Starch shear thinning

Figures 8.37 and 8.38 [9] present velocity and temperature fields across the thickness, respectively, for various values of Br, and forn = 1 and n = 0.6. Griffith calculated the screw characteristic curves for Newtonian and non-Newtonian shear thinning fluids using various power law indices. Figure 8.39 presents these results and compares them to experiments performed with a carboxyl vinyl polymer (n = 0.2) and corn starch (n = 1).

The concentration of lactic acid in the steep has been shown to affect both the yield of starch178 and the quality of the resulting starch.179 Starch yields increased with increasing lactic acid concentration up to —0.5%, and decreased when more than —2% lactic acid was used. Shandera and lackson179 tested starch quality when steeped at 57°C with either 0.2% or 1.5% lactic acid and 0.05% or 0.3% sulfur dioxide. The level of sulfur dioxide had a small effect on starch quality, but the level of lactic acid had a pronounced reduction effect on peak viscosity, shear thinning viscosity, set-back viscosity, pasting viscosity and starch water solubility. 

Figure 1-4 Time-Dependent Shear-Thinning (thixotropic) Behavior of a Tapioca Starch Dispersion Heated at 67°C for 5 min. Data of Tattiyakul (1997).

For the 4% tapioca starch dispersion heated at 70°C for 5 min, values of r]a decreased with y, but those of rj increased with co after an initial decrease (Figure 4-32). For the data shown in Figure 4-32, the critical frequency (a>c) at which the transition occurred was about 100 rad s . In comparison to yc values for shear-thinning to shear-thickening flow behavior of 2.6% tapioca and cowpea starch (Okechukwu and Rao, 1996) dispersions, the value of cuc = 100 rad s is lower. It is possible that the rja versus y data of the 4% dispersion heated at 70°C for 5 min... 

Chamberlain, E. K., Rao, M. A., and Cohen, C. 1998. Shear thinning and antithixotropic behavior of a heated cross-linked waxy maize starch dispersion. Int. J. Food Properties 2 63-77 errata, 2 195-196. 

Unlike the cooked corn flour in the first part of this experiment, the starch polymer remains trapped in the starch granules and the fluid is a concentrated suspension rather than a polymer solution. When the suspension is sheared gently a layer of water between the starch granules lubricates their flow past each other. However, fast deformation forces the water out from between the granules so there is much greater friction between them and they cannot flow. Quicksand behaves in a similar manner. Ice cream mixes, the matrix and ice cream itself are all shear-thinning if these were shear-thickening like the concentrated custard powder suspension, ice cream would be impossible to process, and would become solid every time you tried to chew it ... 

The hydrolysis of starches by amylases Is most conveniently considered In terms of sugar production (for saccharification) or viscosity reduction (for liquefaction). Starch pastes are characteristically pseudoplastic (shear thinning), and thus satisfy, over an Intermediate range of shear rates, a power law relationship between shear stress, T, and shear rate, y, of the form (2.4) ... 

Gum Arabic acts as protective colloid and excellent emulsifier and the molecular aggregation can cause both shear thinning and time-dependent thickening behavior at low shear" Gum Arabic has the ability to create a strong protective film around oil droplets" and is compatible with the most other plant hydrocolloids, proteins, carbohydrates, and modified starches. The viscosity of a solution of a mixture of gum arabic and gum tragacanth tends to be lower than that of either constituent solution. 

Pseudoplastic (shear thinning) adhesives can be prepared by adding polyaciylates such as sodium or ammonium poly aery late. Starch, cel-... 

In addition to investigations on elasticity, also the dynamics of droplets consisting of shear thinning or yield stress fluids has received some attention. Both for shear and extensional flow, it has been found that shear thinning of the droplet fluid reduces the deformation as compared to that of a Newtonian droplet with the same viscosity at the applied shear rate [84,85]. Desse et al. [86] showed that the dependence of the critical Cfl-number on viscosity ratio for a starch suspension droplet with a yield stress deviates substantially from that of Newtonian droplets. In conclusion, the precise relations between the rheological constitutive parameters of droplet and matrix fluid and the droplet dynamics for materials with a complex rheology are far from fully revealed. 

This is one of the most important functional tests for starches because aft analyzing the pasting curve, temp uture at start of gelatinization, peak viscosity, shear thinning, and setback viscosity (retrogradation) can be determined. These properties greatly influence processing characteristics and quality of end products. 

---