Xanthan gum fermentation was studied by locally isolated culture of Xanthomonas cucurbitae PCSIR-52. The , morphological and biochemical characteristics of the isolated cultures were similar to the genus Xanthomonas. The extracellular-polysaccharide produced by the bacterium was also identical to xanthan gum produced by Xanthomonas campestris NRRL B-1459. Nutritional studies of the isolated culture were carried out in shake flasks before scale up production in stirred fermenters of 10, 50 and 100 liter capacity by batch process. Evaluation of carbohydrates, nitrogen sources, both inorganic and organic and carboxylic acids for gum fermentation, were carried out in shake flasks. of all carbohydrates, sucrose was found to be an ideal substrate for the production of xanthan gum and its optimum level was 3.0 per cent. The production of xanthan biopolymer was also encouraging in cane juice, and decreased in the order of cheese whey, beet molasses, hydrol or defatted rice bran when used as raw carbon sources. The pH control near neutrality was maintained by adding calcium carbonate (0.1%).
The inorganic nitrogen sources evaluated were di-ammonium hydrogen phosphate, sodium nitrate, disodium hydrogen phosphate, ammonium phosphate, and ammonium sulphate. The biopolymer production was about the same in the presence of di-ammonium hydrogen phosphate, di-sodium hydrogen phosphate or ammonium phosphate, the concentration of nitrogen was from 0.30 to 0.40 g/l. The addition of organic nitrogen sources, however, greatly improved the conversion of sugar into extracellular-polysaccharide. The organic nitrogen sources tested were urea, thiourea, cornsteep liquor, cabbage extract, penicillin waste mycelium or cotton seed meal (local and imported from U.S.A., i.e. Proflo and Pharmamedia). The production of xanthan gum, however, was greatly stimulated in the presence of cornsteep liquor, cabbage extract or Proflo in the culture medium. Maximum xanthan gum formation however, was found by adding ‚€œProflo extract‚€Ě to the basal medium containing di-ammonium hydrogen phosphate. The amount of pyruvate was doubled in the presence of ‚€œProflo extract‚€Ě or corn-steep liquor as compared with the in organic nitrogen sources and the viscosity of the fermented broth was greatly increased than that obtained by adding inorganic nitrogen sources.
The addition of carboxylic acids or their salts such as sodium pyruvate, potassium citrate, oxalic acid, sodium acetate, DL-malic acid, succinic acid, or tartaric acid increased the conversion of sugar to biopolymer in comparison with the control culture. The production of biopolymer was maximum in the presence of sodium pyruvate or potassium citrate.
Effect of improving oxygen supply by (i) partial replacement of air with oxygen, or (ii) by the addition of hydrogen peroxide solution as oxygen concentrate on the production of biopolymer was investigated in 10 liter stirred fermenter. The rate of gum formation was enhanced as compared with simple aeration system. Studies of biopolymer fermentation by both batch and repeated-batch processes showed that the efficiency of the bacterium was little affected during its continuous growth for 150 hrs., in three cycles (each cycle of 50 hrs.). Production of xanthan gum was greater in sucrose-salt medium than that obtained in simple cane juice. The design of the stirred culture vessel plays significant role in substrate conversion to metabolites and cell mass. The multiple impeller I r , system (disc-turbine) resulted in high yield of xanthan gum due to better agitation and aeration (Oxygen supply).
The parameters such as growth rate (Rx), rate of product formation (Rp) and specific substrate consumption rate or metabolic quotient (q) were also determined from the data obtained in fermenter studies. The rate of gum production, cell growth and metabolic quotient were maximum in 50 liter fermenter.