In the process of researching material for a presentation on Ancient Grains I have become increasingly interested in protein quality, especially as it relates to why modern wheat (T. aestivum) is so good for bread making. The term ‘protein quantity’ is fairly easy to grasp and, in fact, our whole wheat marketing system here in the US (and throughout most of the world) is based on it: producers are paid based on the protein content of their wheat. But ‘protein quality’ is a bit more elusive for me. Of course one must make the standard disclaimer that ‘quality’ is a relative term: relative to what you are making, but using bread as the common denominator allows one to look at little more closely at the term ‘protein quality’



The ‘artisan’ bread movement in the US has been saying, perhaps since the 70’s, that hard red winter wheat (HRW) has protein of a better quality than hard red spring (HRS). And the notion of protein quality is a central theme in Raymond Calvel’s ‘The Taste of Bread’. On the one hand, perhaps tit is obvious, as artisan breads tend to require less automation, and with HRW, on average, being lower in protein than HRS one might just make the argument that it’s really the protein quantity and not so much the quality - end of story. But HRS and HRW protein ranges often overlap, and in years when the HRS crop is low in protein and HRW high (which happened recently - 2005 I believe), HRW replaced much HRS in the wheat mixes at mills in the US. So when you have an HRW flour of similar protein content to HRS how can you account for the difference in the doughs and finished breads they produce?




Well, it seems that ‘protein quality’ is actually quantifiable and is an expression of a wheat’s gliadin to glutinen ratio. What allows wheat flour, when mixed with water and yeast to develop into a CO2 trapping material that can produce a leavened loaf of bread is the presence of gluten proteins. Gluten is composed of the subunits: gliadin and glutenin. A dough is often referred to as a ‘visco-elastic’ material meaning that it has properties of extensibility(or flow) and elasticity. It turns out that the glutenins are responsible for the elastic nature of the dough the and the gliadins the extensible characteristics.

The quote below is from a document called “Wheat for Bread and Other Foods” which is found on the Food and Agriculture Organization of the United Nations website.

Grain protein content in wheat varies between 8 and 17 percent, depending on genetic make-up and on external factors associated with the crop. A unique property of wheat flour is that its insoluble protein forms, when in contact with water, a viscoelastic protein mass known as gluten. Gluten, comprising roughly 78 to 85 percent of total wheat endosperm protein, is a very large complex composed mainly of polymeric (multiple polypeptide chains linked by disulphide bonds) and monomeric (single chain polypeptides) proteins known as glutenins and gliadins, respectively (MacRitchie, 1994). Glutenins confer elasticity, while gliadins confer mainly viscous flow and extensibility to the gluten complex. Thus, gluten is responsible for most of the viscoelastic properties of wheat flour doughs and is the main factor dictating the use of a wheat variety in bread and pasta making. Gluten viscoelasticity, for end-use purposes, is commonly known as flour or dough strength.




Variations in grain protein content may significantly influence the dough strength properties of a wheat variety. Quantity alone, however, cannot always explain quality differences among wheat cultivars. Therefore, protein quality, in terms of the polymeric/monomeric protein ratio and the molecular size of the protein polymer (determined by the presence of specific glutenin subunits), is also important (see Weegels et al., 1996 for a review).




Wheat flour contains roughly the same amounts of glutenins and gliadins, and the unbalance of the glutenin/gliadin ratio may change its viscoelastic properties. The glutenin fraction is, however, the major protein factor responsible for variations in dough strength among wheat varieties. Fu and Sapirstein (1996) recently confirmed this; they observed that most of the variation in dough strength parameters was explained by the amounts of soluble and insoluble glutenin.

As is said, the gliadin-glutenin ratio of wheat flour is 1:1. It is interesting to note that in some of the predecessors of modern wheat (einkorn, emmer, spelt) the ratio is NOT 1:1 and the bread making characteristics of these grains is less desirable. For einkorn it is 3:1 (ie 3 glutinen for every 1 gliadin), emmer is 6.6 to 1 and, perhaps more familiar is spelt at 3 to 1 (Source: Specialty Grains for Food and Feed, 2005. AACC Buy it HERE!). These numbers indicate a less balanced ratio of extensibility to elasticity. For these grains there is more gliadin (extensibility) than glutinen (elasticity). Anyone who has worked with spelt flour will know that the dough is very sticky and extensible (slack).

As these grain are perhaps less familiar to you than wheat you may be surprised that most of these grains, on average, have higher protein contents than wheat. But, as we just saw, they exhibit anything but what we would call a strong dough.