| Summary: | 碩士 === 國立中興大學 === 生命科學系所 === 95 === Lactic acid (LA) and its derivatives are versatile chemicals used in food, pharmaceutical, leather, and polylactic acid (PLA) plastics industries. Because of different optical rotation , lactic acid can be divided into L- lactic acid and D-lactic acid. Polylactic acid (PLA) could be synthesized by polyreaction of D- and L- lactic acid. PLA is a renewable , biodegradable plastic that can be degraded by environmental microorganisms. We can offer it different physical characteristics for specific application by altering the ratio of L- lactic acid and D-lactic acid isomers. Usually lactic acid is produced from the fermentation of starch by lactic acid bacteria , then followed by steps of extraction and purification. The cost of raw material and optical isomers isolation are the main reasons that cause the high price of PLA at present time. If lactic acid could be produced from organic waste material , the producing cost of PLA will be effectively reduced and the problems of handling these organic waste material will be solved simultaneously. Lactobacillus pentosus is a bacterium that can carry out either homo- or hetero-lactic acid fermentation, it produces large amount of lactic acid from hexoses, or produces lactic acid、ethanol and acetic acid from pentoses. In this study, L. pentosus was used to test the possibility of producing lactic acid from a regenerated resource, the wastewater sludge of paper mill. For growing as co-culture, a few Clostridium isolates, which can help to degraded cellulose in compost and sludge, and Bacillus amyloliquefaciens I, which can consume oxygen to make an anaerobic environment for lactobacilli and clostridia, were also inoculated into the culture. Experimental results indicated that the maximal lactic acid production was 2.69 g/L when L. pentosus was grown in a 10-ml batch culture with 0.2 g/ml sludge for 4 days. When L. pentosus was co-cultured with Clostridium M1, the production of lactic acid was increased to 1.35 g/L (including 0.6 g/L D-lactic acid and 0.76 g/L L-lactic acid) after a growth of 34 hours and existed only as L-(+) isomer (0.55 g/L) after 108 hours. When L. pentosus was co-cultured with Clostridium M1 and Bacillus Ⅰ, the production of lactic acid was 0.9 g/L (including 0.36 g/L D-lactic acid and 0.54 g/L L-lactic acid) after a growth of 48 hours and existed only as L-(+) isomer (0.83 g/L) after 155 hours. These results may be helpful to establish a L-lactic acid producing system without optical isomers purification. When nitrogen source supplemented to growth medium, the co-culture of L. pentosus 、Clostridium M1 and Bacillus Ⅰcould product 4.5 g/L of lactic acid after a growth of 48 hours, L. pentosus and the co-culture of L. pentosus and Bacillus Ⅰalso produced 4.8 g/L and 4.6 g/L of lactic acid, respectively, after a growth of 48 hours. All of these concentration of lactic acid were three times as much as those with out nitrogen source. These results also indicated that the wastewater
sludge of paper mill lacks nitrogen source, therefore, perhaps organic waste material rich in nitrogen source could be used together with the wastewater sludge of paper mill to produce lactic acid.
In order to produce pure L- lactic acid and D-lactic acid isomers separately, this study also tried to destroy the d-ldh and l-ldh genes of L. pentosus, respectively. This experiment is still in process at present.
|
|---|
