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In this article we will discuss about:- 1. Extraction and Production of Xanthan 2. Properties of Xanthan 3. Modification 4. Uses 5. Applications.
Extraction and Production of Xanthan:
The primary laboratory and commercial fermentation medium – as has been known for over fifty years for X. campestris growth and xanthan production is a phosphate-buffered (pH ≃ 7) broth containing D-glucose (30 g l-1) (or sucrose, starch, hydrolysed starch), NH4CI, MgSO4, trace salts and 5 g l-1caesin (or soybean) hydrolysate and the fermentation process takes place aerobically at a temperature of ≃ 28°C.
Xanthan production is further stimulated by the presence of pyruvic, succinic or other organic acids. The xanthan produced in this way is very similar to the xanthan produced naturally by the microbes living on a cabbage. In the commercial process the oxygen uptake from the broth is controlled to a rate of 1 mmol 1-1 min -1. Treated in this way the bacterium becomes an extremely efficient enzyme mini- factory converting >70 per cent of the substrate (D-glucose or related substrates) to polymeric xanthan.
The bacterium having done its work is then removed in a rather undignified way by centrifugation and the xanthan precipitated with methanol or 2-propanol at 50 per cent weight concentration. The xanthan slurry is then dried and milled for use. The flow chart for production of xanthan is depicted in Fig. 10.2. The original commercial producer of xanthan was Kelco Ltd. (now MSD-Kelco) and together with other suppliers the annual world-wide production is now over 10000 tonnes.
The biosynthetic process by the bacterium worked out by Sutherland (1989) and later confirmed by others, follows the same basic pattern proposed for other microbial polysaccharides. (Fig. 10.3).
1. Substrate uptake
2. Substrate metabolism
3. Polymerization
4. Modification and extrusion
It involves lipid carriers Fig. 10.3 although there is still uncertainty over their precise role and how the whole process is controlled.
Properties of Xanthan:
Table 10.2 summarizes the fundamental properties of a popular commercial xanthan.
Xanthan is one of the largest of the aqueous soluble polysaccharides. Solutions are correspondingly extremely viscous. The intrinsic viscosity is one of the highest known for a polysaccharide and the dilution solution concentration limit, is very low. The very high viscosity at low concentrations makes it ideal as a thickening and suspending agent. By itself xanthan, however, only forms transient weak gels since the junction zones are weaker than in those used for networking in carrageenan and agarose.
Modification of Xanthan:
The key sites for modification are the first and terminal mannose residues on the trisaccharide side chains and the helical backbone by forming non-covalent interactions with galactomannans. These interactions appear to be also affected by the substitutions in the side chains.
As to the trisaccharide side chains themselves, there are two approaches for alteration or control- one is changing the physiological conditions of fermentation; the other is the use of different pathovars or strains of campestris. The pathovar p. phaseoli and oryzae yield virtually acetyl-free or pyruvate-free xanthan respectively. The other approach has been to look at the genetics of the enzymes, controlling the biosynthetic pathway, attempt to produce and isolate in sufficient quantity.
Genetic mutants deficient or defective in one or more of these enzymes to give ‘polytetrame’ (i.e. lacking in the terminal mannosyl or pyruvalated mannose group) and ‘polytrimer’ (lacking in addition the adjacent glucuronic acid residue). Xanthan reacts with carrageenan and agarose synergistically and give stronger gels. Deacetylating the xanthan side chains seems to enhance these synergistic interactions.
Uses of Xanthan:
Xanthan which has food grade approved by the USA Food and Drug Administration, makes it not only attractive as a food product but also for use in packaging material in contact with food, pharmaceutical and biomedical applications that involve ingestion. Its uses chiefly derive from its solubility in hot and cold water, it’s very high thickening, suspending potential which in turn derives from the very high viscosity of its suspensions. Despite the high viscosity, xanthan suspensions exhibit high shear thinning which means they also flow easily.
Applications of Xanthan:
(a) Food Industry:
Besides high viscosity, thickening and suspending ability, xanthan suspensions have high acid stability. This makes them highly popular in sauces, syrup, toppings and salad dressing. In drinks, the addition of xanthan together with carboxymethylcellulose adds ‘body’ to the liquid and assists with uniform distribution of fruit pulp etc. It is also to add body to dairy product.
The high freeze – thaw stability of xanthan suspensions makes them particularly attractive for the frozen food industry. The high suspending and stability properties are also taken advantage of the animal feed industry for transporting liquid feeds with added vitamins and other supplements that would otherwise sediment out with transport or storage time. Specific uses of xanthan gum in food industry are given in Table 10.3.
(b) Pharmaceutical and Cosmetic Uses:
Microencapsulation Xanthan has recently been added to the list of hydrophilic matrix carriers, along with chitosan, cellulose ethers, modified starches and scleroglucan. Tablets containing 5 per cent xanthan gum were shown under low shear conditions to give successful controlled release of acetaminophen into stomach fluid, and tablets containing 20 per cent xanthan successfully carried a high loading of the drug theophylline.
The high suspension stability is made use of in pharmaceutical cream formulations and in barium sulphate preparations. This high cream stability takes advantage of cosmetic industry including toothpaste technology, where the toothpaste will hold its ingredients (high viscosity) and then easily brush onto, off the teeth (high shear thinning). Uniform pigment dispersal, along with other ingredients and long-term stability, make xanthan a good base for shampoos.
(c) Oil Industry:
Xanthans are used in a number of aspects of well-bore technology such as-
(i) A proppant into fissures for rock fracturation,
(ii) The suspension and removal of debris from the bore area even in the presence of harsh environment (sea water included), pipelines,
(iii) Enhancing oil recovery using the technique of polymer ‘flooding’.
(d) Wallpaper Adhesives:
The high stability of suspensions makes xanthan an ideal base for suspending adhesive agents for wallpapers.
(e) Textile Industry:
Like alginates the suspension stabilizing property of xanthan makes them ideal for producing sharp prints from dyes with a minimum risk of running, for application to textiles and carpets.