Continuous Simultaneous Saccharification And Co-Fermentation Of Lactic Acid With Lignocellulose As Substrate

Compared to batch fermentation,continuous fermentation can obtain a higher volumetric productivity by reducing the switching time of process vessels for emptying,cleaning,sterilizing.The fermentation of bulk fermentation products such as lactic acid is more economically feasible under continuous mode.In order to achieve higher product concentration and efficiency,the production of cellulosic lactic acid usually needs to be carried out under high solid content,and the most commonly used production process is batch simultaneous saccharification and co-fermentation(SSCF).However,the most significant disadvantage of SSCF is the mismatch between the optimal temperatures of cellulase enzymes and microbial cells.As a result,most of the hydrolysis and fermentation of SSCF have to be carried out at two different temperatures,which makes it difficult to carry out the continuous SSCF process based on lignocellulosic raw materials.In this paper,the continuous SSCF for high chiral purity cellulosic L-lactic acid production based on the dry bio-refinery processing platform was investigated and optimized.The lignocellulosic raw material was wheat straw pretreated with dry acid and bio-detoxificated.An engineered thermophilic L-lactic acid bacterium Pediococcus acidilactici ZY271 with pentose assimilation pathway and antibacterial activity was applied for continuous SSCF.This strain could efficiently metabolize total sugar from lignocellulose at 48℃ and pH 4.8-5.5,in which fermentation temperature was close to the optimal temperature of cellulase(50℃),and the fermentation pH was consistent with the optimal pH of cellulase(5.5).Firstly,in order to establish an effective continuous SSCF process,saccharification and fermentation temperatures were unified in this research without an external saccharification tank.Continuous SSCF of lactic acid was carried out in a single-stage reactor under different conditions of temperature(42-50℃)and dilution(0.008-0.025 h-1).The average L-lactic acid titer,yield,and productivity at the steady state reached to 104.8±3.5 g/L,0.24±0.01 g/g DM and 0.87±0.03 g/L/h,respectively,at 30%(w/w)solid content,45℃constant temperature and 0.008 h-1 dilution.When the dilution rate was further increased to 0.025h-1,the yield of lactic acid increased to 2.21 ±0.11g/L/h,however the titer and yield decreased by 15.7%and 1.25%.This indicate that the worse hydrolysis of materials under the condition of high dilution generated the decreased titer and yield of lactic acid.In view of the problem of incomplete saccharification and fermentation in continuous SSCF due to the short retention time of materials in a single bioreactor,the series of multi-stage reactors were used to prolong the retention time of materials.After the optimization of fermentation solid content(20-30%,w/w)and dilution(0.008-0.012 h-1),the optimal conditions were determined as 25%solid content(w/w),fermentation temperature at 45℃ and dilution of 0.008 h"1 in three-cascade bioreactors with the L-lactic acid titer,yield,and productivity reached to 107.5±1.1 g/L,0.29 ± 0.01 g/g DM,and 0.89±0.01 g/L/h,and the chiral degree of lactic acid was 99.3%,there was no bacterial contamination in the fermentation process.Compared with continuous SSCF in a single-stage bioreactor,a higher lactate yield was obtained with lower solid content.This indicates that the strategy of multi-stage continuous SSCF can effectively improve the saccharification of substrate and obtain better fermentation results.Finally,the material balance of the whole process of bio-refining was calculated in this paper,based on the results of multi-stage continuous SSCF fermentation in the optimal mode.Without considering the residual sugar,the yield of lactic acid from the detoxification material was 74.3%,and the yield of lactic acid under the influence of removing residual sugar was 77.3%.The efficient continuous SSCF production process established in this study unified the saccharification and fermentation temperatures,and the lactic acid chiral purity,titer and productivity had a good fermentation performance,providing technical means for the continuous production of cellulosic lactic acid under real industrial scenarios.