One of the challenges in biofuel production from lignocellulosic wastes is to improve its conversion to solvents; therefore, new strategies to enhance xylose uptake are required due to be the secondary abundant sugar. In this context, a novel fermentation strategy integrating a co-culture of Clostridium acetobutylicum and Saccharomyces cerevisiae with pH control was developed. Initially, two different buffers, ammonium acetate and calcium carbonate, were tested under pH_min > 4.8 by fermenting 60 g L⁻¹ of glucose with the C. acetobutylicum monoculture. Ammonium acetate was selected for fermenting media as butanol production was increased from 9.8 to 10.9 g L⁻¹ over the calcium carbonate test. Comparing with the spontaneous acetone-butanol-ethanol (ABE) fermentation with C. acetobutylicum when no xylose consumption was observed, xylose consumption was efficiently increased by controlling pH_min > 4.8. The xylose consumption was > 47% either by using a 45:15 g L⁻¹ glucose:xylose mixture or with rice straw (RS) hydrolysate. Clostridium monoculture using RS hydrolysate and pH_min > 4.8 produced a butanol (ABE) concentration of 6.5 (9.5) g L⁻¹. While it increased to 7.0 (13.1) g L⁻¹ when the co-culture with S. cerevisiae was used using same pH regulation strategy mainly due to ethanol increase up to 2.7 g L⁻¹. Moreover, the xylose uptake doubled to 94% due to amino-acid secretion by yeast. Overall, this combined strategy was a very effective method for promoting sugar consumption and ABE solvent production from lignocellulosic waste.

Acetone–butanol–ethanol (ABE) fermentation is influenced by external conditions. This work aimed to study the influence of pH regulation on monosaccharide composition in batch and continuous fermentation processes to determine butanol production and productivity. Batch fermentations with ammonium acetate or calcium carbonate combined with minimum pH control (pH ≥ 4.8 or 5.1) were assessed with pure xylose and glucose/xylose mixtures (ratios of 1:1 and 3:1). Continuous two-stage fermentation was developed using plastic rings to retain the biomass. Although batch fermentations with pure xylose performed better without active minimum pH control with both buffers, minimum pH control was necessary to metabolize xylose in the presence of glucose. Xylose uptake was favored by the use of calcium carbonate and pH ≥ 5.1 at a ratio of 1:1, while ammonium acetate and a pH ≥ 4.8 was the best option for a 3:1 ratio. The best butanol production and productivity values with sugar mixtures in batch reactors were 8.8 g L⁻¹ and 0.61 g L⁻¹ h⁻¹ with an ammonium acetate pH ≥ 4.8 (ratio 3:1). The glucose/xylose ratio combined with pH regulation thus modulated xylose metabolism and solvent production in batch modes. Immobilized cells combined with operating at D = 0.333 h⁻¹ and pH regulation increased butanol productivity almost fourfold up to 2.4 ± 0.2 g L⁻¹ h⁻¹.

BACKGROUND: This study describes the operation of continuous packed‐bed reactors containing Clostridium beijerinckii DSM 6422 attached to granular activated carbon to improve butanol productivity and sugar consumption from rice straw hydrolysate.RESULTS: A single‐stage fermenter, which was evaluated at dilution rates ranging from 0.3 to 1.8 h⁻¹, achieved a maximum butanol productivity of 4.44 g L⁻¹ h⁻¹ at a dilution rate of 1.2 h⁻¹, corresponding to a sugar consumption of 40%. The performance of the two‐stage fermentation plant with in‐line gas stripping and effluent recycling was studied at a dilution rate of 1.2 h⁻¹ for each bioreactor, resulting in a butanol productivity of 4.16 g L⁻¹ h⁻¹ in the first stage and 0.98 g L⁻¹ h⁻¹ in the second. Nutrient supplementation was identified as an effective method of enhancing butanol productivity by up to 50% in the second stage. The overall sugar consumption of the two‐stage packed‐bed reactor with in‐line gas stripping was 69%.CONCLUSION: These findings show considerable promise for the future development of lignocellulosic biorefineries, as they demonstrate a potential to achieve economically profitable butanol production. © 2024 The Author(s). Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

The sharp rise in the worldwide production of biodiesel has created an excess in the crude glycerol market, so it is essential to develop new added-value alternatives for crude glycerol. This paper describes a study on fermenting high concentrations of two types of medium-pure crude glycerol to solvents by Clostridium pasteurianum. The effect of media composition (iron, yeast extract, and vitamins) on solvents production was assessed by a full factorial design with pure glycerol. Granular activated carbon (GAC) adsorption was highly effective in removing impurities from crude glycerol. Following GAC pretreatment, fermentation of glycerol at initial concentration as high as 60 g L⁻¹ was possible, resulting in a butanol production of ~9 g L⁻¹. Based on these results, a batch fermentation with in situ gas stripping and pH controlled at ≥6.5 was shown to be the best alternative to enhance biomass growth, glycerol uptake, and solvent production. The combination of controlling pH in the early stages of fermentation with in situ butanol removal stabilised the metabolism of the strain and showed that the fermentation performance with crude glycerol is very similar to that of pure glycerol. With a notable uptake of glycerol (>83%), solvent production was >11 g L⁻¹ butanol (yield > 0.21 g g⁻¹_{glycerol consumed}) and >6 g L⁻¹ 1,3-propanediol (yield > 0.13 g g⁻¹_{glycerol consumed}). Setting the fermentation conditions to achieve a high uptake of high levels of glycerol with a similar product distribution is of great interest for the viability of the industrial processing of crude glycerol into chemicals via biological conversion.