.

Both properties can, for example, be measured by the falling

number method or the amylograph test. The falling number is

inversely related toα-amylase activity. An important parameter

in the amylograph test is amylograph peak viscosity, which is

correlated positively with starch content and negatively withα-

amylase activity. It should be noted, however, that arabinoxylans

can influence falling number and amylograph characteristics of

rye flours. At similarα-amylase activity levels, higher falling

numbers (Weipert 1993a, 1993b, 1994) and higher amylograph

peak viscosities (Nilsson et al. 1997b) can be measured due to

higher arabinoxylan contents. The arabinoxylans increase the

viscosity, but also protect the starch granules against enzymic

breakdown by hindering and retarding the activity ofα-amylase

(Weipert 1993b, 1994).

For rye bread-making suitability, a minimum starch gela-

tinization temperature of 63◦C and, under specified experimen-

tal conditions, a minimum amylograph peak viscosity of 200

amylograph units (AU) are required. Rye shows optimal bread-

making quality at a starch gelatinization temperature of 65–69◦C

and an amylograph peak viscosity of 400–600 AU (Weipert

1993a). Maximum bread-making quality is associated with in-

termediate values of falling number (Rattunde et al. 1994). Rye