Development of a Low Cost, Industrially Suitable Medium for Production of Acetic Acid From Glucose by Clostridium Thermoaceticum

Calcium magnesium acetate (CMA), an environmentally friendly, non-corrosive road deicer, has been investigated for use as a possible replacement for NaCl or CaCl2, due to their corrosive and environmentally damaging nature. The main obstacle preventing widespread use of CMA is its high cost, 15-20 times as much as NaCl. The most expensive ingredient for the production of CMA is acetic acid, which accounts for 97% of the cost of CMA. The organism most commonly evaluated for biological production of acetic acid is Clostridium thermoaceticum. The main advantage of C. thermoaceticum is that it can quantitatively convert a wide range of substrates into acetic acid. Unfortunately, this organism has extensive and diverse nutrient requirements and most investigations performed with this organism have used an expensive, chemically defined medium. Such a medium (e.g. modified Ljungdahl\u27s medium [MLM]) would be prohibitively expensive to use on an industrial scale. Therefore, the objective of this project was to develop a low cost, industrially suitable medium that would allow for large scale biological production of acetic acid by C. thermoaceticum. To meet this end, two waste byproducts from the production of fuel ethanol from corn, heavy stillage (HS) and corn steep water (CSW), were evaluated in flask trials for their ability to replace the ingredients of MLM. Through stepwise eliminations it was found that HS was able to replace all major ingredients in MLM except for the buffers and still maintain 100% or greater of the acetic acid production characteristics (productivity and yield). Batch and fed-batch fermentations were performed in a 5L fermentor to compare MLM with the stillage medium under conditions of constant pH control. In batch fermentations the optimum stillage concentration (13.3% weight/volume) in the medium gave nearly equal maximum acetate productivity (0.30 g/L/h) as MLM (0.33 g/L/h). In fed-batch fermentations in stillage medium, maximum acetate productivity (0.33 g/L/h maximum acetate productivity) was equal to that in MLM; however, maximum acetic acid production ended at significantly lower acetate concentrations in the stillage medium (5-10 g/L acetate) than in MLM (15-20 g/L acetate). However, the stillage medium had equal acetic acid productivity at higher concentrations of acetate (above 20 g/L) making it equal to MLM for the purpose of continuous fermentation. Throughout the course of this research, yields in stillage medium were often above the theoretical maximum, 100%. This was likely due to unquantifiable substrate in the stillage. This free substrate is another advantage of the stillage medium, as it would significantly reduce substrate costs and thereby reduce the cost of acetic acid