Quantitative methods are those tests in which the amount of a
constituent is determined. The total amount of protein has a very important influence
on the baking properties of wheat flour. Fig. 40 shows that the protein content
increases with the degree of extraction, but there is no increase in the gluten
content that enhances the properties of the dough and also the baking properties.
The increase results from the protein contained in the aleuron layer, which is
especially protein-rich.
This effect indicates that it is not just the protein content in
absolute terms that matters. In wheat flour there are several protein fractions
that differ in respect of their solubility. It is the glutenforming protein
fractions of the endosperm that determine the baking properties of wheat flours.
The amount of these present can only be established by testing for the wet
gluten content. The proteins from the outer layers and the germ have quite different properties. Although they are
included in the determination of the protein content, they are soluble in water
or brine and therefore have no influence on the properties of a wheat dough.
1. Determining Total Protein
The total protein content of the wheat is determined by the Kjeldahl
method. The organic constituents are oxidized in the presence of a catalyst. The
ammonia formed after another step is distilled and titrated. The amount of nitrogen
ascertained by titration is multiplied by a factor specified for each food. For
wheat and rye and the products for human nutrition made from these cereals
the factor for converting the nitrogen values found from the protein is 5.7 (See also Number 2 and Number 2).
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| Fig. 43: Device for automatically determining the protein content by
the Dumas combustion method (source: Elementar Analysensysteme GmbH) |
A more modern way of determining the nitrogen content is the practically
automatic "Dumas combustion method" (Fig. 43) in which the sample is
incinerated under oxygen, the resulting nitric oxide converted into nitrogen
and the heat conductivity of helium and nitrogen subsequently measured against
pure helium. This procedure is faster and less complicated than the Kjeldahl
method usual in the past.
The method for determining the crude protein content of cereals and
cereal products for human nutrition is specified in ICC Standard 105/2. ICC
Recommendation 202 favours determining the protein content by nearinfrared spectroscopy
(NIR); see also the section on determining the moisture content.
2. Determining the Wet Gluten
Content
The wet gluten content is a measure of the amount of swollen gluten in
the wheat flour that can be determined by forming a paste from a flour sample
and washing it out. The principle of the method is that a dough is made with a
buffered solution of common salt (for adjustment to a pH of 5.95) and then washed
out to remove the starch and the water-soluble constituents of the gluten. The
gluten is then dried and subsequently weighed. The amount of gluten thus
determined is the wet gluten content in percent by weight. The methods for
washing the sample differ greatly. The gluten content can be determined under
running water or in a gluten washer. But reproducibility (repeatability of the
results) can only be achieved with these methods if the test is carried out by
very experienced persons. ICC Standard 106/2 uses a gluten washer with an eccentric plate and a gluten press. Mechanical
determination of the wet gluten content of wheat flour (ICC Standard 137) is
carried out with the Glutomatic equipment.
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| Fig. 44: Perten gluten washer, with the Gluten Index centrifuge
(source: Perten Instruments AB) |
Evaluation is carried out by weighing the pieces of gluten after
centrifuging. The result has to be converted to correspond to a flour moisture
content of 14%. If a special sieve is used for certrifugation it is possible to
determine the Gluten Index. The centrifugal force presses some of the gluten
through the sieve. The higher the proportion of gluten that has not passed
through the sieve, the higher is the index and the better the gluten.
3. Determining the
Sedimentation Value
The swelling properties of wheat flours are tested by determining the
sedimentation value (sedimentation means the sinking of a solid in a liquid).
Since gluten is insoluble in water and comes from the floury kernel of the grain,
the method measures the volume of the swollen gluten proteins. On the one hand
this volume of sediment (settled solids in a liquid) shows the amount of
gluten, and on the other it reflects the swelling properties of the gluten. Since
the gluten-forming proteins of the wheat differ in their ability to bind water,
the sedimentation volume is not solely a function of the quantity of protein.
The sedimentation value combines qualitative and quantitative elements of the
measurement of the wheat gluten.
To carry out the test, small quantities of wheat flour (3.2 g, with
14% moisture content) are placed in a sedimentation cylinder with 50 mL of
bromophenol blue solution (sedimentation solution I) and shaken first by hand
for 5 s and then with the mechanical shaker for 5 minutes. After this, 25 mL of
sedimentation solution II (lactic acid and isopropyl alcohol with distilled water)
are added and the constituents mixed intensively for a further five minutes in
the shaking device. The sedimentation cylinder is left to stand without moving
on a level tabletop for another five minutes, then the amount of sediment is
read off exactly at the end of the time.
The sedimentation value is stated in millilitres. It has proved possible
to predict the resting time of the dough, its gas retention capacity and the
volume yield of the baked products by the sedimentation value. The more
millilitres of sediment that can be read off the measuring cylinder, the more
suitable is the flour for making bakery products that require strongprotein flours.
Determination of the sedimentation value is carried out according to
ICC Standard 116. The result may lie between 8 mL for an extremely low protein
content and 78 mL for a flour with very strong gluten.
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