Optimum pH and temperature are determined in an extractive fermentation system using Ca-alginate entrapped Zymonomas mobilis, and some solvents's productivity and toxicity on the free cells and immobilized cells are tested.
To increase ethanol productivity in the product inhibition system, an extractive fermentation method combined with ethanol fermentation was applied to the immobilized fermentor. Immobilization is one of the methods to utilize enzymes or microorganisms effectively. It holds many advantages, while some problems remain. Mass transfer resistance in supporting materials like gel is one of the problems. Overall reaction rate of immobilized cell can be estimated by regarding it as a homogeneous system. Extractive fermentation is shown to greatly improve the performance of Zymomonas mobilis in batch and continuous cultures during the conversion of concentrated substrates to ethanol, and it is also used to eliminate the oscillatory behavior often exhibited by z. mobilis in conventional fermentations.
As ethanol concentration increased, the distribution coefficients of each solvent increased, the same manner in temperature decrease and pH increase.
When the glucose concentration was 200 g/L, 30℃, pH 6.0 and the Oleyl alcohol/medium was 1, extractive fermentation present higher productivity and final ethanol concentration, 5.75 g/L·hr and 89 g/L, respectively than their counterparts, 4.3 g/L·hr and 76 g/L, in general fermentation. The reason for this enhancement is that a product inhibition effect is less due to the improved extraction capacity of the solvent at pH 6.0 and the immobilized cells have relatively constant activity over pH range.
Sustained oscillations of biomass, substrate and product concentrations are routinely observed in continuous cultures with Zymomonas. The strong fluctuations of cell viability observed during Zymomonas cultures suggests that viability is a major parameter which unfortunately has never been completely taken into account. If a structuration of the cell population as viable, dead, and non viable cells (unable to divide but still able to produce ethanol) is introduced into a simple mathematical model, any situation from completly stable to completly unstable continuous cultures can be described. Experimental observations suggest that this approach may represent the biological reality more closely than the structured model proposed by Jobses et al.(1985-1987).
