Shear stress relaxation properties of wheat flour doughs were analyzed with an equation derived from Letherisch's analogical model, i.e., a Kelvin body (G and eta(k)) in series with a dashpot eta. Assuming eta (k) is much less than eta, the relaxation curves may be described by considering that eta alone varies with the shear stress sigma according to a power law. The corresponding constant shear modulus G decreases when the shear strain increases. G does not depend strongly on flour strength, dough water content, mixing time, or temperature, except at temperatures beyond 45-50 C, where G strongly increases. The addition of urea or sodium sulfite does not appear to alter G significantly. It is inferred that hydrophobic interactions play a major role in dough elastic properties. Results published by Bloksma and Meppelink are showen to be consistent with an almost invariable value of G at a given shear strain and with decreasing values of G when the shear strain increases. The power-law model is compared with Peleg's equation and with the theory of cooperative flow proposed by Bohlin and Carlson. The stress relaxation kinetics derived from this theory is similar to the one predicted by the power-law model, but the fitting parameters do not have the same physical meaning. | |