Bioconversion Of Waste Cellulosic Material To Produce Lactic Acid

Fermentation of rice straw to produce lactic acid was studied in this paper. The purpose of the research is to produce lactic acid from renewable rice straw, alleviate the environmental pollution caused by combustion of rice straw and bring down the production cost of lactic acid. This paper contains three parts: (I) selection and breeding of cellulose-degrading microorganisms. (II) production of cellulase by liquid state fermentation of mixed two strains and solid state fermentation. (III) production of lactic acid by solid state fermentation and simultaneous saccharification and fermentation. The contents and main conclusions are as follows:(1) 12 strains of bacteria, 9 strains of actinomycete, 11 strains of fungi were isolated from the soil samples. Two strains with higher cellulose-degrading activity were finally obtained through primary and secondary screening test. One strain is bacterium, Cellulomonas; Another strain is fungus, Trichoderma viride.(2) Co-fermentation of above two strains were done to investigate the enzyme production conditions. Results showed the optimum temperature for producing FPase (Filter Paper Cellulase) and CMCase (Carboxyl Methyl Cellulase) was 30℃. The optimum initial pH of the media are different for production of FPase and CMCase with FPase at pH7.0 and CMCase at pH6.5, respectively. Addition of 0.5%(w/v) yeast extract benefited the production of both of FPase and CMCase. The highest FPase activity was obtained with addition of 0.2%(w/v) Tween 80, while for CMCase, addition of 0.4%(w/v) was best. Metal ions also have a significant effect on the enzyme production. Production of FPase and CMCase are affected strongly by Mg²⁺ and Ca²⁺, respectively.(3) To investigate the production of cellulases from Trichoderma viride, solid state fermentation was performed by using different ratios of rice straw (RS) and wheat bran (WB) as substrate. The mixed support inoculated with spores was incubated under static conditions for 6 days and the enzyme extracts obtained at different time intervals were analysed The results showed that the activities of FPase, CMCase andβ-glucosidase were significantly affected by substrate mixture. The maximum activity of FPase andβ-glucosidase were produced at 96 when the ratio of RS and WB is 3:2 and 1:4, respectively. While for CMCase, the highest activity was obtained at 72h with ratio of 4:1. Ratio of RS and WB also influence the patterns of extracellular protein concentration, reducing sugars and pH during the time course of cultivation.(4) In order to make full use of rice straw (RS) produced at large quantity in China and to reduce the production cost of _L(+)-lactic acid, Attempts were made to utilize the hydrolysate of RS as sole carbon source and the lignocellulose as inert support for producing _L(+)-lactic acid using solid state fermentation. The pretreated rice straw was enzymatically hydrolyzed by cellulase and hydrolysate containing reducing sugars supplemented with minimum (NH₄)₂SO₄, MnSO₄ and yeast extract was used as moistening agent to impregnate 5g of RS, which was used as inert support in solid state fermentation. Maximum _L(+)-lactic acid production of 3.467g/5g support was obtained at 37℃using Lactobacillus casei as inoculum after 5 days of fermentation with optimized process parameters such as 72% moisture content, 4 g/5g support of reducing sugars, 2.5ml/5g support of inoculum size, 0.5g/5g support of yeast extract 3g CaCO₃ and pH 6.5.(5) In the bioconversion of cellulosic materials to produce lactic acid, the enzymatic hydrolysis of cellulosic materials to produce reducing sugars is rate-limiting step. Higher enzyme loading are needed to obtain a higher product yield, which makes the large-scale utilization economically difficult. Addition of non-ionic surfactants and poly(ethylene glycol) (PEG) to the enzymatic hydrolysis of lignocellulosics has been found to increase the conversion of cellulose into fermentable sugars. We have found that adding 0.2g/g substrate of Tween 80 and PEG 6000 increased the sugar yield by 22% and 12% when enzyme loading at 10FPU/g at 24h, respectively. Tween 80 behaved better than PEG 6000 when different substrate concentration, temperature and enzyme loading were investigated. Tween 80 provides an opportunity to reduce enzyme dosage while still remain same extent of hydrolysis. We also investigated the effect of Tween 80 on the simultaneous saccharification and fermentation of pretreated rice straw to lactic acid. Results showed that addition of 0.7g/L Tween 80 improved the lactic acid production by 24% compared to the reference without Tween addition at 72h.