2009_Book_FoodChemistry

15.4 Baked Products 715
Fig. 15.29. Extensograms of a normal (I) and weak
dough (II). For quality assessment the following parameters
are determined: resistance to extension, height
of the curve at its peak (B–C) given in extensogram
units (EU); extensibility, abscissa length between A–C
in mm; extension area (A–B–C–A, cm 2 ) is related to
energy input required to reach the maximum resistance;
extensogram number (overall dough quality) is the ratio
of extension resistance to extensibility
ble 15.41), form, crumb structure and elasticity,
and the taste of the baked product are evaluated.
A baking test is performed with 1000 g flour for
each product.
When the effects of expensive and not readily
available flour constituents and/or additives are
tested or a new cultivar is assessed, of which
only several hundred kernels are available, a “micro
baking test” is used, with 10 g flour for each
baked product (cf. Fig. 15.35). If even less material
is available, 2 g are sufficient. The sample is
then kneaded in a mixograph and baked in a capsule.
15.4.1.2 Rye Flour
The Falling Number test (cf. 15.4.1.1.1) and an
amylographic assay are the most important tests
to assess the baking properties of rye flour. These
tests depend to a great extent on gelatinization
properties of starches and the presence of α-amylase.
The higher the α-amylase activity, the lower
the Falling Number.
An amylograph is a rotational torsion viscometer.
It measures the viscosity change of an aqueous
suspension of flour as a function of temperature.
The recorded curve, called an amylogram
(Fig. 15.30), shows that with increasing temperature
there is an initial small fall followed by
a steep rise in viscosity to a maximum value. The
steep rise is due to intensive starch gelatinization.
Fig. 15.30. Amylograms of two rye flours (according to
H. Stephan, 1976)
Gelatinization Gelatinization α-Amylase
maximum (peak) temperature
Flour I 720 AU 67 ◦ C high
Flour II 520 AU 73.5 ◦ C low
AU: amylogram units.
The viscosity value and temperature at maximum
viscosity (i. e., the temperature reflecting the end
of gelatinization) are then read.
In rye flour with balanced baking properties,
an optimal relationship should exist between
α-amylase activity and starch quality. The extent
of enzymatic starch degradation influences the
stabilty of the gas-cell membranes which are
formed by gas released in the dough and which
consolidate during baking into an elastic crumb
structure. These membranes contain pentosans,
proteins and intact starch granules in addition
to gelatinized and partially hydrolyzed starch.
High α-amylase activity in rye or a large difference
between the temperatures needed for
enzyme inactivation (close to 75 ◦ C) and those
required for termination of starch gelatinization
will produce poor bread since too much starch
will be degraded during breadmaking. The
gas-cell membranes are liquefied to a great
extent; so the gas can escape. This gas will
then be trapped in a hollow space below the
bread crust (I in Fig. 15.30). Low α-amylase
activity, especially in conjunction with low starch
gelatinization, leads to a firm and brittle crumb
structure.