Synthesis and Production of Biopolymers

The enzyme glycoprotein releases fructose from sucrose and transfers the glucose residue to the reducing end of the growing dextran chain on an acceptor molecule, which is bound to the enzyme . During polymerization, the growing dextran chain is always bound to the enzyme. The degree of polymerization increases until an acceptor molecule releases the polymer chain from the enzyme.  

Dextran can be produced either directly by batch fermentation or indirectly by the use of the enzyme complex dextransucrase mentioned above. Direct production is, of course, the simpler process, but molecular weights of dextrans are varying. The fermentation process is performed in stirred and aerated bioreactors with typical volumes of up to 200 m. Bacterial growth requires amino acids and growth factors. Fermentation is started at 25 - 30 C and a pH of 6.5 - 7.0, dropping to about 4.5 at the end of the production cycle (after about 48 hours) due to formation of lactic acid as a by-product. Maintenance of an adequate dissolved oxygen concentration is problematic due to the non-Newtonian fluid behavior of the nutritional 

For dextran production, 10% w/v sucrose is supplied as substrate, the incubation is conducted at 15 °C or below, in the presence of about 30 units/ml of dextran sucrase. Molecular weight of dextran produced can be controlled by sucrose concentration, enzyme concentration, and by the temperature and time used for incubatioiL 

After its production, dextrans are precipitated by addition of either methanol or acetone (1 °C), and the supernatant is decanted. Precipitated dextran is resolubilized in distilled water at 60 to 70 C and precipitated again for cleaning purposes. The typical yield for dextran is about 60-70% of the glucose part of sucrose. About 2000 tons of dextranes are consumed worldwide per year. Depending on product qualities they are sold for a price of35-2800 US/kg. 

Tabie 2. Different properties of Xanthan used in Foods (3) 

---