Polysaccharides pseudoplasticity

The alternative large scale recovery method to precipitation is ultrafiltration. For concentration of viscous exopolysaccharides, ultrafiltration is only effective for pseudoplastic polymers (shearing reduces effective viscosity see section 7.7). Thus, pseudoplastic xanthan gum can be concentrated to a viscosity of around 30,000 centipoise by ultrafiltration, whereas other polysaccharides which are less pseudoplastic, are concentrated only to a fraction of this viscosity and have proportionally lower flux rates. Xanthan gum is routinely concentrated 5 to 10-fold by ultrafiltration. 

Many microbial polysaccharides show pseudoplastic flow, also known as shear thinning. When solutions of these polysaccharides are sheared, the molecules align in the shear field and the effective viscosity is reduced. This reduction of viscosity is not a consequence of degradation (unless the shear rate exceeds 105 s 1) since the viscosity recovers immediately when die shear rate is decreased. This combination of viscous and elastic behaviour, known as viscoelasticity, distinguishes microbial viscosifiers from solutions of other thickeners. Examples of microbial viscosifiers are ... 

At 1-atm pressure in the surroundings, polysaccharide deformation and flow are normally initiated either by gravity or an applied shear rate (y) solvent (water) only flows under temperature (T) and concentration (c,) gradients. When T)i is constant or independent of the rate of shear (y in s 1) or stress (t), the flow is Newtonian. Very dilute polysaccharide dispersions are characterized mostly by Newtonian flow. At moderate concentrations, ti, may decrease (shear-thinning synonymous with pseudoplastic) or increase (shear-thickening synonymous with dilatant) nonlinearly with y for these dispersions, is replaced with (the apparent viscosity). Low DP and uniform distribution of substituents are conducive to tH high DP and nonuniform distribution are conducive to. A high T a is believed to elicit the human oral sensation of thickness. ... 

A polysaccharide with the commercial name Diutan is highly pseudoplastic and proposed for use as a rheology modifier in concretes, grouts, and mortars. Diutan has the same backbone structure as gellan, but has a disaccharide side unit [28]. 

All the polymers that have been suggested as mobility control agents are pseudoplastic, i.e., they are shear-thinning. The extent of pseudo-plasticity is different for polyacrylamides, polysaccharides, and hydroxy ethyl cellulose. Viscosities of these polymers are plotted against shear rate in Figure 12. One model that describes the shear-thinning behavior observed is the Ost-wald-de Waele relationship ... 

Polyacrylamides are extremely non-Newtonian particularly at low salt conditions, while polysaccharides and hydroxy ethyl cellulose show less pseudoplasticity. The non-Newtonian behavior of all of these solutions can be described by a power law model. Many variables such as salt concentration and polymer concentration influence the extent of pseudoplasticity. 

Although pectins are not employed as thickening agents, pectin solutions exhibit the non-Newtonian, pseudoplastic behavior characteristic of most polysaccharides. As with solubility, the viscosity of a pectin solution is related to the molecular weight, DE, and concentration of the preparation and the pH and presence... 

Tara gum is the galactomannan obtained from the seed of Tara shrub, Caesalpinia spinosa, mostly found in northern Africa and Peru of South America. The main polysaccharide chain is formed of (1 4)-llnked 3-D-mannopyranosyl units, with single a-D-galactopyranosyl units attached with average three main chain units via (1 6]-linkages. Molecular weight of tara gum may fall between 300,000 and 1,000,000. It is 70% soluble in cold water and hydrated 100% at temperature above 80°C showing pseudoplastic behavior in solution. 

Definition High m.w. hetero polysaccharide gum produced by a pure-culture fermentation of a carbohydrate with Xanthomonas campestris-, contains D-glucose, D-mannose, and D-glucuronic acid and is prepared as the sodium, potassium, or calcium sait Properties Wh. to cream-colored powd., si. organic odor, tasteless sol. in hot or cold water producing vise, highly-pseudoplastic sol ns. insol. in oils, most org. soivs. vise, 600 cps min. unaffected by high or low pH... 

Microbial polysaccharide prod, by Alcali-genes ATCC 31555 by aerobic fermentation. Solns. V. viscous and pseudoplastic used as an additive in oil and natural gas recovery. Also in specialised cement and concrete applications. 

Special Comments Excellent stability in the presence of acids and salts and at low pH. Excellent pseudoplastic rheology. Anionic polysaccharide Supplier Calgon, TIC Gums, Rhone-Poulenc, Vanderbilt... 

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