Continuous fermentation and in-situ reed separation of butyric acid for higher sugar consumption rate and productivity

For a couple a decades, in the frame of bio-based chemicals and materials, there has been focus on biological butyric acid production due to the wide application of butyric acid in chemical, pharmaceutical and food industries. Major challenges for biological production are strain selection and development, and process improvement for higher yield, productivity and selectivity. Compared with other microbial strains Clostridium tyrobutyricum has been well characterised, exhibits higher yield and selectivity and can utilize glucose and xylose simultaneously. However, a prerequisite for cost effective production of butyric acid is the use of cheap feedstocks as carbon source as well as a process allowing for increased productivity. The present work focuses on butyric acid fermentation of pre-treated and hydrolysed wheat straw (PHWS), consisted of around 72 and 55 g/L glucose and xylose, respectively and 21% (w/w) total solids.
An adapted strain of C. tyrobutyricum was developed after one year of serial adaptation and selection. The strain could grow in up to 80% (v/v) PHWS in batch mode and was capable of fermenting both glucose and xylose. The developed strain was used for continuous butyric acid fermentation coupled with Reverse Enhanced Electro-Dialysis (REED) at D=0.0417 h-1 (1 day HRT) in experiments with a mixture of glucose and xylose in synthetic growth medium as well as with increasing concentrations of PHWS (up to 100%). Data obtained from experiments with synthetic medium showed that disconnection of the REED system resulted to much lower (48 and 83% for glucose and xylose, respectively) sugars consumption rates and consequently lower butyric acid production rates. It was also noticeable that continuous operation, even without the REED system, resulted to higher glucose consumption rates than batch operation (0.85 compared to 0.31 g/L/h). Similar conclusions were drawn from the experiments with increasing concentrations of PHWS. It should also be emphasized that fermentation of 100% PHWS proceeded unhindered with just urea and K2HPO4 added (to making up for nutrient deficiency) and resulted in a butyric acid productivity and yield of 1.31g/L/h and 0.44 g/g, respectively at 1 day HRT.

Acknowledgements: This work is a part of EU-7th Framework programme supported project SUPRABIO (FP7-cooperationproject no 241640).