1.4.8. Hot Phase of the
Bread-Making Process
Whereas the focus of attention in the cold phase of bread making is on
the swelling substances of the flour, particularly the proteins, it is starch
and its pasting behaviour that dominate tests to show the behaviour of doughs
in the hot phase, i.e. the actual baking. Starch only starts to swell
intensively at elevated temperatures; it binds water and gelatinizes, losing
its crystalline structure. But the gelatinizing and already gelatinized starch is
exposed to enzymatic breakdown through the activity of α-amylase. In the quick
breakdown process of the enzymes the starch loses its ability to bind and hold
water; this results in bread with an inelastic, soft, wet and very often unchewable crumb
which makes the product inedible. On the other hand, optimum enzyme activity is
necessary for optimum results in the baked product. So it is essential to
determine the activity of the α-amylase in a flour or any other ground product
in order to achieve the desired result. If the enzyme activity is too high,
activity-inhibiting agents are added or suitable measures taken; if it is too
low it can be optimized by adding enzyme preparations.
1.4.9. Amylograph and Falling
Number
The Amylograph is a rotational viscometer with a measuring system
consisting of a round vessel, in which the flour-and-water slurry is heated under
controlled conditions, and a sensor to record the changes in viscosity during
the measuring time. The pins of the measuring device cause turbulences in the slurry;
these are necessary to prevent sedimentation of the starch, but they make it
impossible to calculate the viscosity precisely in absolute physical units. The
viscosity of the slurry is therefore stated as torque in Brabender or Amylogram
units. The measurements that can be read off include the temperature and the
viscosity at maximum gelatinization; these provide more differentiated information
than a viscosity value alone.
The Amylogram, a viscosity curve showing the gelatinization of the
starch in the flourand- water slurry, reflects the changes in water binding
capacity of the swelling and pasting starch and the enzymatically and mechanically
decomposed starch gel. In the way the standard Amylograph method is used it
offers a suitable means of describing the pasting properties of rye flour
slurries. Rye starch gelatinizes at lower temperatures than wheat starch, especially
if it is sproutdamaged as a result of poor environmental conditions. Then the
task is to find out whether the rye is suitable for baking by determining the
temperature and viscosity at the pasting peak of the Amylogram. This applies to
wheat too, but only if it is assumed to be sprout-damaged. The starch of the flour
slurry from a wheat lot that is not sprout-damaged normally gelatinizes later and
at higher temperatures, towards the end of the temperature range of an Amylogram
or the measurements from an Amylograph. However, a very high viscosity at the
pasting peak yields little information in relation to the amount of time that
has to be invested. For such wheat flours the quick and simple determination of
the Falling Number is sufficient.
Falling Number determination is a simple and quick method in which the
viscosity of a flourand- water slurry is stated as the number of seconds a
pestle takes to penetrate the starch gel. Measurement of the viscosity in a
Falling Number tube does not start until 60 seconds after stirring, when the
viscous properties of the gelatinized starch slurry have already been changed
by the α-amylase present in the flour and the mechanical force of stirring. The
Falling Number is therefore a one-point measurement of the residual viscosity
of the starch gel, not a continuous measurement like that of the Amylograph.
The Falling Number method can be used for both wheat and rye, although the limits
of the measurements differ. This results not least from the different water
binding capacities of the swelling substances of wheat and rye. In rye flours too, the Falling Number can be used with sufficient
accuracy for indirect determination of α-amylase activity and the suitability
of a flour for baking.
There is not a close enough relationship between the Amylograph and
Falling Number methods to permit a direct comparison of the measurements.
Firstly, the ratio of flour to water (concentration of the slurry) and the time/temperature
gradient of the heating differ; secondly, the Amylograph method is a continuous
measurement over a period of up to 45 minutes, whereas in the case of the Falling
Number the measurement of residual viscosity does not start until after 60
seconds of stirring. For these reasons it is not possible to allocate an Amylogram
value to a corresponding Falling Number. Nevertheless, a numerical relationship
between the two methods can be achieved by comparing a large number of
measurements and calculating a mathematical-statistical regression. But this relationship
only applies to the harvest of a single year and has to be reviewed or
recalculated for each new harvest.
