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Xanthan gum is a polysaccharide used as a food additive and rheology modifier ^[1]. It is produced by a process involving fermentation of glucose or sucrose by the Xanthomonas campestris bacterium.

[edit] Chemical structure

The backbone of the polysaccharide chain consists of two β-D-glucose units linked through the 1 and 4 positions. The side chain consists of two mannose and one glucuronic acid, so the chain consists of repeating modules of five sugar units. The side chain is linked to every other glucose of the backbone at the 3 position. About half of the terminal mannose units have a pyruvic acid group linked as a ketal to its 4 and 6 positions. The other mannose unit has an acetyl group at the 6 positions. Two of these chains may be aligned to form a double helix, giving a rather rigid rod configuration that accounts for its high efficiency as a viscosifier of water. The molecular weight of xanthan varies from about one million to 50 million depending upon how it is prepared.

[edit] Biosynthesis

Synthesis originates from glucose as substrate for synthesis of the sugar nucleotides precursors UDP-glucose, UDP-glucuronate, and GDP-mannose that are required for building the pentasaccharide repeat unit. This links the synthesis of xanthan to the central carbohydrate metabolism. The repeat units are built up at undecaprenylphosphate lipid carriers that are anchored in the cytoplasmic membrane. Specific glycosyltransferases sequentially transfer the sugar moieties of the nucleotide sugar xanthan precursors to the lipid carriers. Acetyl and pyruvyl residues are added as non-carbohydrate decorations. Mature repeat units are polymerized and exported in a way resembling the Wzy-dependent polysaccharide synthesis mechanism of Enterobacteriaceae. Products of the gum gene cluster drive synthesis, polymerization, and export of the repeat unit.^[2]

[edit] Preparation

The polysaccharide is prepared by inoculating a sterile aqueous solution of carbohydrate(s), a source of nitrogen, dipotassium phosphate, and some trace elements. The medium is well-aerated and stirred, and the polymer is produced extracellularly into the medium. The final concentration of xanthan produced will vary greatly depending on the method of production, strain of bacteria, and random variation. After fermentation that can vary in time from one to four days, the polymer is precipitated from the medium by the addition of isopropyl alcohol and dried and milled to give a powder that is readily soluble in water or brine.

[edit] History

It was discovered by an extensive research effort by Allene Rosalind Jeanes and her research team at the United States Department of Agriculture, which involved the screening of a large number of biopolymers for their potential uses. It was brought into commercial production by the Kelco Company under the trade name Kelzan in the early 1960s. (Whistler p. 486) It was approved for use in foods after extensive animal testing for toxicity in 1968. It is accepted as a safe food additive in the USA, Canada and Europe, with E number E415.

[edit] Uses

One of the most remarkable properties of xanthan gum is its capability of producing a large increase in the viscosity of a liquid by adding a very small quantity of gum, on the order of one percent. In most foods, it is used at 0.5% and can be used in lower concentrations. The viscosity of xanthan gum solutions decreases with higher shear rates; this is called pseudoplasticity. This means that a product subjected to shear, whether from mixing, shaking or even chewing, will thin out, but once the shear forces are removed, the food will thicken back up. A practical use would be in salad dressing: The xanthan gum makes it thick enough at rest in the bottle to keep the mixture fairly homogeneous, but the shear forces generated by shaking and pouring thins it so it can be easily poured. When it exits the bottle, the shear forces are removed and it thickens back up so it clings to the salad. Unlike other gums, it is very stable under a wide range of temperatures and pH.

In foods, xanthan gum is most often found in salad dressings and sauces. It helps to prevent oil separation by stabilizing the emulsion, although it is not an emulsifier. Xanthan gum also helps suspend solid particles, such as spices. Also used in frozen foods and beverages, xanthan gum helps create the pleasant texture in many ice creams, along with guar gum and locust bean gum. Toothpaste often contains xanthan gum, where it serves as a binder to keep the product uniform. Xanthan gum is also used in gluten-free baking. Since the gluten found in wheat must be omitted, xanthan gum is used to give the dough or batter a “stickiness” that would otherwise be achieved with the gluten. Xanthan gum also helps thicken commercial egg substitutes made from egg whites to replace the fat and emulsifiers found in yolks. It is also a preferred method of thickening liquids for those with swallowing disorders, since it does not change the color or flavor of foods or beverages.

In the oil industry, xanthan gum is used in large quantities, usually to thicken drilling mud. These fluids serve to carry the solids cut by the drilling bit back to the surface. Xanthan gum provides great “low end” rheology. When the circulation stops, the solids still remain suspended in the drilling fluid. The widespread use of horizontal drilling and the demand for good control of drilled solids has led to the expanded use of xanthan gum. Xanthan gum has also been added to concrete poured underwater, in order to increase its viscosity and prevent washout.

In cosmetics xanthan gum is used to prepare water gels usually in conjunction with bentonite clays. Is also used in oil-in-water emulsions to help stabilise the oil droplets against coalescence. It has some skin hydrating properties.

[edit] Allergy

Some people are allergic to xanthan gum, with symptoms of intestinal gripes, diarrhea, temporary high blood pressure, and migraine headaches.^{[citation needed]}

Evaluation of workers exposed to xanthan gum dust found little evidence that respiratory symptoms were associated with exposure to xanthan gum dust.^[3]

Since xanthan gum is produced by a bacterium that is fed corn to grow, some people allergic to corn will also react to it. Yellow Phrygian Husk is a common source of bacterium in which xanthan gum is created.^[4] However, some xanthan gum is non corn-derived.

[edit] References and footnotes

1. ^ Davidson, Robert L. (1980). Handbook of Water-soluble Gums and Resins. McGraw Hill. ISBN 0070154716.
2. ^ Becker and Vorholter (2009). “Xanthan Biosynthesis by Xanthomonas Bacteria: An Overview of the Current Biochemical and Genomic Data”. Microbial Production of Biopolymers and Polymer Precursors. Caister Academic Press. ISBN 978-1-904455-36-3.
3. ^ Sargent EV, Adolph J, Clemmons MK, Kirk GD, Pena BM, Fedoruk MJ. (1990). “Evaluation of flu-like symptoms in workers handling xanthan gum powder“. J Occup Med. 32 (7): 625. doi:10.1097/00043764-199007000-00014. http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=2391577.
4. ^ Pollick, Michael (2007). “What is Xanthan Gum?”. http://www.wisegeek.com/what-is-xanthan-gum.htm. Retrieved on 2007-11-21.

• Whistler, Roy, L, and BeMiller, James N., eds Industrial Gums: Polysaccharides and their Derivatives Academic Press (1973) ISBN 0-12-746252-x.