Methods for Testing the Quality of Wheat Gluten

As we already mentioned in connection with the sedimentation value, the quality of the wheat gluten is extremely important. Since it is a genetic attribute, it is given great attention in the breeding of new varieties. The quality of the gluten is the basis for assigning the wheat varieties to quality groups. For investigating the dough properties dependent on the gluten it has become usual to employ methods that show water absorption and mixing stability and also record the extensibility of the dough. The changes to the dough during fermentation and baking can also be detected with special instruments. Suitable devices for this purpose are the Maturograph and the Oven Rise Recorder.

1. Determining Water Absorption and the Mixing Behaviour of Wheat Doughs with the Farinograph

In the Farinograph (Fig. 45) a dough is prepared under standardized conditions from 50 or 300 g of flour, depending on the size of the mixer. While the dough is being made up, its resistance to the mixing paddles is measured and recorded continuously. In order to achieve uniform dough properties it is first necessary to measure the water absorption of the dough. To do this, water is added to the flour, which has a moisture content of 14%, from a burette until the dough reaches a consistency of 500 FU¹⁴. This amount of water is a measure of the water absorption of the flour, since a consistency of 500 FU corresponds to the average firmness of a dough. It is often necessary to make several attempts to determine the amount of water needed, as it is rarely possible to add the liquid to the mixer so quickly the first time that no dough softening occurs.

Fig. 45: Farinograph - E (source: Brabender OHG)

Once the amount of water has been determined, another dough is made from the same flour by adding all the necessary water at once. This dough is then mixed for 12 min. On the graph paper or the VDU¹⁵ there appears a mixing curve from which it is possible to read off the development time, the stability and the softening of the dough.

These Farinograph data can be used for preparing the dough. The water absorption – largely dependent on the quality of the gluten – indicates the dough yield; the doughdevelopment time permits conclusions on the speed of swelling; the fall in consistency (degree of softening) shows whether the flour is strong (slight fall) or weak. The Farinogram curves are characteristic of the gluten properties of a flour.

Farinogram curves with a broad band (bandwidth) and only a slight fall over the mixing time indicate elastic doughs that do not become weaker even when exposed to intensive mechanical stress. These flours are most suitable for bread rolls and toast slices. In flours with very strong gluten the consistency may even be found to increase during mixing. Subsequent swelling of the gluten causes the dough to become firmer. Narrow curves with very obvious dough softening result from weak flours (flours with weak gluten), which should preferably be used for making biscuits. Testing of wheat flours with the Farinograph conforms to ICC Standard No. 115/1. Since there are other methods of evaluating the curves (e.g. according to Brabender), the method used should be stated. Simple adaptation of the equipment and connection to a personal computer with the appropriate software makes it possible to evaluate and store the measurements electronically. With the addition of a printer, the software can produce a complete report on the results if required. The water absorption for certain standard baking tests is also determined in the Farinograph. For the baking test for white pan bread the measurement used should be 350 FU, for the Rapid Mix Test and the wholemeal wheat baking test it should be 500 FU. The Farinograph can also be used for standardizing rye doughs. The dough consistency for the sour dough baking test with rye flours should be 300 FU. The dough for measuring stretching properties with the Extensograph is also prepared in the Farinograph.

2. Determining the Stretching Properties of Wheat Doughs with the Extensograph

The Extensograph (Fig. 46) has proved to be a useful instrument for determining the stretching properties of wheat flour doughs. To do this a dough is made up of flour, water and salt under standard conditions in the Farinograph and rolled out in the rounder and shaping unit. After 45, 90 and 135 min the piece of dough held by clips is pulled by the stretching hook until it ruptures.

