The Effect of Culture Media
Culture media is probably the most important operation parameter in the optimization of the ethanol yield [25,32,33,21]. Kundiyana and coworkers recently reported two studies on the effect of culture media during the syngas fermentation utilizing Clostridium ragsdalei. In one study they reported [32] the effect of nutrient limitation and two-stage continuous fermenter design on productivities, whereas in the other study the role of the buffer presence in the media was evaluated [25]. In the first study three nutrients, calcium pantothenate, vitamin B12 and cobalt chloride (CoCl2), on syngas fermentation using Clostridium ragsdalei were determined using serum bottle fermentation techniques. These studies indicated that three-way interactions between the three limiting nutrients, and two-way interactions between vitamin B12 and CoCl2 had a significant positive effect on ethanol and acetic acid formation. In general, ethanol and acetic acid production ceased at the end of nine days, corresponding to the production of 2.01 and 1.95 g L-1 for the above interactions. Reactor studies indicated the three-way nutrient limitation in two-stage fermenter, which showed improved acetic acid and ethanol yields compared to treatments in single-stage fermenter. As a result of these studies Kundiyana and coworkers concluded [32] that their results further support the hypothesis that it is possible to modulate the product formation by limiting key nutrients during C. ragsdalei syngas fermentation.
As the fermentation medium is a critical factor in the biocatalysis route, there are several reports on using experimental and theoretical techniques in the optimization of pH and composition parameters. In one example, Plackett-Burman applied central composite designs to optimize the medium for ethanol production by Clostridium autoethanogenum in a medium containing NaCl, KH2PO4, CaCl2, yeast extract, MgSO4, and NH4Cl [33]. The optimum ethanol yields predicted by response surface methodology (RSM) and an artificial neural network-genetic algorithm (ANN-GA) were 247.48 and 261.48 mg/L, respectively. Furthermore, Guo et al. reported that these values are similar to those obtained experimentally under the optimal conditions suggested by the statistical methods (254.26 and 259.64 mg/L). It is interesting to note that the fitness of the ANN-GA model was higher than that of the RSM model. Additionally, as a result of optimizations in the culture media, yields obtained in this study were substantially higher than those previously reported (60-70 mg/L) with the same organism [33], and this result clearly established the importance of using optimum nutrient concentrations in the fermentation broth.