Non-carbon loss long-term continuous lactic acid production from mixed sugars using thermophilic Enterococcus faecium QU 50

In this study, a non-sterile (open) continuous fermentation (OCF) process with no-carbon loss was developed to improve lactic acid (LA) productivity and operational stability from the co-utilization of lignocellulose-derived sugars by thermophilic Enterococcus faecium QU 50. The effects of different sugar mixtures on LA production were firstly investigated in conventional OCF at 50°C, pH 6.5 and a dilution rate of 0.20 hr⁻¹. The xylose consumption ratio was greatly lower than that of glucose in fermentations with glucose/xylose mixtures, indicating apparent carbon catabolite repression (CCR). However, CCR could be efficiently eliminated by feeding solutions containing the cellobiose/xylose mixture. In OCF at a dilution rate ca. 0.10 hr⁻¹, strain QU 50 produced 42.6 g L⁻¹ of l-LA with a yield of 0.912 g g⁻¹-consumed sugars, LA yield of 0.655 g g⁻¹ based on mixed sugar-loaded, and a productivity of 4.31 g L⁻¹ hr⁻¹ from simulated energy cane hydrolyzate. In OCF with high cell density by cell recycling, simultaneous and complete co-utilization of sugars was achieved with stable LA production at 60.1 ± 3.25 g L⁻¹ with LA yield of 0.944 g g⁻¹-consumed sugar and LA productivity of 6.49 ± 0.357 g L⁻¹ hr⁻¹. Besides this, a dramatic increase in LA yield of 0.927 g g⁻¹ based on mixed sugar-loaded with prolonged operational stability for at least 500 hr (>20 days) was established. This robust system demonstrates an initial green step with a no-carbon loss under energy-saving toward the feasibility of sustainable LA production from lignocellulosic sugars.