Pilot scale demonstration of d-lactic acid fermentation facilitated by Ca(OH)2 using a metabolically engineered Escherichia coli

•Ca(OH)2 significantly facilitated homo-d-lactate fermentation by E. coli HBUT-D.•The facilitation of Ca(OH)2 resulted in a tripled volumetric productivity (6.35 g L−1 h−1).•The enhancement of Ca(OH)2 was further demonstrated in a pilot scale fermentation.•Pilot fermentation achieved d-lactate of 126 kg ton−1 and productivity of 6 kg ton−1 h−1.

In this study, a genetically engineered Escherichia coli strain, HBUT-D (ΔpflB Δpta ΔfrdABCD ΔadhE Δald ΔcscR), was initially evaluated on a laboratory scale (7 L) in a glucose (130 g L−1) mineral salts medium for d-lactic acid fermentation using 6 N KOH, Ca(OH)2 or NH4OH as the neutralizing agent. Fermentations neutralized by Ca(OH) 2 achieved a volumetric productivity of 6.35 g L−1 h−1, tripling that achieved by KOH (1.71 g L−1 h−1) and NH4OH (1.5 g L−1 h−1). The facilitative effect of Ca(OH)2 neutralization was then demonstrated on a pilot scale (6 ton vessel, 130 kg glucose ton−1), resulting in a volumetric productivity of 6 kg ton−1 h−1, a titer of 126 kg ton−1, a yield of 97%, and an optical purity of 99.5%. These results demonstrated that E. coli HBUT-D is a promising strain for large scale d-lactic acid fermentation using mineral salts medium and Ca(OH)2 for neutralization.