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:-•*    ELASTIC PROPERTi_Es 1

Ot

V = V»/^W(°) fZ\^H R"⁺V

^Ai li \^2L/ '«+!

He applies this eąuation with some HRfl I data obtained by Herschel and Bulkley _0n^{S to} \ benzene sols; but it is doubtful if the find much further application. In addition p has developed ąuite a number of other compfe^ ⁶¹ eąuations.    ^x

It begins to be elear that the failure of the BW principle in some systems is due to the elastic ^ perties of the materiał in ąuestion. When sys£?' which are morę solid still are considered, the elasj* properties preponderate. A good materiał f_G^C experiment is flour dough, sińce the liąuid and soljj properties in this are about evenly balanced. ^ mechanics of flour dough is discussed at length i_{n a} series of papers by Schofield and Scott Blair, and _a similar method of experimentation was later used by Issoglio, and also by Bohn and Bailey. When _a piece of dough is extended, held under tension and then released, part of the deformation is recovered and part retained. Schofield and Scott Blair propose a number of models which, while not claiming to give a literał picture of the intemal structure of flour dough, do explain many of its properties. For example, consider two springs terminating in straight rods, the rods being held together by a clamp. If these springs are extended, there will be a certain amount of slip at the clamp, and the rods will slide past one another. The friction at the clamp corre-sponds to viscosity, and the slipping to viscous flow. It is elear that, if the clamp is tightly fastened, most of the extension produced on stretching will be recovered on release ; and also that the softer.the

the lower their modulus), the less will sP^r^^gSc£esś at the clamp for any given extension, ^Jhence the less the slip. Materials which show a S percentage of recovery Hę said to bave agood Spring/’ and this is seen to be associated with high liscosity and Iow modulus, i.e., sińce t_r — j]\n, with Sgh relaxation time. Halton and Scott Blair show that this is true, in practice, of flour doughs. Unfor-tunately, ł_r is not a constant independent of stress, sińce neither the viscosity nor the shear modulus is constant: comparisons must therefore always be madę under standard stress-strain conditions. Moreover, sińce both viscosity and shear modulus vary with such factors as the moisture-coirtent, the age of the dough, etc., it is important to find some function of them that remains fairly constant with these factors. For flour dough it is found that ij/n² is comparatively independent of the moisture-content or the age, provided that 1 a^d w are determined always under the same conditions; and tj/«² certainly gives a good criterion of ‘ spring," which is probably the most important factor in determining the baking ąuality of a dough.

COOH), because of its structure, can link on to two main protein chains, thus

The idea that protein chains really do behave like coiled springs has been highly developed by Astbury, Shorter and others. These authors explain the elastic properties of hairs, silk, wool, etc. in terms of coiled, spring-like molecules, whose length and width, coiled and uncoiled, they are able to measure from X-rays. An interestmg link with the work on flour doueh is to be found in Astbury’s comments