A modified hypothesis for the structure and rheology of glutelins

Abstract

Cereal storage proteins, it is suggested, are more tolerant of genetic mutations than other proteins. Evolutionary tendencies may have been towards decreasing the cystine content and increasing the content of amino acids which are easily synthesised, especially those which are rich in nitrogen. The appearance of stress in molecular structure so that an intrachain SS bond is no longer able to close, thus creating interchain linkages, may have led to the origin of the glutelins.It is argued that a viscoelastic dough can be formed only if protein particles, which originate in the form of protein bodies, are able to form a continuous structure whether maintained by covalent bonds orland secondary forces. The constituent polypeptide chains of glutelins could be joined in a linear or a branched mode. A number of difficulties arise if the branched model is used to explain the properties of dough at the molecular level. In attempting to reconcile fact and theory, it is speculated that polypeptide chains of wheat, rye and barley glutelins form linear concatenations with two disulphide bonds connecting each chain to the next: continuity of structure in their doughs depends on entanglement and interaction by secondary forces. The glutelin molecules of oats and maize are unable to form a continuous network, hence the absence of viscoelasticity from doughs of these cereals. Two reasons are suggested: more probably the molecules are highly cross-linked in the branched mode and are thus incapable of significant entanglement; or if the molecules were cross-linked in the linear mode, poor solubility would ensure that protein-protein contacts in protein bodies were maintained, thus preventing dispersal of the protein and subsequent molecular entanglement. The glutelin molecules of sorghum appear to be too small to create a network by molecular entanglement. The importance of the disulphide interchange reaction is thought to lie in stress relaxation, but not in the creation of a dough structure. It is suggested that SS groups in viscoelastic doughs are sterically protected except when exposed by stress and consequently stress relaxation by SS interchange tends to occur only when it is needed.

Functional properties of proteins in foods: A survey

“…The physicochemical basis of dough formation and structure has been extensively reviewed by Ewart, 130 " 133 Buskuk, 78 Bietz et al 49 Holme, 218 Kasarda, 240 Mecham, 297 Krull and Wall, 25S and Wall. 458 The dough forming ability of wheat gluten has been ascribed to a combination of its gliadin-glutenin ratio, composition and amino acid sequence of the gluten proteins, critical juxtapositioning of specific amino acid residues, high glutamine level, conformation and intramolecular bonding, ability to engage in thioldisulfide interchange, and the large degree of hydrophobic association which occurs between gluten proteins in the hydrated state.…”

“…These have been reviewed in some detail by Krull and Wall 25S and Ewart. 130 " 133 The functional role(s) of chemical bonds in dough formation and behavior was summarized by Wehrli and Pomeranz. 468 The active participation of particular amino acids in specific reactions, i.e;, cysteine and cystine in thiol interchange, and the importance of amide groups of glutamine and asparagine and apolar amino acids in hydrophobic associations (associated with elastic properties), as well as the significance of the polar and hydroxy amino acids in hydration and hydrogen bonding, were discussed by Krull and Wall 2 s s and Wall and Beckwith.…”

Biaxial Extensional Viscosity of Sheeted Noodle Dough

“…The hydrated glutenin with a molecular weight above a certain value in wheat flour dough appears to determine its extensional properties (Singh and MacRitchie 2001). Ewart (1968Ewart ( , 1972Ewart ( , 1979 has proposed the structure of the glutenin in dough for dough development as linear (unbranched) glutenin molecules built up from subunits joined by disulfide linkage and continuity of the glutenin network depends on the secondary force (noncovalent cross-links). Ewart (1990) also pointed out that when the extensional stress is applied to the dough, the viscosity of the sheeted dough depends largely on cooperating intermolecular associations between protein molecules.…”

Correlating dough elastic recovery during sheeting with flour analyses and rheological properties

“…The 'Linear glutenin hypotheses' of Ewart (1968Ewart ( , 1972Ewart ( , 1978, proposed that glutenin contained molecules joined by disulfide bonds into linear polymers, with only a limited amount of branching. These molecules were responsible for elasticity.…”