Study On Biological Pretreatment Of Corn Stalks And L-lactic Acid Fermentation Process

As one of the three major organic acids,lactic acid was widely used in food,medicine,chemical industry,textile,cosmetics and other fields.The demand for lactic acid has been increasing year by year.The cost of lactic acid was one of the main reasons for limiting its large-scale production.The use of starch and other raw materials for lactic acid would cause competition with people for grain,and starches required processes such as liquefaction and saccharification,which greatly increased the production cost.Lignocellulosic materials were used to produce lactic acid,and conventional methods used acid or alkali pretreatment,which not only produced a large amount of wastewater,but also produced many inhibitors such as furans and phenols,which would inhibit the subsequent enzymatic hydrolysis and fermentation process.The pretreatment method of straw biomass biological micro-storage pretreatment originated from silage,which is easy to operate and would not produce a large number of inhibitors.Bacillus coagulans could grow at(45?-50?),with low nutritional requirements,weak carbon metabolism inhibition,and homozygous fermentation to synthesize lactic acid with high optical purity.BCS could synthesize lactic acid with high optical purity by homogenous fermentation.In this study,the screened Bacillus coagulans BCS was used as the fermentation strain,while the biologically treated corn stover was used as the raw material to explore the lactic acid fermentation process.(1)Different concentrations of monosaccharide(glucose,xylose,arabinose)and mixed sugars were fermented on the screened Bacillus coagulans.BCS could metabolize hexose and pentose at the same time,while the fermentation capacity of sugar is that glucose was greater than xylose than arabinose.BCS performed well for relatively low concentration(<60g/L)sugar fermentation,no other by-products,and had good fermentation potential.When the initial glucose concentration was 49.36g/L and 64.53g/L,the lactate concentration was47.47g/L and 45.35g/L,the conversion rate reached 99%,and the lactate yield was 0.99g/g and 0.90g/g.When the initial sugar concentration was 40.87 g/L,the lactic acid concentration was 21.43 g/L,the conversion rate was 98%,and the yield was 0.4 g/g.When the ratio of60g/L mixed sugar(glucose:xylose)was(5:5),(7:3)and(9:1),the lactic acid yields were29.97g/L,35.23g/L,40.38g/L,respectively,the conversion rate>92%,the maximum yield(9:1)was 0.71g/g.(2)Mixed flora(Bacillus coagulans,Lactobacillus and Enterococcus)were added to corn stover for 7 weeks of microbial storage.The components of corn stover raw materials and 7-week micro-storage corn stover were determined.Scanning electron microscopy(SEM),Fourier transform infrared spectroscopy(FT-IR),X-ray diffraction analysis(XRD),and thermogravimetric analysis(TGA or DTG)were used to characterize straws with different microstorage times,and high-throughput sequencing was used to analyze the microbial community during micro-storage.Based on the changes of components and p H during the micro-storage process,the lactic acid content in the micro-storage material was 2.03%,the acetic acid content was 0.53%,and the p H was 4.47 in the 4th week.The straw biological treatment entered a stable period,and the 4th week was selected as the best micro-storage time.The scanning results of electron microscope showed that the straw structure changed obviously.The results of FT-IR,XRD and DTG showed that the content of hemicellulose,cellulose and lignin changed,while the internal structure of the straw was destroyed during the micro-storage process,which was beneficial to the subsequent saccharification and fermentation of lactic acid.After micro-storage,the microorganisms on the surface of corn stalk were analyzed,which the additional Enterococcus and Lactobacillus became the dominant flora.The relative abundance of fermented Enterococcus reached more than 50% in the first 3 weeks,and it was 10.62% in the 4th week.The relative abundance of the Lactobacillus reached a maximum of 44.90% in the fourth week.High-throughput sequencing showed that the added exogenous bacterial species gradually evolved into a dominant population,producing lactic acid and acetic acid,inhibiting the growth of harmful bacteria in the raw material,and destroying the structure of the raw material,which was conducive to subsequent enzymatic hydrolysis and fermentation.(3)Enzymatic hydrolysis and fermentation were carried out on the raw materials stored for 4 weeks.The substrate concentration was controlled at 5%,while the enzymatic hydrolysis rate could reach more than 96%,which was better than the acid-base pretreatment method.The concentration and yield of lactic acid obtained by final fermentation were18.54g/L and 0.86g/g,while the optical purity was over 97%.It was proved that the pretreatment method of anaerobic micro-storage was feasible in biomass utilization and conversion,which provided data support for the biological pretreatment of corn stover to convert fermented lactic acid.