Xanthans

In packed beds of particles possessing small pores, dilute aqueous solutions of hydroly2ed polyacrylamide will sometimes exhibit dilatant behavior iastead of the usual shear thinning behavior seen ia simple shear or Couette flow. In elongational flow, such as flow through porous sandstone, flow resistance can iacrease with flow rate due to iacreases ia elongational viscosity and normal stress differences. The iacrease ia normal stress differences with shear rate is typical of isotropic polymer solutions. Normal stress differences of anisotropic polymers, such as xanthan ia water, are shear rate iadependent (25,26). 

Superbase mix of rice maltodextrin, starch xanthan, and whey protein Excel... 

Xanthan, although a gum, is derived from the pure culture fermentation of an organism, TCanthomonas campestris. The organism is filtered from the growth medium and the gum recovered by alcohoHc precipitation, followed by drying. It is composed primarily of D-glucose and D-mannose units. 

Xanthan Gum. Xanthan gum [11138-66-2] is produced by industrial fermentation of a carbohydrate under aerobic conditions by culturing the... 

Concentrations above 0.3% form a gel with borate which is reversible upon the subsequent addition of mannitol (a sequestrant for borate) or of acid. Usefiil combinations are formed with carrageenan (63) and xanthan gum (64) and agar. In many appHcations, it is used in combination with these gums at considerable cost savings. 

Xanthan Gum. As a result of a project to transform agriculturally derived products into industrially usefiil products by microbial action, the Northern Regional Research Laboratories of the USDA showed that the bacterium TCanthomonas campestris - noduces a polysaccharide with industrially usefiil properties (77). Extensive research was carried out on this interesting polysaccharide in several industrial laboratories during the eady 1960s, culminating in commercial production in 1964. 

Properties. Xanthan gum is a cream-colored powder that dissolves in either hot or cold water to produce solutions with high viscosity at low concentration. These solutions exhibit pseudoplasticity, ie, the viscosity decreases as the shear rate increases. This decrease is instantaneous and reversible. Solutions, particularly in the presence of small amounts of electrolyte, have exceUent thermal stabiHty, and their viscosity is essentially constant over the range 0 to 80°C. They are not affected by changes in pH ranging from 2 to 10. 

Xanthan gum dissolves in acids and bases, and under certain conditions, the viscosity remains stable for several months. Xanthan gum has exceUent StabiHty and compatibUity with high concentrations of many salts, eg, 15% solutions of sodium chloride and 25% solutions of calcium chloride (79). 

Health nd Safety Factors. The toxicological and safety properties of xanthan gum have been extensively investigated (82). On the basis of these studies, the EDA issued a food additive order in 1969 that allowed the use of xanthan gum in food products without specific quantity limitations. 

Uses. The unique properties of xanthan gum make it suitable for many appHcations for the food, pharmaceutical, and agricultural industries (79). 

Welan has similar properties to xanthan gum except that it has increased viscosity at low shear rates and improved thermal stabiUty and compatibihty with calcium at alkaline pH (90). The increased thermal stabiUty has led to its use as a drilling mud viscosifter especially for high temperature weUs. The excellent compatibihty with calcium at high pH has resulted in its use in a variety of specialized cement and concrete appHcations. 

Other thickeners used include derivatives of ceUulose such as methylceUulose, hydroxypropylmethylceUulose, and ceUulose gum natural gums such as tragacanth and xanthan (see Cellulose ethers Gums) the carboxyvinyl polymers and the poly(vinyl alcohol)s. The magnesium aluminum siHcates, glycol stearates, and fatty alcohols in shampoos also can affect viscosity. 

Solutions of welan are very viscous and pseudoplastic, ie, shear results in a dramatic reduction in viscosity that immediately returns when shearing is stopped, even at low polymer concentrations (230). They maintain viscosity at elevated temperatures better than xanthan gum at 135°C the viscosity half-life of a 0.4% xanthan gum solution is essentially zero, whereas a welan gum solution has a viscosity half-life of 900 minutes (230). The addition of salt to welan solutions slightly reduces viscosity, but not significantly. It has excellent stabiUty and theological properties in seawater, brine, or 3% KCl solutions... 

Applications. The high heat tolerance and good salt compatibiUty of welan gum indicate its potential for use as an additive in several aspects of oil and natural gas recovery. Welan also has suspension properties superior to xanthan gum, which is desirable in oil-field drilling operations and hydraulic fracturing projects. It is compatible with ethylene glycol, and a welan—ethylene glycol composition that forms a viscous material useful in the formulation of insulating materials has been described (244). 

Xanthan gum [11138-66-2] is an anionic heteropolysaccharide produced by several species of bacteria in the genus Aanthomonas A. campestris NRRL B-1459 produces the biopolymer with the most desirable physical properties and is used for commercial production of xanthan gum (see Gums). This strain was identified in the 1950s as part of a program to develop microbial polysaccharides derived from fermentations utilizing com sugar (333,334). The primary... 

Structure of xanthan has been determined by chemical degradation and methylation analysis (335,336) it is composed of repeating units consisting of a main chain of D-glucopyranosyl residues with trisaccharide side chains made up of D-mannopyranosyl and D-glucopyranosyluronic acid residues. 

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