Rheology and The Problem with Durum Gluten

Baking professionals in Italy commonly refer to the strength of flour by its W index, describing this as its “force”. Italian Millers provide much data in their flour specifications often including rheological properties recorded by the Chopin-Alveograph. Testing performed by the Alveograph involves forcing air into a piece of dough causing it to expand like a balloon until it bursts, at which point the test is complete. The data is recorded on a graph as a line measured in millimetres. The pressure required and therefore resistance of the dough is measured as P. The final size of the dough is measured as L, this being its extensibility. The area under the line is represented by W which indicates overall strength.

The index of W.

Up to W170 (weak):
Biscuits, waffles, and tender baked sweets. Béchamel and thickened sauces.

From W180 to W260 (average):
French bread, bread rolls, pizza, pasta.

From W280 to W350 (strong):
Classic bread, pizza, pasta, baba, brioche.

Above W350 (very strong):
Made with selected types of wheat, that are used to reinforce weaker flours. Ideal for highly enriched doughs subjected to long leavening, often referred to as “Manitoba”.

Extensibility, elasticity and quality.

Gluten consists of proteins gliadin and glutenin, the latter providing the main structure which supports the dough. Flour richer in gluten is attributed to strong flour, but the quantity of gluten does not determine everything, its characteristics are also important. Strength is more dependent on the compositional properties of gluten. Therefore, two flours may have the same amount of gluten, though one may be stronger and the other weaker.

Gliadin in contact with the water forms a fluid sticky mass, while glutenin absorbing water, forms a compact mass, elastic and resistant. When these proteins are hydrated, their properties interact to provide the viscoelastic properties of gluten. In the case of strong flour, it must contain a majority of glutenin as these proteins provide the resistance within the elastic property. Flour that is high in gluten, but consisting mainly of gliadin cannot be very strong however.

Resistance extensibility ratio (P/L) and Durum

Instead of referring to them individually the balance between extensibility and resistance can be expressed with the P/L ratio, the optimal being between 0.5 – 0.6. A P/L ratio higher than 0.7 characterises flour that is resistant, while values lower than 0.4 are weak and extensible. Soft wheat flour is naturally more extensible while hard wheat flours are naturally more tenacious, this is especially true with durum, the hardest wheat of all, where a typical P/L ratio can be above 1 and even exceeding 2.

Durum wheat doughs are distinguished by a high resistance to deformation and consequently limited extensibility. To be judged sustainable for bread making the semolina must have a protein content >12%, a good farinographic stability and alveograph P/L index value below 1. However it should be noted that the bread-making process preferred in the case of durum wheat requires the use of sourdough. A case in point is the pane di Altamura. The proteolytic activities of semolina and/or of the lactobacilli may produce considerable changes in the rheological properties of the gluten network, reducing the natural excessive elasticity of dough made from durum wheat.

Numerous factors brought about by fermentation and inclusion of other ingredients will affect the properties of gluten, even water quality. Excessively hard water, rich in minerals will make gluten more tenacious reflecting an increase in the P/L ratio. Soft water will encourage the opposite effect and make dough softer and more extensible which would display a lower P/L ratio.