Studies on mechanical factors affecting dough development *

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.

Section: Improver Actionsupporting

confidence: 81%

A modified hypothesis for the structure and rheology of glutelins

Ewart¹

1972

“…54,59 Wilson et al 54 suggested that 'it may be that higher work inputs are required for higher mixing speeds (shorter mixing times) due to time dependent hydration effects.' They noted that Kilborn and Tipples 29 used a slow-speed premix of the ingredients, thereby allowing time for hydration, and found that the work input required for peak dough development did not change with mixing speed, which was in contrast with the findings of Frazier et al, 61 who did not use a slow-speed premix in their work.…”

Section: Introductionmentioning

confidence: 99%

Dough aeration and rheology: Part 1. Effects of mixing speed and headspace pressure on mechanical development of bread dough

Chin¹,

Campbell²

2005

A laboratory-scale Tweedy-type mechanical dough development mixer was modified to monitor work inputs of dough during mixing through a computerised data acquisition system. Recorded torque showed a curve that increased to a peak about midway through mixing. Using this system, the effects of mixing speed and pressure on the development of doughs made from strong and weak flours mixed to a work input of 40 kJ kg −1 were studied. Mixing was characterized in terms of peak torque and torque at the end of mixing (end torque), and in terms of the number of blade revolutions at these two reference points. The peak torque and the end torque both increased with increasing mixing speed. The peak torque increased with increasing mixer headspace pressure, but the end torque did not. The number of blade revolutions to both the point of peak torque and to the end of mixing decreased with increasing mixing speed for both flours, implying dough development at higher speeds was more efficient. Increasing headspace pressure reduced the number of revolutions to the peak, but not to the end of mixing. These results provide evidence that achieving dough development is not independent of mixing speed in Tweedytype mixers, and that dough aeration, as affected by mixer headspace pressure, affects dough rheology and hence development in the mixer.

A hypothesis for the structure and rheology of glutenin

Ewart¹

1968

It is suggested that because of the rapid lowering of the viscosity of its solutions by reducing agents the major, important fraction of glutenin consists of linear molecules made up of polypeptide chains linked to one another difunctionally by S.S bonds. When dough is stretched the natural tendency of the polypeptide chains to return to a contracted state of low free energy accounts for the elasticity. The interchain S.S bonds are essential for elasticity because inter-chain adhesion between individual polypeptide chains will not overcome the stronger intra-chain forces unless reinforced by the S.S bond. If extension exceeds the elastic limit, viscous flow occurs because steric hindrance and molecular slip will prevent a return to the original conformation. Disulphide interchange is believed to play an important part in stress relaxation. Mechanical scission of S.S probably occurs when molecules at their elastic limit are subjected to too much stress; this may explain the work maximum in the Chorleywood Bread Process. Explanations are advanced for mechanical and activated dough development and evidence in favour of the hypothesis is discussed. IntroductionGlutenin molecules show a wide variation in size, but their weight-average molecular weight is high, exceeding 1 million according to Jones et aZ.I.2 They are assemblages of polypeptide chains of mol. wt. 20,000, which are joined to one another by S.S bonds from evidence provided by Nielsen et aZ.3 It was concluded by these last workers that the elastic and cohesive properties of glutenin depended on the integrity of the S.S crosslinked structure. The polypeptide chains, referred to throughout this paper as 'chains', though of similar mol. wt., do not appear to be identical because after reductive scission of S.S bonds they show a range of mobilities