Abstract
(1) For the purpose to obtain microorganisms having high dextran producing ability, five strains were isolated from various sources. These were lactic acid bacteria which were able to produce slime from sucrose.
(2) Morphological and physiological investigations of these strains were carried out and a gummy material obtained from sucrose was identified as dextran. These strains were decided to be assigned to Leuconostoc mesenteroides.
(3) Dextran production by these strains was examined and it was confirmed that strain N-4 was the most satisfactory strain. Optimum conditions for dextran formation with this strain are as follows: (a) Optimum pH ranged between 6.8 and 7.4. (b) Peptone concentrations of 0.05-O.1% were optimum for the production of dextran. (c) A high yield of dextran production was obtained in low sucrose concentrations, and this decreased in a concentration of more than 15% sucrose. About 10% sucrose concentration was considerded to be suitable for dextran production. (d) K2HPO4 0.1% and NaCl 0.1% were found to be optimum for dextran production. (e) The use of yeast extract was very effective and fermentation was completed within 24-48 hours using a 0.05% concentration. (f) The production of dextran by shaking culture was found to be inferior to stationary culture.
(4) Dextran fermentation was carried out in a large scale using a 200L fermentor. (a) The viscosity of the broth reached a maximum after 24 hours then decreased rapidly. (b) pH showed 4.4 after 24 hours then lowered slowly to 3.7 and maintained this value throughout the fermentation. (c) Total acid increased rapidly up to 48 hours, and then a slight increase was observed. (d) Dextran formation reached a maximum during 24-30 hours' incubation and the yield showing about 42.5% of sucrose. (e) Reducing sugars were produced in proportion to dextran formation, and a slight decrease was observed after it reached a maximum during 24-30 hours. (f) Fractional precipitation of native dextran which were obtained from both the 24 hours' and 7 days' culture broth respectively, was performed and it was found that the molecule of native dextran separated from the latter was lower than that of the former, so dextran once produced seemed to degraded by prolonged incubation.
