Fermentation Conditions Of Aureobasidium Pullulans G-58 And Its Kinetic Study

Pullulan is a major polysaccharide produced by strains of Aureobasidium pullulans. The pullulan has the characteristics of water-soluble,no-gelation, no-aging and without any toxicity.Pullulan is one of the neutral water-soluble microbial polysaccharides that can be produced in large quantities by fermentation. It can be widely used in food, cosmetic, and pharmaceutical industries. It can be decomposed by certain microorganisms and will not pollute the environment.The objective of this dissertation is to develop a fermentation bioprocess with high yield, high yield coefficient, and high productivity by Aureobasidium pullulans G-58. The research contents include; (1) culture conditions of pullulan shake-flask fermentation; (2) process optimization and strategy of temperature, dissolved oxygen and pH during pullulan batch fermentation; (3) kinetic model of pullulan batch fermentation. Main results are as follows: (1) The proper culture conditions of shake-flask were as follows: sucrose 50 g/L, (NH4)2SO4 0.652 g/L, K2HPO4 5.892 g/L, MgSO4·7H2O 0.5 g/L, NaCl 4 g/L, yeast extract 0.4 g/L, seed age 36h, inoculum size 3%, initial pH value 6.0, fermentation temperature 28℃, fermentation medium size 50mL/250mL. Under the optimum condition, the pullulan yield could reach 24.652g/L and was increased by 28.4 %. Moreover, it was found that both (NH4)2SO4 and K2HPO4 had a substantial influence on pullulan yield by the Placket-Burman design.(2) Based on the above optimum condition, batch fermentation process of pullulan in 5L bioreactor was investigated. The result showed that 300r/min agitation and pH6.0 were good for cell growth and pullulan synthesis. At a higher temperature (34℃), the cell capability of Aureobasidium pullulans was enhanced. On the other hand, at the lower temperature (28℃) , the production capability of pullulan was promoted. Based on the kinetic parameter analysis, a two-stage temperature controlling strategy, in which temperature was controlled at 34℃in the first 48 h and then shifted to 28℃, was suggested and experimentally verified. Following this strategy, the following results were obtained that the maximum yield, productivity and yield coefficient was increased by 22.8%, 50.3% and 8.87%, respectively, by comparison with that of constant temperature 28℃.(3) On the basis of the experimental data of batch fermentation in the 5L bioreactor, the modeling of the kinetics of pullulan fermentation process was studied. Mathematic models of cell growth, product generation and substrate depletion in batch fermentation were proposed according to the Logistic equation and the Luedeking-Piret equation. The parameters of the models were established by using 1stopt software with experimental data and the models. The modes consist of:Cell growth model:Production formation model: P ( t)= 1.245X(t)-4.374+2.037ln(7.842+0.759et) Substrate depletion model: S ( t)= 58.99-1.237X(t)-3.284ln(7.842+0.759et)The above models could fit the profiles of cell growth, production formation and substrate depletion well. It might be helpful to understand and guide further optimization of pullulan fermentation in the future.