 Fig. 46: Extensograph (source: Brabender OHG)

The force exerted during stretching and the length of the extension are recorded on graph paper. Both resistance to extension, measured as the force required for stretching, and extensibility (stretching length to rupture) are important variables for assessing wheat flour doughs. The curves of an Extensogram give a clear indication of the results to be expected when the flour tested is baked. If the resistance to extension is low, i.e. the curve is rather flat, the dough has weak characteristics and the baked volume will be small. If the curve of the Extensogram rises steeply and is quite pointed at the top, the dough may be assumed to have short characteristics; such doughs are also said to be "bucky". In this case it is difficult to influence the structure of the dough; the gas formed by the yeast is not sufficient to achieve the necessary leavening. The best Extensograms for making bread and rolls are those with a well-balanced curve. The following curve (Fig. 47) shows an example of a flour that is well suited for the mechanical production of bread.

 Fig. 47: Extensogram with a "balanced" curve (source: Brabender OHG)

The Extensograph offers a simple method of testing the effect of additives on the properties of the gluten. For example, the amounts of ascorbic acid to be added to improve wheat flour should first be determined in the Extensograph. Too much ascorbic acid results in Extensogram curves characterized by high resistance to extension and rapid rupture of the dough. All substances acting on the gluten (cysteine, cystine, proteinases etc.) can be tested in this way for their effects on the dough made from a flour.

Evaluation of the curve drawn on the graph paper by the plotting device yields a number of measurements. First, the resistance of the dough to extension and its extensibility should be established. Then the area below the curve can be determined as an indicator of fermentation tolerance. The area is stated in cm² and referred to as energy. ICC Standard 114 does not provide either for information on the energy or for calculation of the Ratio Number, but this data should always be used for rating flours. The Ratio Number is calculated by dividing resistance by extensibility. The different measuring units are disregarded. High Ratio Numbers indicate a gluten with short dough properties and baked products with a low volume. Flours with Ratio Numbers below 2 are not suitable for making bread rolls.

3. Determining the Extensibility of Wheat Doughs with the Alveograph

The Alveograph (Fig. 48) is used chiefly in the Mediterranean countries, parts of South America and the former French colonies in Africa for testing the extensibility of dough. The principle is based on an extension test in which a disc of dough in a holder is blown up into a bubble (alveolus). The aim is to investigate the stretching properties of the dough up to the point where the bubble bursts. The process of extension and the volume achieved are recorded as a curve from which it is possible to read off the baking properties of the wheat flour. Testing of flour with the Alveograph is described in ICC Standard No. 121.

 Fig. 48: Alveograph and Consistograph (source: Mühlenchemie GmbH, Ahrensburg)

4. Mixograph for Determining the Water Absorption and Mixing Times of Wheat Doughs

In the USA the Mixograph (Fig. 49) is often used for testing the baking properties of wheat flours. In the newly developed computerized Mixograph it is possible to determine the water absorption of flours with as little as 10 g of flour.

Fig. 49 Mixograph with mechanical plotting of the measurements (source: Oregon State University )

The Mixograph curve also reveals the mixing tolerance (MT) of the dough. The dough development time and a fall in the viscosity of the dough can be read off the curve with certainty. As with the Farinograph, the extent of the "tail" of the curve indicates the quality of the flour.

5. Method for Testing the Aggregation Behaviour of Wheat Gluten for the Production of Wafer Batters

In the gluten aggregation test (Fig. 50) a slurry of wheat flour is tested for its suitability for making flat wafers. The instrument measures the electric current consumed by the mixer. For the test a thin wheat flour/water suspension is beaten at high speed. The gluten thus formed increases the shear forces, and this in turn alters the power consumption of the mixer. The aggregation time, recognizable by the time of greatest power consumption, and the aggregation area described by the curve are used in the evaluation, as are the maximum, mean power consumption and the rise in temperature.

 Fig. 50: Equipment for performing the gluten aggregation test (source: Brabender OHG)

Aggregation times up to 80 s indicate soft gluten structures that go hand in hand with high water absorption and average baking potential. Samples with a higher protein content and corresponding gluten structure formation have aggregation times of about 100 - 250 s. In this case very good baking properties can generally be expected. The flour quality is less suitable if the aggregation times are longer than 300 s.

Note :

¹⁴ FU = Farinograph Units; also referred to as Brabender Units (BU)

¹⁵ VDU = Visual display unit

¹⁶ EU = Extensograph Units; also referred to as Brabender Units (BU)