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

Abstract

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.

Batch and Continuous Lactic Acid Fermentation Based on A Multi-Substrate Approach

“…Panesar et al [37] reported the production of 33.73 g•L −1 L-LA using L. casei, while Taleghani et al [38] reported a value of 32.1 g•L −1 when whey was fermented by L. bulgaricus. Molasses derived either from sugarcane or sugar beets fermentation to lactic acid has also been investigated, using various LAB strains [39][40][41]. In the recent work of Sun et al [41], a microbial consortium in which Clostridium sensu stricto was the predominant strain was tested for sugarcane molasses fermentation to lactic acid.…”

Production of Lactic Acid from Carob, Banana and Sugarcane Lignocellulose Biomass

“…Recent studies suggested that continuous fermentation process is advantageous for high LA productivity due to dilution of the produced LA in the broth resulted from feeding new medium and high operational stability by achieving steady state of cell growth. The LA production, and sugar consumption, compared with batch or fed-batch fermentation processes where usually difficult to sustain homogeneities in large scales [15,16,46]. It was suggested that continuous SSF with enzyme addition and cell recycle can solve the problems caused by feedback, substrate and end-product inhibition compared to batch or fed-batch fermentation while resulting in higher LA productivities, yield, and concentration [47].…”

“…It was suggested that continuous SSF with enzyme addition and cell recycle can solve the problems caused by feedback, substrate and end-product inhibition compared to batch or fed-batch fermentation while resulting in higher LA productivities, yield, and concentration [47]. Several recent studies showed that continuous fermentation with cell recycle resulted in a higher LA productivity, and when the cells reached a steady-state, a high LA level was maintained for longer time and the utilization of the sugars was at maximum to produce LA (7,15). In addition, these studies have shown that, the lag phase in the cells was reduced, the operational cost was lower, and resulted in the increase of LA production (7,15).…”

“…Several recent studies showed that continuous fermentation with cell recycle resulted in a higher LA productivity, and when the cells reached a steady-state, a high LA level was maintained for longer time and the utilization of the sugars was at maximum to produce LA (7,15). In addition, these studies have shown that, the lag phase in the cells was reduced, the operational cost was lower, and resulted in the increase of LA production (7,15). The pilot scale using carob biomass which is just a lignocellulose bagasse when fermented even without yeast extract supplementation resulted in high yield of 0.84 g LA•g −1 sugars, and productivity of 2.30 g LA•L −1 •h −1 which is very promising process for industrial production of LA.…”

Dynamic Simulation of Continuous Mixed Sugars Fermentation with Increasing Cell Retention Time for Lactic Acid Production using Enterococcus mundtii QU 25

“…xylose as substrates during the single and mixed sugar fermentation. Substrates and production inhibitions were observed in the batch fermentation experiments with E. mundtii QU 25 [22,23]. The cell growth was defined by Monod kinetics with substrate and production inhibition terms as shown in…”