During the Second World War (1940), with the discovery of dextran polysaccharides,the usefulness of microbial polysaccharides was recognized which was used a blood plasma extender. It was after two decades i.e. in the year 1960 that another useful microbial polysaccharide, XANTHAN GUM, was developed as one of the most widely used industrial polysaccharides, received GRAS listing (Generally Regarded As Safe) for food use in the US after its initial discovery in the USDA laboratories in Peoria and its development by KELCO. Subsequently, the polysaccharide received approval in UE. This polymer exhibits three desirable properties:
(1) High viscosity at low concentrations
(3) Insensitivity to a wide range of temperature, pH and electrolyte variations.
Because of its special rheological properties, xanthan is used in food, cosmetics,
pharmaceuticals, paper, paint, textiles, adhesives and oil and gas industry. The flow characteristics of xanthan, coupled with its stability to salts and extremes of pH, gives it a technical advantage over most polymers used in drilling. The greatest potential for xanthan gum appears to lie in the enhanced oil recovery operations. Discovery and industrial acceptance of xanthan gave boost to further research and development in the field of microbial polysaccharides.
Multinational manufacturers of microbial polysaccharides have made a strong case in support of applications, advantages and cost effectiveness of their products, compared to plant polysaccharides. To a large extent they have been able to create a big market for these products. Microbial gums e.g. xanthan have certain advantages over plant gums and their use in India shall increase. Currently about 30,000 tonnes are produced annually for food and non-food uses in the US, UK and elsewhere in the EU ,China and in other countries. Food industry in India is fast developing and the use of a food additive is also increasing. Besides the indigenous guar gum, now food grade Xanthan gum and CMC are exported by India. Alliance Global, N. B. Enterprise and H. B. Gum Industries Pvt. Ltd are the major market players in India for Xanthan gum production.
Xanthan gum was first described by Rogovin et al. as a commercially promising polysaccharide product from cultures of Xanthomonas campestris. Since then, considerable effort has been devoted to improving the production strain. Many other pathovars of X. campestris effeciently produce exopolysaccharides, including the pathovars phaseoli, malvacearum, carotae, citrumelo, juglandis, as well as some other members of the genus Xanthomonas (X. fragariae, X. oryzae ).
The strains for xanthan gum production are selected and improved by several conventional methods. The purpose of genetic modification could be to have improvements of the properties as required by the down stream application, or to suit with the medium supplied, or to improve the product yield, or to improve the performance by reducing the fermentation time, or to simplify the recovering and purification in following processes.
Production of most microbial polysaccharides involves growth in stirred tank fermenters using media with glucose or sucrose as the carbon and energy source.
Synthesis is often favoured by high C : N ratios. Because of the high viscosity of the fermentation broths, efficient mixing and aeration are required together with
considerable energy input.
Xanthan gum is produced commercially by submerged aerobic batch fermentation of a pure X. campestris culture. Commercial production of xanthan gum uses glucose as the substrate, and generally batch production instead of continuous production due to the batch process having been proven to work successfully. Most commercial production of xanthan gum uses glucose or invert sugars, and most industries prefer batch instead of continuous. Quality assurance and easier of control are reasons why the xanthan gum production uses invert sugars, instead of polysaccharides, and batch process instead of continuous
Acids are produced during fermentation and pH is maintained near neutral by periodic addition of sodium hydroxide. This is crucial to the success of the process as Xanthan gum production ceases below Ph 5.
Usually commercial fermentations are maintained at approximately 28ºC, taking into account that cell growth is most rapid between 24-27 ºC and xanthan yield is highest at 30-33 ºC.
One of the major problems in the industrial production of xanthan is the growing viscosity of the culture during fermentation, which affects the availability of oxygen and nutrients. X. campestris campestris strains are strictly aerobic. Therefore, the oxygen-transfer rate also affects the xanthan yield. Moreover, the molecular mass of xanthan decreases under oxygen limitation. Owing to the growing viscosity during fermentation, suitable aeration and mixing of the culture have to be guaranteed to avoid stagnant areas. Therefore, mechanically agitated fermentation tanks instead of air-agitated tanks are usually used for the industrial production of xanthan.
Oxygen mass transfer is influenced by both agitation speed and air-flow rate. The aeration rate must higher than 0.3 (v/v), and the specific power input for agitation higher than 1 kW/m3 . The fermentation process is carried out for about 100 h and converts an approximately 50% of the glucose into the product.
RECOVERY AND PURIFICATION
After the fermentation stage, multi steps downstream processes would follow. When industrial grade xanthan is required, the post fermentation process treatment may be started with pasteurisation on the fermented broth to sterile the bacterial and to deactivate the enzymes. This process usually uses a large amount of alcohol to precipitate the xanthan gum, and the precipitated xanthan gum is then sprayed dry or maybe re-suspended on the water and then
re-precipitated. When cell-free xanthan gum is required, cecentrifugation is facilitated by diluting the fermentation broth to improve the cell separation. The cell separation by dilution process from highly viscous xanthan solution is a cost-intensive process.
In the petroleum industry, xanthan gum is used in oil drilling, fracturing, pipeline cleaning, and work-over and completion. Due to xanthan gum is excellent compatibility with salt, and resistance to thermal degradation, it also useful as an additive in drilling fluids. The pseudoplasticity of its solutions would provide low viscosity at the drill bit where the shear rate is high and high viscosity in the annulus where shear is low. Therefore, xanthan would serve a dual purpose by allowing faster penetration at the bit and suspending cuttings in the annulus. For every barrel of oil produced, approximately two remain in the ground. Therefore,
enhanced oil recovery (EOR) will be an important use of xanthan gum in the next decades. The basic principle applied is to improve the separation of water and oil thereby would increase oil recovery. However, the quality of xanthan gum is a critical consideration as high impurities would increase the difficulty when refining the oil. Xanthan gum is used in micellar-polymer flooding as a tertiary oil recovery operation. In this application, polymer-thickened brine is used to drive the slug of the surfactant through porous reservoir rock to mobilise residual oil; the polymer prevents bypassing of the drive water through the surfactant band and ensures good area sweeping . In both applications, the function of polymers is to reduce the mobility of injected water by increasing its viscosity.
Xanthan has been used to improve the flow-ability in fungicides, herbicides, and insecticides formulations by uniformly suspending the solid component . The unique rheological properties of xanthan gum solution also reduce drift, and increase pesticide cling and permanence. Recently, various "tolerance exemptions" were issued by U.S. Environmental Protection Agency for use of xanthan gum as the surfactant in pesticide formulations.
Xanthan's shear thinning characteristics can be utilised in the paint industry. Paints containing xanthans are highly viscous at low shear rates, and thus will not drip from a brush. However, the shear stress produced by brushing, thins the paint and allows for easy application. Because of its ability to disperse and hydrate rapidly, is non-polluting and gives a good colour yield, xanthan is also used in jet injection printing. Recently, in the formulation of new generations of thermo-set coatings, xanthan gum has been introduced to meet the challenges of producing environmental friendly products.
The other specialty applications employed the xanthan gel is in removing rust, welding rods, wet slag, and cleaning other debris from gas pipelines.
It is used as suspending and thickening agent for fruit pulp and chocolates. United States Food and Drug Administration have approved xanthan on the basis of toxicology tests for use in human food. Many of today's foods require the unique texturization, viscosity, flavour release, appearance and water-control properties. Xanthan gum improves all these properties and additionally controls the rheology of the final food product. It exhibits pseudoplastic properties in solutions, and has less 'gummy' mouthfeel than gums with more Newtonian characteristics.
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FINAL YEAR MTECH BIOTECHNOLOGY STUDENT