No abstract

The experiments were based on multivariate statistical concepts, and response surface methodology (RSM) was applied to optimize the fermentation medium for the production of ribonucleic acid (RNA) by Candida tropicalis no. 121. The process involved the individual adjustment and optimization of various medium components at shake flask level. The two-level Plackett-Burman (PB) design was used to screen the medium components, which significantly influenced RNA production. Among seven variables, the concentrations of molasses, ZnSO 4 , and H 3 PO 4 were found to be the important factors that significantly affected RNA production (confidence levels above 95%). These factors were further optimized using a central composite design (CCD) and RSM. The optimum values for the critical components were as follows: molasses 47.21 g/L: ZnSO 4 0.048 g/L; H 3 PO 4 1.19 g/L. Under optimal conditions, RNA production was 2.56 g/L, which was in excellent agreement with the predicted value (2.561 g/L), and led to a 2.1-fold increase compare with that using the original medium in RNA production.

During ribonucleic acid fermentation, the fermentative processes were researched at pH controlled at 4.0 and under natural conditions. Unstructured models in a 50-L airlift fermentor were established for batch RNA production at pH 4.0 using the Verhulst equation for microbial growth, the Luedeking-Piret equation for product formation and a Luedeking-Piret-like equation for substrate uptake. Parameters of the kinetic models were determined using origin 7.5. Based on the models estimated above, another batch fermentation experiment was conducted in a 300-L airlift fermentor, which demonstrated that the models could be useful for RNA production on an industrial scale. Additionally, continuous fermentation based on kinetic models was proposed to make full use of substrates and reduce the cost of waste water treatment. As a result, although the DCW and RNA concentration were 11.5 and 1.68 g L(-1), which were lower than that of batch fermentation, the sugar utilization increased by 14.3%, while the waste water decreased by more than 90%.

The production of cyclic adenosine monophosphate (cAMP) by Arthrobacter sp. A302 was studied in a 5 L stirred tank fermentor under a range of pH values (6.5-8.0) and glucose feeding rates. In batch fermentation under a controlled pH, the optimum pH for cell growth was 7.5 with dry cell density (X) of 11.43 g L, and the optimum pH for cAMP accumulation was 7.0 with cAMP concentration of 7.41 g L. In order to achieve the high X and cAMP yield simultaneously, a pH-shift control strategy was proposed based on kinetic analysis of specific cell growth rate (μ) and specific cAMP formation rate (q ( s )). In this method, pH was controlled to 7.0 for the first 30 h of fermentation, and then subsequently shifted to 7.5 and maintained until the end of the process. Application of this approach significantly enhanced the cAMP concentration. Thereafter, cAMP production was further improved by combining the above-mentioned pH-control system and fed-batch process with glucose at a constant feeding rate of 1.0 g L(-1 )h(-1). Under optimum conditions, the final cAMP production was 10.87 g L, which is 110.0, 46.7, and 27.7% higher than that of the pH-uncontrolled, pH-controlled, and pH-shift controlled methods, respectively.

A glucose utilizing strain, Arthrobacter A302 was used for cyclic adenosine monophosphate (cAMP) production in batch modes. The non-structured model in a 5 l stirred tank bioreactor for understanding, controlling, and optimizing the fermentation process was proposed using the logistic equation for microbial growth, the LuedekingPiret equation for product formation and LuedekingPiret-like equation for substrate uptake, respectively. The production of cAMP was a mixed-growth-associated pattern. Based on model prediction, a comparison of calculated value using the parameters evaluated above with another experimental data in 30 l bioreactor was used to test the model. The results predicted from the model were in good agreement with the experimental observations in 30 l bioreactor, which demonstrated that the model might be useful for the development and optimization of production of cAMP in industrial scale. Based on estimated kinetic parameters, three different fed-batch modes, constant rate and intermittent (once and repeated), were adopted in order to obtain more cAMP accumulation. Furthermore, the final production of cAMP reached 11.24 g l -1 after 72 h incubation using three stages feeding strategy. In particular, the cAMP productivity (0.156 g l -1 h -1 ) was successfully improved by 22.83, 11.43 and 9.86%, respectively, compared with the modes of the batch, constant rate fed-batch and intermittent fed-batch once.