Study On Lactic Acid Production From Inedible Biomass By Bacillus Coagulans

Lactic acid(LA)was widely used in food,pharmaceutical,cosmetic,textile and chemical areas,and in recent years the production of biodegradable polylactic acid has increased the demand for industrial LA.Microbial fermentation was the main method for industrial LA production,but the cost of raw materials and fermentation process were the main factors that restricted the large-scale biosynthesis of LA.In this study,Bacillus coagulans IPE22 was used as the starting strain,and inedible biomass was used as raw material to study the law and mechanism of open LA fermentation.(1)For starchy biomass,the mechanism of LA fermentation by B.coagulans IPE22 was firstly studied using soluble starch as carbon source.It was found that IPE22 strain can secrete amylase(29.47 mU/mg)and synthesize LA though the productivity is low(0.80 g/(L h)).Furthermore,aiming at the cumbersome operation caused by the inconsistent temperature of enzymatic hydrolysis and fermentation in the conventional LA production process,various amylases were adopted to carry out fermentations with different enzymatic hydrolysis strategies.It was found that one step simultaneous liquefaction,saccharification and fermentation(SLSF)with the addition of mesothermal a-amylase and glucoamylase was the optimal mode with conversion rate,yield and productivity of 89.77%,0.99 g/g and 1.72 g/(L h)respectively.Finally,inedible starchy biomass(cassava and sorghum flours)were used instead of soluble starch to carry out one-step open LA fermentation,and no significant difference was found.At the same time,abnormal phenomenon of decreased LA yield and increased by-products accumulation appeared occasionally during open LA fermentation from starch without the addition of exogenous enzyme,which was mainly caused by the contamination of B.thermoamylovorans and B.licheniformis.In order to avoid the microbial contamination during open fermentation,systematic research was carried out and different contamination prevention and control strategies were developed.The results showed that the addition of glucose or sodium lactate(10 g/L)at the initial of open fermentation reduced the accumulation of by-products by 45.15%and 60.81%respectively.However,the co-fermentation of newly isolated B.thermoamylovorans 48-3 with B.coagulans IPE22 was the most effective strategy,which not only reduced 95.90%by-products formation also increased the LA yield(0.90 g/g)by 11.11%(compared to single strain fermentation(0.81 g/g)).(2)Although starchy inedible biomass could be used to produce LA by one-step open fermentation,some of them traditionally considered inedible have been currently developed as food.Thus,seeking other non-starch inedible biomass was necessary.Lignocellulosic materials attracted widely attention in recent years,however the ratio of hexose(glucose)to pentose(xylose and arabinose)of different lignocellulosic hydrolysates(H:P)varied greatly(0.08-6.50),and their effects on LA production were unclear still.In this study,sugars(glucose,xylose and arabinose)of 60 g/L were utilized as carbon source separately,and results proved that glucose tended to promote LA production while xylose preferred to enhance cell growth.Then,fermentations of simulated sugar mixtures(60 g/L)with various H:P ratios were studied.With the increase of pentose content,maximal cell density(13.10 to 7.61)and LA productivity(3.08 to 2.05 g/(L h))decreased,while similar LA concentrations(50 g/L)were obtained.At last,corn cob hydrolysate with 60 g/L sugars was used to produce LA,and its productivity(2.97 g/(L·h))was higher than that of single glucose utilization(2.34 g/(L·h)).In all,lignocellulosic biomass derived from various sources could be applied to produce LA,and they were good alternatives of fermentation substrates.In addition,corncob acid hydrolysate neutralized by sodium hydroxide contained a large amount of sodium sulfate(2%),which inhibited the growth of B.coagulans.In this study,the hydrolysate was desalted by bipolar membrane electrodialysis(BMED),and the removal rates of sulfate and sodium ions were 79.07%-86.31%and 84.63%-90.90%respectively.The acid solution recovered from desalination by BMED was recycled through acid hydrolysis of corn cob,and a total of 37.02%sulfuric acid was saved after four recycles.Therefore,inorganic salt inhibitors were removed effectively by BMED technology,and the acid and alkali recovered from salt inhibitors were recycled meanwhile,which reduced pollution generation and resource waste in the whole process.In summary,the one step open SLSF process for starchy biomass and the open fermentation and acid hydrolysate integrated utilization process for lignocellulosic biomass were both established in this study,which was of great significance for open LA fermentation.