| Summary: | 碩士 === 大葉大學 === 環境工程學系碩士班 === 91 === In this study we investigated the effects of pH, aeration and agitation on γ-polyglutamic acid(γ-PGA) productivity in a 10-L fermenter . In addition, the changes of carbon source ( citric acid, glutamic acid, glycerol), dissolved oxygen, cell density, andγ-PGA production were monitored during fermentation process.
The results showed that the cell growth is most suitable at pH=6.0, but the highest yield of γ-PGA is at pH=6.5. Because of low mass-transfer rate and low cell growth at 100rpm , theγ-PGA yield is low. Although mass-transfer rate is high at 300rpm, the γ-PGA yield at 300rpm is very low as well. Good yield ofγ-PGA was obtained when the agitation was at 200rpm. In addition, it was found that theγ-PGA yield is highest at aeration rate of 3L/min. In conclusion, when B.licheniformis CCRC 12826 was cultivated in medium F (NH4Cl 7.0(g/L) , K2HPO4 0.5(g/L) , MgSO4․7H2O 0.5(g/L) , FeCl3 ․6H2O 0.04(g/L) , CaCl2․2H2O 0.15(g/L) , MnSO4․4~6H2O 0.104(g/L), Citric acid 22(g/L), Glutamic acid 65(g/L), Glycerol 170(g/L)), pH 6.5, 200rpm, 3L/min, the highest yield of γ-PGA was obtained ；it was 25.93g/L. The yield increased 23% from 21g/L in shake flask culture to 25.93g/L in 10-L fermenter.
In addition to investigation of γ-PGA production in fermenter, we also investigated the antifreeze activity of γ-PGA. Various enatiomeric isomers, metals salts and molecular weights of γ-PGA, produced by B.licheniformis CCRC 12826 were prepared and their antifreeze activities were studied by differential scanning calorimetry(DSC).
The antifreeze activity of γ-PGA was significant；it increased as its molecular weight decreased. However, the antifreeze activity ofγ-PGA was indifferent to its enatiomeric content. The antifreeze activity was cation dependent；it decreased in the order Mg salt > Ca salt ≈ Na salt >K salt. This trend agrees with that for inorganic chlorides；that is high ionic charge leads to high antifreeze activity. The mechanism by which the cryoprotective effects of γ-PGA can be explained is still yet to be determined.
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