| Summary: | 碩士 === 國立成功大學 === 化學工程學系 === 104 === In recent years, extensive usage of fossil-fuel-based plastics has left an undesirable impact on the environment and caused accelerated global energy depletion. Microbial production of bio-plastics is being looked upon as the best alternative, since both the production process and the end product are eco-friendly.
Lactic acid has received increasing attention all over the world, due to its widespread applications in cosmetic, food, pharmaceutical and chemical industries. In particular, lactic acid has been extensively used for the synthesis of bio-based plastics, acting as the monomer of polylactic acid (PLA), which is the most promising biodegradable and environmentally friendly material on market. Lactic acid can be produced by microbial fermentation, but higher production costs are always a concern. Enhancement of lactic acid production efficiency and reduction of substrate costs are important issues that needs to be addressed to produce lactic acid by microbial fermentation.
In this study, the Lactobacillus plantarum 23 was used to produce lactic acid. The cells were used as suspended cells and the experiment was conducted in batch mode. Anaerobic culture conditions, pH value of 5.5 and glucose concentration of 40 g/l were selected as the optimal conditions and the lactate production, yield, and productivity were 26.46 g/l, 0.66 g/g, and 4.28 g/l/h, respectively. The strategy of immobilized cells was used to enhance lactic acid fermentation by Lb. plantarum 23. Lactic acid production (37.93 g/l), yield (0.91 g/g), and productivity (4.96 g/l/h) were enhanced 1.43, 1.39, and 1.12 times, respectively, compared with fermentation using suspended cells. A cell concentration of 5.25 g/l in Poly Vinyl Alcohol (PVA) for immobilization and particle loading of 12.5% were selected as optimal condition for PVA-immobilized cells fermentation. Carbon sources other than glucose, like xylose and sucrose were tested for lactic acid production in batch fermentation using PVA immobilized cells. The results indicated that using sucrose as carbon source gives high productivity (6.39 g/l/h) compared to other sugars. To reduce the cost of modified MRS medium and enhance the rate of lactic acid fermentation, the use of diluted modified MRS medium was investigated. 50% dilution of modified MRS medium had comparable productivity with the undiluted medium and was decided as optimum medium composition.
Two fed-batch strategies were used for lactic acid production, including residual glucose concentration control and cyclic glucose concentration feeding. The lactic acid produced from immobilized cells by cyclic glucose concentration feeding could produce higher lactic acid concentration of 108 g/l. In-situ removal of lactic acid by ion-exchange resin could alleviate the problem of end-product inhibition and enhance the rate of fermentation and conversion of glucose to lactic acid.
Continuous lactic acid production was conducted to achieve higher productivity. Continuous production with particle loading of 12.5% and HRT of 4 h had lactic acid production of 23.17 g/l, yield of 0.90 g/g, and productivity of 5.79 g/l/h. Increasing the particle loading from 12.5% to 50 % had higher production of 31.75 g/l, yield of 0.93 g/g and productivity of 7.94 g/l/h with glucose consumption of 96.62%. Continuous production was efficiently improved by 50% of particle loading and could be stably operated for more than three months. It is important that lactic acid has to be produced from cheaper feedstock to reduce substrate costs of fermentation. In this direction, microalgal biomass based reducing sugars were used for lactic acid fermentation. The lactic acid yield was close to the theoretical yield of 99%, and productivity was enhanced to 9.93 g/l/h. The results showed that microalgae biomass as feedstock is advantageous, because of reduced production cost, enhanced lactic acid fermentation and it has the potential to replace refined sugars in such fermentation processes.
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