| Summary: | 碩士 === 國立中興大學 === 食品暨應用生物科技學系 === 97 === Lactic acid is a versatile chemical, used as an acidulant flavor and preservative in the food, pharmaceutical, leather and textile industries, and for polymerization to biodegradable polylactide(PLA) that is degraded to nontoxic molecules, carbon dioxide and water. Lactic acid exists as two optical isomers, D-lactic acid and L-lactic acid. PLA with different properties can be produced depending on the composition of optical isomers. It is mainly produced by fermentation with lactic acid bacteria, whereas through fermentation, both D-(-) and L-(+) isomers are produced. Thus, it is necessary to confirm the ratio of lactic acid isomers in the ferment.
All fermentations were carried out in 5 L-fermentor with automatic pH, temperature and agitator controller. In this study, Lactobacillus casei is used to produce lactic acid from yeast extract and glucose by batch or fed-batch fermentation, and investigate the change of the components of ferment during fermentation. Besides, in order to reduce the production cost of lactic acid, we also use soybean extract and molasses as nitrogen and carbon source, respectively, and investigate the change of the components of ferment during batch fermentation.
The results show that initial glucose concentration of 130 g/L, the final lactic acid concentration can achieve 96.04 g/L during 36 hours by batch fermentation (the yield is 81.61%); and it takes 84 hours to achieve 139.97 g/L by fed-batch fermentation (the yield is 78.82%). The results show that using low cost material, soybean extract and molasses, as nutrient source, the carbohydrate in molasses (glucose, fructose and sucrose) can be used, and produced lactic acid of 43.53 g/L. It can arrive the stationary phase of growth curve at the 10th hours (the plate count was approximately 9.5 log CFU/mL) and maintained for 42 hours.
The inhibitory effect of substrate can be overcome by fed-batch fermentation. In the end, fed-batch fermentation is still limited by end-product inhibition, result in the stop of lactic acid fermentation. To modify this restriction, we try to dilute the ferment with new substrate (glucose concentration of 50 g/L) when the lye was stopped adding into the fermentor. The results show that diluting the ferment can modify the end-product inhibition and enhance the effective fermentation processing. Finally, using low cost material, soybean extract and molasses, as nutrient source can supply the nutrients that is used to the growth of Lactobacillus casei.
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