By using nomograms in a double-logarithmic system it is possible to make
up optimized mixtures from rye flours with different Falling Number and
Amylogram data (Weipert, 1987c). However, the percentages of the enzyme-active
components with low data depend on the height of the data of the lowenzyme components;
as a rule these percentages tend to be low.
1.4.10. Rapid Visco Analyzer
Whatever the advantages for which the Brabender Amylograph (and the
Viscograph, intended chiefly for the starch industry) is appreciated, it has
disadvantages too. Firstly it requires a very large sample for testing, and secondly
the recording of a pasting curve is time-consuming. Several attempts have recently
been made to develop and market a "micro-Amylograph". The development
of the Rapid Visco Analyzer (Newport Scientific, Sydney, Australia) was and is
still the biggest and most successful step towards simplifying and broadening
the investigation and description of the pasting properties of starch and products
containing starch (Weipert, 1998a). Because of its versatility it is in general
use in the field of food analysis (milk, soups, sauces, salad dressings etc.).
The Rapid Visco Analyzer is also a rotational viscometer that unites
the advantage of requiring only a small sample (2-4 g) with the possibility of
setting to any desired temperature gradient. The temperature profile of a test
can be adjusted in such a way that the test starts at any chosen temperature,
which rises slowly or rapidly and remains constant for a time before cooling down in the
desired steps. In practice this means that the test can be performed at a
constantly high temperature in the manner of the Falling Number test, with slow
or faster heating in the manner of an Amylograph or with controlled heating and
cooling as in a Viscograph. A close correlation has been found between the
measurements from these two methods and the results of the Falling Number and
Amylograph tests; this correlation enables the Rapid Visco Analyzer with the already
standardized methods ICC "stirring number" (similar to the Falling Number)
and "rapid pasting" (similar to the Amylograms and Viscograms) to be
adopted by cereal laboratories and used "seamlessly" (Weipert,
1998a).
Interpretation of Brabender viscograms and the RVA rapid pasting
curves:
 |
Fig. 60: Temperature ramps (upper set of curves) and pasting curves
(lower set of curves) of a wheat flour recorded in the Rapid Visco Analyzer at
different temperature gradients:
SN: constant 95 °C;
ST1: 12 °C/min;
ST2: 6 °C/min;
ST3: 3 °C/min;
ST4: 1.5 °C/min
|
The possibility of programming and determining the duration of a test
for the pasting behaviour of starch in a starch/water or flour/water slurry
oneself, according to needs, and thus monitoring the behaviour of the starch in
the relevant process is very much appreciated by users of the Rapid Visco
Analyzer. A quick method can doubtless yield a reliable result as a guide, but
the process of making and baking bread takes rather longer. In order to
describe the pasting behaviour of wheat and rye starches in flours for baking
and to assess it in the manner of an Amylogram, the measuring time in which the
starch swells and gelatinizes must be taken into account. The starch grains have
time to absorb and bind the water, to swell, to be "annealed", and
finally to gelatinize completely or incompletely, depending on the amount of
free water available. One and the same flour/water slurry shows different viscosities
and temperatures at the pasting peak according to the length of the measuring time.
The shorter the measuring time, the higher the viscosity; it is therefore
highest in the "stirring number" method, similar to the Falling Number
(Fig. 60). To save time by shortening the measuring period may mean a loss of information
(Weipert, 1998a), especially if the quality data from the time-consuming Amylograph
method are still used on the grounds of experience. Nevertheless, a "quick
test" of this kind may serve as an initial guide. Farther-reaching
decisions require the introduction of new critical values for each of the
suggested temperature profiles in the course of measurement. The Rapid Visco Analyzer
can measure both fast and slowly.
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