Physical Chemistry of Foods
Thixotropy. In most systems the latter effect is not directly reversible: at constant strain rateZatends to decrease with time, ...
purely elastic material. The material is instantaneously deformed upon applying the stress and it instantaneously returns to its ...
FIGURE5.8 Various kinds of behavior of a material under stress. (a) Stress applied as a function of time. (b) Resulting strain a ...
higher, than those obtained for Newtonian liquids. This implies that it is difficult to reach a high elongation rate during flow ...
FIGURE5.9 Illustration of dynamic (oscillatory) measurement of rheological properties. In (a) the applied shear strainðgÞis show ...
total strain. Theloss tangenttandis a measure of the nature of the material. For very small tand, the material is solidlike (‘‘e ...
time scale of observation what we observe or, more precisely, on the Deborah number: De: relaxation time observation time ð 5 : ...
If De 5 1 we call the material a liquid; and at De 4 1 the material appears solid. It is only if De is of order unity that we ob ...
Yield Stress. Curve (c) is characteristic for many viscoelastic materials. At small stress, the material behaves elastically, as ...
An order of magnitude calculation is as follows. The kinetic energy is of order mv^2 (Section 4.3.1), wheremis the mass of the e ...
5.2.1 Brownian Motion When observing a dilute dispersion of small particles (order of 1mm) under the microscope, one observes—as ...
i.e., the mean of the distances covered by a large number of identical molecules—the linear displacement xof a molecule is zero ...
5.2.2 Mass Diffusion So far, we have tentatively assumed that all molecules and particles are randomly distributed throughout th ...
depends on the boundary conditions, i.e., the geometrical constraints. For the fairly simple case of diffusion through an infini ...
cube mentioned in a weak polysaccharide gel, which does not allow convection, while hardly slowing down diffusion (see Section 5 ...
Some Complications. Above it is implicitly assumed that the diffusion coefficient in the Stokes–Einstein relation (5.16) equals ...
Answer In first approximation, Eq. (5.17) would be directly applicable. From Table 5.3 we derive that for sucroseD¼ 4 : 7? 10 ^ ...
5.2.3 Heat Transfer If a temperature gradient exists in a liquid—and this is also valid for a solid—heat motion of the molecules ...
diffusion to the center of the loaf of about 4 hours, far too long. The viscosity of the dough is far too high to allow signific ...
During several processing operations, like drying, extraction, and soaking, diffusion is the rate determining step. When a food ...
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