Acetone-butanol fermentation, growth

Growth Inhibition Kinetics for the Acetone-Butanol Fermentation... 

The inhibitory effect of each fermentation product on the cell growth rate and the kinetics of product formation was studied for the acetone-butanol fermentation with Clostridium acetobutylicum ATCC 824. Inhibition of cell growth was studied by challenging cultures with varying concentrations of each product. There was a threshold concentration which must be reached before growth inhibition occurred. This concentration was found to vary with each inhibitor. Above the threshold concentration, there was a linear decrease of the growth rate with an increase in product concentration. 

Product Challenged Growth Studies. To study the inhibitory factors of the acetone-butanol fermentation, the growth rates of Cl. acetobutylicum in the presence of each fermentation product were determined. The end products used in this study included ethanol, butanol, acetone, acetic acid, and butyric acid. From the slopes of the least squares regression lines of optical density vs. time data, the maximum specific growth rates in the presence of varying concentrations of each inhibitor ()j ) were determined. The results for each fermentation product are shown in Figures 1 - 3. There appears to be a threshold concentra-... 

Similar growth challenge experiments were performed using tert-butyl alcohol and n-hexanol. Although tert-butyl alcohol and n-hexanol are not products of this fermentation, the Up and Pq values obtained for these two alcohols were found to lie on the straight line shown in Figure 4, Linden et al. (3) have shown that the end product toxicity in the acetone-butanol fermentation occurs by altering membrane functionality. The linear relationship between and P ay indicate that the inhibition of each of these various compounds occurs by the same mechanism. 

From these studies of growth inhibition and fermentation kinetics in the acetone-butanol fermentation, the following conclusions may be made ... 

Lemmel SA, Datta R, Frankiewicz JR (1986) Fermentation of xylan by Clostridium acetobutylicum. Enz Microbiol Technol 8 217-221 Lenz TG, Moreira AR (1980) Economic evaluation of the acetone-butanol fermentation. Ind Engin Chem Prod Res Devel 19 478-483 Lepage C, Fayolle F, Hermann M, Vandecasteele J-P (1987) Changes in membrane liquid composition of Clostridium acetobutylicum during acetone-butanol fermentation effects of solvents, growth temperature and pH. J Gen Microbiol 133 103-110... 

Production of bulk chemicals. The production of solvents is normally characterized by a general inhibition phenomenon which has been mainly attributed to the changes in membrane permeability, or to the toxic effects on the metabolic pathway. Aqueous two-phase systems have been shown to be effective as media for the extractive fermentation of a number of solvents which include ethanol, acetone-butanol and acetic acid (3). Improved productivity has been achieved in most of the cases as compared to the conventional fermentations, which is significantly due to the elimination of product inhibition. However, there is an indication that changes in the microenvironment of the microbial cells due to the presence of non-metabolizable polymers could also contribute, in the initial phases, to the increased production. The addition of PEG and dextran to a growth medium, for instance, was shown to give increased initial ethanol yields, as a result of decrease in the chemical potential of water (8). 

In principle, the same carbohydrates and their degradation products formed after hydrolysis of wood can be recovered from sulfite spent liquors. However, this requires complicated and expensive separation techniques. The industrial use of sulfite spent liquor components is mainly limited to fermentation processes. The most common product is ethyl alcohol which is produced from hexose sugars by yeast (Saccharomyces cerevisae) and separated from the mixture by distillation. Even the carbon dioxide formed in the process can be recovered. Other fermentation products, including acetone, n-butanol, and lactic acid, can be produced by certain microorganisms. Because some contaminants, for example, sulfur dioxide, inhibit the growth of the yeast, they must be removed from the liquor prior to the fermentation. 

The production of both acetone and butanol by C. acetobutylicum is a characteristic of this species, and the two products are responsible for the organism s continued usefulness during and after World War 1. Interestingly, under certain growth conditions, a different mode of fermentation... 

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