1. Determining Acidity in
Flours
The acidity of a flour is an important indicator of its freshness. The
fats and phosphatides (lecithin) are broken down enzymatically during storage
of the flour. This breakdown is accelerated by a high water content, high
temperatures and a high degree of extraction. The lipases and phosphatases
naturally present in the cereal cause an increase in the percentage of free
fatty acids and phosphate.
To a certain extent this breakdown is desirable as a maturing of the
flour, but once the tolerances are exceeded the flours become inedible. By
determining the acidity of the flour, taking the degree of extraction into
account, it is possible to monitor the progress of maturing and the possible
start of deterioration. Since the changes in the flour during storage consist
mainly in an increase in the free fatty acids, the sample is suspended in
ethanol (67% alcohol) before filtration. The filtrate is then titrated withsodium hydroxide solution to a pH of 8.5. The amount of 0.1 N (mol/L)
of sodium hydroxide solution required (multiplied by two) is a measure of the
degree of acidity. Titration is sometimes carried out using the indicator dye
phenolphthalein, but the point at which the colour changes from yellow to pink
is difficult to determine precisely. It has therefore become established
practice to use a pH meter.
2. Determining the Falling
Number as a Measure of α-Amylase Activity
The gelatinization properties of the starch in wheat or rye dough depend
greatly on the α-amylase activity. Since rye starch gelatinizes at low
temperatures, it is more easily broken down by this enzyme. It is therefore
especially important to establish the gelatinization properties of rye flours.
Determination of the Falling Number and the Amylogram have established
themselves as standard methods. The Amylogram is used mainly for testing rye,
whereas the Falling Number serves as a measure for both cereals.
To determine the Falling Number, 7 g of flour are heated with 25 mL of
distilled water in a water bath for one minute to approximately 95 °C (modified
Falling Number: approx. 80 - 95 °C). The viscosity of the starch gel thus
obtained is then determined by measuring the time the stirring rod takes to
sink through the gel to the bottom of the measuring cylinder. The Falling
Number is the sum of the stirring and sinking time. It is stated in seconds.
The minimum Falling Number is 60 s. When the flour is weighed, its moisture
content is assumed to be 15%. If the water content differs from this value, the
amount weighed in must be corrected by calculation or according to a special
table.
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| Fig. 41: Equipment for determining the Falling Number (source:
Mühlenchemie GmbH & Co. KG) |
Nowadays the test is performed with a Falling Number device (Fig. 41)
that carries out the stirring and measuring procedure automatically. If the
enzymatic activity is high, the starch is broken down very rapidly during
gelatinization. The stirring rod falls through the relatively liquid paste in a
short time. The Falling Number is low. If the activity of the amylases is low
it takes much longer for the rod to cover the distance and trigger the signal
for the end of the process. This equipment can also be used to adjust the amylase activity of flour mixtures or determine the amount of malt to
be added. With the aid of a diagram it is easy to read off the mixing ratios.
Conversely, it is possible to make a precise adjustment to the desired Falling
Number. But blending with other flours is preferable, since enzyme-active malt
flours contain the whole spectrum of cereal enzymes as well as the starch-degrading
enzymes. In other words, they are able to break down the other constituents to
an undesirable extent. Determination of the Falling Number is an internationally recognized method that has long been used as ICC
Standard No. 107.
3. Determining the
Gelatinization Properties of Starch with the Amylograph
The Amylograph (Fig. 42) enables continuous measurement of the changes
of viscosity in a flour-and-water suspension during heating. For heating, a
temperature gradient is selected that corresponds to the rise in temperature
during the baking process; this means that the Amylograph can be used to make
important predictions about the baking properties of the flour. Besides the
gelatinization characteristics of the starch it also shows the effect of the amylases.
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| Fig. 42: Amylograph (source: Brabender OHG) |
To obtain an Amylogram, flour (14% moisture) and water are transferred
to the stirring bowl, ensuring that there are no lumps. The bowl contains rods
that project upwards with the suspension between them. When the bowl has been
placed in the appropriate opening in the device, the sensor is inserted into it
from the top. The sensor is fitted with similar rods, projecting vertically
downwards. During heating, the liquid has to flow between the rods projecting
into it from above and below, since the bowl is rotated continuously. The shear
forces that occur between the rods as gelatinization progresses are recorded on
a diagram as changes in viscosity; this results in the typical curve of an
Amylogram.
When gelatinization starts, the suspension reaches the optimum
temperature for α-amylase activity. The rise in viscosity to be seen on the
evaluation diagram is the product of the increase caused by gelatinization and
liquefaction by the amylases. Besides the maximum viscosity, the curve also
shows the temperature at the start of gelatinization and at its maximum. A
viscosity of at least 200 AU (Amylogram units) alone is not enough for rye to
be classified as bread rye; a temperature of at least 63 °C in the suspension
must also have been reached at this time.
Testing of wheat and rye flours with the Amylograph conforms to ICC
Standard 126/1. It is especially important for assessing rye flour products.
This method is also suitable for investigating the effects of enzyme-active
ingredients.
A further measure of the breakdown of starch is the maltose content of
the flour. Being a degradation product of starch, maltose – also called malt
sugar – may indicate increased sprouting of the grain. The method is based on
the enzyme activity of the flours and measures the fermentable sugar formed
within an hour at 27 °C. The amount of maltose measured is influenced chiefly
by ß-amylase activity, but the starch damaged during milling may also
contribute to a high "maltose number". If a value of 2.3% for rye or
1.8% for wheat is exceeded, it must be assumed that there is too much activity
of starch-degrading enzymes or too much damage to the starch.
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