C.W. Wrigley, Food Science Australia and Wheat CRC, North Ryde (Sydney), NSW 1670, Australia; F. Békés, CSIRO Plant Industry, Canberra, ACT 2600, Australia; W. Bushuk, Food Science, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
Gliadin and Glutenin: The Unique Balance of Wheat Quality
The simplicity with which gluten can be purified from flour by water-washing made it one of the first proteins to be isolated in reasonably pure form. This achievement was first reported by Beccari of Bologna, in 1728 (see Bailey 1941), but it was still many years before the chemical constitution of proteins was elucidated, and before the term “protein” was coined. This occurred in 1838 when Berzelius wrote to Mulder; see Hartley (1951). The term “gliadine” predated even this, being suggested by G. Taddei in 1819, based on the demonstration by Einhof, in 1805, that gluten could be separated into two fractions, based on the extractability of gliadin in aqueous ethanol. The insoluble residue was named “zymom” by Taddei, “plant albumin” by Berzelius in 1826; also “glutin” by de Saussure in 1833 and by Dumas and Cahours in 1843 (Osborne and Vorhees 1893; Wrigley 1993). By the end of the nineteenth century, the terms “gliadin” and “glutenin” were established to describe the two halves of gluten that were extractable and residual, respectively, using 70% aqueous ethanol (Osborne and Vorhees 1893). Significantly, the chemical distinction between these two fractions was demonstrated on the basis of their respective contents of proline and glutamic acid, and the degree of amidation (Osborne and Clapp 1906). Nevertheless, there was the erroneous assumption that each of these components was a pure homogeneous protein.
Researchers in America, Australia, England and France pursued the concept that variations in gluten quality (and thus in dough properties) could be explained by varying the balance between these two major components of gluten. Pursuit of this hypothesis led to the appearance of several conflicting reports of the ratio between gliadin and glutenin in the literature of the late nineteenth century, with values for this ratio ranging from 0.59 to 4.0 (see Tracey 1967). Reasons for these wild variations appear to relate mainly to differences in extraction procedures, but also to difficulties in obtaining reproducible results with extraction as the method of fractionation.
Nevertheless, the results appear to have shown initial promise, with a higher proportion of glutenin relating to dough strength in the words of Guthrie (1896): ‘Flours in which glutenin predominates yield strong, tough, elastic, non-adhesive glutens. Increased gliadin content produces a weak, sticky, and inelastic gluten.’ Today, we would agree with these conclusions, but further research led Guthrie (1912) to abandon this approach: “Further work on this subject has convinced me that the relationship is not as simple as I at first thought; nor is the separation and accurate determination of the two proteins quite satisfactory. This method has, therefore, been abandoned in this laboratory, and is not, I believe, any longer recognised. The question – what is the cause of [dough] strength – still remains to be solved.”
The subsequent century of research in cereal chemistry has revealed the great complexity of the gluten complex, the genetic control of the many component polypeptides, and the importance of the various bonds between the protein chains (see Chapters 2–7). Nevertheless, the concept of a critical balance between the complementary roles of the gliadin and glutenin components is still central to our understanding of gluten function. However, this balance is now more likely to be seen as being due to their distinct contributions to molecular-size distribution, which in turn explains the original distinction of their extractabilities into solution.