A Preliminary Study On Pretreatment Of Corn Stover Via NaOH-Urea And Fermentation Production Of D-lactic Acid

The abundant straw resources are an important source of raw materials for the lignocellulosic biorefinery industry.Straw is mainly consist of cellulose,hemicellulose and lignin.Among them,the six-carbon sugars produced by cellulase hydrolysis of cellulose,and the five-carbon sugars produced by the enzymatic hydrolysis of hemicellulose can be used for fermentation.Applying straw as raw material to abstract ethanol,diol,locust glycolipid,lactic acid and other chemical products is straw bioconversion research hotspots.Not only can it improve the added value of straw,but also it is able to alleviate the waste of resources and environmental pollution caused by straw waste and burning.However,there are still many by-products in D-lactic acid production process,which leads to low straw saccharification rate and low yield.To solve the above problems,this this study first carried out sodium hydroxide-urea pretreatment of corn stover to improve the efficiency of enzymatic hydrolysis of cellulose.Then,the fermentation conditions of Lactobacillus bulgaricum were optimized,and then the acid-producing bacteria were immobilized,and the preliminary research on the process of producing D-lactic acid by fermentation of corn stover as raw material was carried out.The main findings were as follows:1.The best pretreatment process was obtained using sodium hydroxide-urea solution to treat corn stover: temperature 80°C,sodium hydroxide concentration 6%,Urea concentration 12%,corn stover mass fraction 9%,and treatment time 25 min.The pretreated corn stover showed 77.1% cellulose digestibility and 83.64% lignin removal after enzymatic digestion.Fourier infrared spectroscopy(FTIR),scanning electron microscopy(SEM),X-ray(XRD)and gas chromatography-mass spectrometry were used to prove that the lignin of corn stover was degraded after pretreatment,and the cellulose crystallinity increased.2.The optimal fermentation conditions were obtained by single-factor and orthogonal experiments using a positive mutant strain of Lactobacillus bulgaricum CICC21101 with a temperature of 41°C,an initial glucose concentration of 50 g/L,and an inoculum of 5%,under which the fermentation was carried out for 72 h.The yield of D-lactic acid reached 62.7±0.9 g/L with timely sugar replenishment in the middle of the fermentation.3.The microencapsulation of Lactobacillus bulgaricum was fixed by using low acyl gellan gum,so that the strain could be reused,and the results showed that the microcapsules would maintain good activity during the reuse process and could be reused at least three times.The acid production of three consecutive batches of fermentation medium in the carbon source of corn stover enzymatic digest was45.23±0.4 g/L,43.24±0.6 g/L and 37.96±2.9 g/L,respectively.