Exopolysaccharides applications

SAQ7.12 Use the information given in section 7.9 to complete the Important Physical Properties column of the following table. Exopolysaccharide Application Important Physical Properties ... 

Commercial applications for polysaccharides include their use as food additives, medicines and industrial products. Although plant polysaccharides (such as starch, agar and alginate) have been exploited commercially for many years, microbial exopolysaccharides have only become widely used over the past few decades. The diversity of polysaccharide structure is far greater in micro-organisms compared to plants and around 20 microbial polysaccharides with market potential have been described. However, microorganisms are still considered to be a rich and as yet underexploited source of exopolysaccharides. 

The method used is governed by the market application of the exopolysaccharide. In general, the food industry has a requirement for a dry powder, whereas for several other applications, such as enhanced oil recovery, a liquid product is required and the ultrafiltration concentrate is preferred. 

For food and pharmaceutical applications, the microbial count must be reduced to less than 10,000 viable cells per g exopolysaccharide. Treatment with propylene oxide gas has been used for reducing the number of viable cells in xanthan powders. The patented process involves propylene oxide treatment for 3 h in a tumbling reactor. There is an initial evacuation step before propylene oxide exposure. After treatment, evacuation and tumbling are alternated and if necessary the reactor is flushed with sterile nitrogen gas to reduce the residual propylene oxide level below the Food and Drug Administration permitted maximum (300 mg kg 1). The treated polysaccharide is then packaged aseptically. 

The concentrate derived from ultrafiltration is usually a thick colourless gel containing about 4-8% solids. This must contain an antimicrobial agent to inhibit microbial growth and biological degradation. The type of antimicrobial agent used depends on the particular application for the exopolysaccharide. For example, the nature of file antimicrobial agent is less critical for industrial applications, such as enhanced oil recovery, than for use in cosmetics. 

Microbial exopolysaccharides are widely used in industry as viscosifiers and as gelling agents. In this section we will consider, in general, the rheology of exopolysaccharides in solution and their ability to form gels. Specific properties of individual microbial exopolysaccharides and applications which exploit these characteristics are considered later in this chapter. 

The unique properties of exopolysaccharides are exploited in a wide range of industrial applications. They are sold either as powders or as concentrates (approx 8% w/v). Although more expensive to transport, concentrates offer several advantages they can be pumped any lumps formed during make up of the solid are avoided the viscosity at a given concentration is higher than from solids. 

Xanthan has some unique properties and high activity at low concentrations and is commercially the most important exopolysaccharide. Food products account for approximately 60% of xanthan use, 15% is accounted for by toothpaste, textiles and crop protection products, 10% in the oil industry and the remainder in miscellaneous industrial/consumer applications. 

Xanthan gum is the most important exopolysaccharide used commercially, its unique properties making it ideal for an extraordinary range of applications Table 7.5. 

Several new exopolysaccharides such as welan and rhamsan produced by Alcaltngettes spp may supercede xanthan for some industrial applications. These are based on the same repeat tetrasaccharide backbone of glucose, glucuronic acid, glucose and rhamnose but differ in the substituents rhamsan has a disaccharide side chain and welan a monosaccharide. Both are stable at high temperature and have excellent pseudoplastic properties. 

Exopolysaccharides are used in lotions and gel formation is exploited in encapsulated drugs. The latter application also takes advantage of the mouth feel and flavour neutrality, qualities also vital for the food industry. 

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