Production of Ammonium L-Lactate by Culture of Rhizopus in an Air-lift Bioreactor

• Main Authors: Chih-yu Liu, 劉芝妤
• Other Authors: Dey-chyi Sheu
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/37684427918499060014

碩士 === 大同大學 === 生物工程學系(所) === 97 === Interest in L-lactic acid production has increased due to its ability to serve as raw material for the manufacture of biodegradable and biocompatible polymers. Traditionally, L-lactic acid is produced through the fermentation of glucose or starch with Rhizopus strains, using calcium carbonate as neutralizing agent. In the downstream process for the purification of lactic acid, large amount of calcium sulfate, a solid waste, is formed. When calcium carbonate, the neutralizing agent during the fermentation, is replaced by ammonia water, the product is mainly ammonium L-lactate and the formation of calcium sulfate during the purification of lactic acid is avoided. Ammonia can serves as a nitrogen source for the fungus. Furthermore, ammonia may be recovered from ammonium L-lactate by means of ion exchange or electro-dialysis. Ammonia is toxic to the fungus, leading to a decreased production of lactic acid. Firstly, the main component in ammonia water is ammonium hydroxide, which is caustic and thereby inactivates germs and even viruses. Secondly, high concentration of ammonium ions in the fermentation medium might reduce the bioactivity of the fungus, slowing down the formation of lactic acid. In this work, L-lactic acid fermentation was carried out in an 3.5-L air-lift bioreactor by Rhizopus species including Rhizopus arrhizus BCRC 33071 and Rhizopus oryzae ATCC 9363. A mixture of ammonium bicarbonate, urea and ammonia water served as the neutralization agent through a porous glass device. Both ammonium bicarbonate and urea provide buffer effect against ammonia water, reducing the damage of ammonium hydroxide on fungal cells. Urea was converted into ammonia and carbon dioxide through the catalysis of urease on fungal cells. Optimum production of L-lactic acid was achieved when the fermentation was controlled at pH 5.25. After a 32-h fermentation with R. oryzae or a 34-h fermentation with R. arrhizus, final concentration of lactic acid at 92 g/L or 102 g/L was obtained, respectively.