Butyric acid is a valuable building-block for the production of chemicals and materials and nowadays it is produced exclusively from petroleum. The aim of this study was to develop a suitable and robust strain of Clostridium tyrobutyricum that produces butyric acid at a high yield and selectivity from lignocellulosic biomasses. Pretreated (by wet explosion) and enzymatically hydrolysed wheat straw (PHWS), rich in C6 and C5 sugars (71.6 and 55.4 g l−1 of glucose and xylose respectively), was used as substrate. After one year of serial selections, an adapted strain of C. tyrobutyricum was developed. The adapted strain was able to grow in 80% (v v−1) PHWS without addition of yeast extract compared with an initial tolerance to less than 10% PHWS and was able to ferment both glucose and xylose. It is noticeable that the adapted C. tyrobutyricum strain was characterized by a high yield and selectivity to butyric acid. Specifically, the butyric acid yield at 60–80% PHWS lie between 0.37 and 0.46 g g−1 of sugar, while the selectivity for butyric acid was as high as 0.9–1.0 g g−1 of acid. Moreover, the strain exhibited a robust response in regards to growth and product profile at pH 6 and 7.

The economic feasibility of biochemical conversion of wheat straw to butyric acid was studied in this work. Basic process steps included physicochemical pretreatment, enzymatic hydrolysis and saccharification, fermentation with in-situ acids separation by electrodialysis and product purification. Two scenarios (S1 and S2) were examined assuming a plant with an annual capacity of 10,000 tonnes of product installed in India (due to significantly lower feedstock prices). S1 resulted in a product of 89% butyric acid mixed with acetic acid and S2 produced butyric acid of 99% purity. Unit production cost was estimated at 2.75 and 3.31 $ per kg product for S1 and S2 respectively. The main part of production cost was attributed to steam for the purification step and electricity for the in-situ acids separation. This unit production cost combined with an estimated butyric acid selling price (year 2014) at 3.50 and 3.95 $ per kg product (for S1 and S2 respectively) and a plant capacity of 10,000 tonnes indicated an internal rate of return of 14.92% and 12.42 % and payback time of 4.28 and 4.70 Highlights (for review)

The present study focused on fermentative butyric acid production by Clostridium tyrobutyricum from pre-treated and hydrolysed wheat straw (PHWS) based on continuous operation mode and in situ acids extraction by reverse electro enhanced dialysis (REED). Different dilutions of PHWS in a synthetic medium (60–100 % v/v) were tested. It was found that continuous fermentation of PHWS greatly enhanced the sugar consumption rates and butyric acid productivity compared to batch tests, while application of REED enhanced them even further. Specifically, applying combined continuous operation mode and REED system for the fermentation of 70 % PHWS resulted in 19- and 53-fold higher glucose (1.37 g L−1 h−1) and xylose (0.80 g L−1 h−1) consumption rates, respectively, compared to those obtained by batch processing. Fermentation of 100 % PHWS continued unhindered with just urea and K2HPO4 added with butyric acid production rate, yield and selectivity being 1.30 g L−1 h−1, 0.45 g g−1 sugars and 0.88 g g−1 acids, respectively. These results were also confirmed in a 20 L pilot plant bioreactor system.

In the present study, the effect of potassium ions and increasing concentrations of glucose and xylose on the growth of a strain of Clostridium tyrobutyricum, adapted to wheat straw hydrolysate, was investigated. Application of continuous fermentation of a mixture of glucose and xylose and in situ acid removal by reverse electro enhanced dialysis (REED) was investigated as a method to alleviate potassium and end-product inhibition and consequently enhance the sugar consumption rates and butyric acid productivity. It was found that glucose and xylose were not inhibitory up to a concentration of 50 and 37 g L−1 respectively, and that they were consumed at comparable rates when fermented alone. However, continuous fermentation of a mixture of glucose and xylose resulted in a significantly decreased xylose consumption rate compared to that of glucose alone, supporting the conclusion that C. tyrobutyricum has a lower affinity for xylose than for glucose. Potassium ions negatively affected the effective maximum growth rate of C. tyrobutyricum at concentrations higher than 5 g L−1 exhibiting a non-competitive type of inhibition. Continuous fermentation of a glucose and xylose mixture with simultaneous acid removal by REED resulted in a two to threefold increase of the glucose consumption rate, while the xylose consumption rate was enhanced sixfold compared to continuous fermentation without in situ acid removal. Similarly, butyric acid productivity was enhanced by a factor of 2–3, while the yield remained unaffected.