(Selective) Isolation of acetic acid and lactic acid from heterogeneous fermentation of xylose and glucose

The present study focuses on the isolation of acetic and lactic acid from a fermentation broth produced by heterofermentative microorganisms using glucose and xylose as substrate. Especially xylose fermentation to lactic acid leads to unwanted by-product formation of acetic acid. Reactive liquid-liquid extraction is an energy-efficient downstream process, where the use of green solvents such as d-limonene opens the way to a more sustainable production. To find the optimum solvent/reactive extractant pairing, the reactive extractants trioctylamine, trioctylphosphine oxide, Aliquat 336, and tributyl phosphate were used diluted in 1-octanol, 1-decanol, d-limonene or a deep eutectic solvent. The phosphine-based extractants proved to be most effective for both acids. In the first extraction step, 19% of acetic acid and only 3% of lactic acid are extracted with trioctylphosphine oxide/1-octanol when the xylose feed is used without pH adjustment. The pKa value of the acids is responsible for the difference in extraction efficiency leading to the proposal of a two-step extraction separating first acetic acid and after pH adjustment, lactic acid can be extracted. The combination of trioctylphosphine oxide diluted in d-limonene leads to a surprisingly high selectivity for lactic acid isolation in the second extraction step. The present study shows for the first time that green solvents lead to similar extraction efficiencies compared to conventional solvents such as 1-octanol. For back extraction n-heptane and p-cymene are used as disintegration agents and water as stripping phase; n-heptane results in a lactic acid back extraction efficiency of up to 82% and p-cymene up to 70%. This shows that also for the back extraction p-cymene as a green disintegration agent leads to similar results as fossil-based n-heptane.