Studies On Bioconversion Of Renewable Cellulosic Material To Produce Lactic Acid

Cellulosic material is the most abundant renewable carbon source in the world. Cellulose may be hydrolyzed using cellulase to produce glucose, and glucose can be used for lactic acid production. The utilization of renewable biomass can not only save the foodstuff but also reduce the environmental pollution.Since the production of cellulase was the major contribution to the bioconversion process, the submerged fermentation by Trichoderma reesei ZU~04 was studied. Using corn cob residue from xylose factory as the carbon source, the obtained cellulase activity was comparable with that using purified cellulose on the same cellulose basis. The bean cake was used as nitrogen source instead of peptone or yeast extract with the C/N ratio 10.0. The fermentation was scaled up in a 30 m3 stirred fermenter, and the activity of cellulase reached 8.16 FPIU/ml (331.7 FPIU/g cellulose) after 4 days. This research work greatly reduced the production cost, and laid a good foundation for industrial application.A high yield cellobiase producing strain (Aspergillus niger LR-12) was screened. Using solid-state fermentation, the activity of cellobiase could reach 438.3 CBIU/g dry koji under optimized conditions, and this result was very attractive compared with those reported in recent literature. It was found that the spores of this strain were rich in cellobiase. By entrapping the spores into calcium alginate gels, the cellobiase was immobilized efficiently. The entrapped spores would not germinate at hydrolysis temperature (50 C), but could release cellobiase slowly into the gel beads. Comparing with the traditional immobilization of pure enzyme protein, this new method was more convenient and economical. The activity of enzyme was not destroyed, and the immobilized cellobiase were quite stable with a long half-life. The immobilized cellobiase could accelerate the synergetic hydrolysis process of cellulosic biamass.During the saccharification of cellulosic material, the main influence factors including character and concentration of substrate, enzyme dosage, and composition of cellulase system were studied. Since the cellulase system from T. reesei was poor in cellobiase (CBA / FPA was 0.03), the hydrolysis yield of corn cob residue was only 67.4%. When cellobiase was added into the system to increase CBA/FPA to 0.42, the hydrolysis yield could be improved to 83.6%. Under the synergetic reaction of T. reesei cellulase and immobilized cellobiase, the yield of hydrolysis was raised to 88.2%.Lactic acid is an important organic acid and applied in broad fields. Using cellulose instead of starch for lactic acid production was significant for development of national economy. In this work, Lactobacillus delbrueckii was immobilized byentrapping the cells into calcium alginate gels. Comparing with free cells, the fermentation term of immobilized cells was shortened, and the yield of lactic acid was improved. The immobilized cells could utilize cellulosic hydrolysate to produce lactic acid efficiently, the yield of lactic acid reached 90.7%.A three-step coupling bioreactor was set up by coupling the cellulose hydrolysis, the immobilized cellobiase and the immobilized L. delbrueckii cells together. In this coupling bioreactor, the feedback inhibition to cellulase reaction caused by the accumulation of cellobiose and glucose was eliminated, the hydrolysis of cellulosic material was promoted, and the yield of lactic acid from cellulose reached 90.6%. The new reactor was stable and efficient, and the immobilized cellobiase and cells could be repeatedly used for a long time. Under fed-batch process, the final concentration of cellulosic substrate and lactic acid were increased to 200 g/L and 105.2 g/1, respectively. The utilization of cellulase and the productive efficiency of lactic acid were both unproved.Further, the spores of A. niger containing cellobiase and the cells of L. delbrueckii were entrapped together into calcium alginate gels to form a coimmobilized system. The coupling bioreactor with coimmobilized system showed a good p...