Lactic acid production by Rhizopus oryzae in submerged systems
Abstract
Interest in lactic acid has been increasing because it has the potential to be one of the raw materials for the manufacture of the environmentally benign polymers. For the purpose of polymerization, the production of optically pure lactic acid is highly preferred. However, synthetic methods and most bacterial fermentation currently used can only produce mixtures of DL-lactic acids. Rhizopus oryzae has the ability to produce optically pure L(+)-lactic acid with high productivity. The potential usage in polymers makes the fungal fermentation a viable alternative for the production of optically pure lactic acid with high purity. The objectives of the research are to investigate the physiology of R. oryzae and to develop processes for lactic acid fungal fermentation. One major problem arises with most fungal fermentation. Cells usually grow as clumps or large pellets which may result in nutrition- and oxygen-limitation inside them. Changing the carbon source from glucose to xylose in the cultivation medium allows R. oryzae to grow as small and uniform pellets with a size of about 1 mm. The growth pattern of R. oryzae is unique with two growth phases; one of them follows the exponential law, and the other follows the cubic root rule. A modified Monod equation with a substrate inhibition term can successfully characterize both phases. Among the factors studied, undissociated lactic acid is the most important in affecting lactic acid production. Neutralization or continuous removal of the undissociated lactic acid is essential to improve the fermentation. Poly 4-vinylpyridine is known to adsorb undissociated lactic acid but not lactate. With the simultaneous lactic acid adsorption by the PVP resin, the fermentation can be performed as well as pH controlled at 5 $\sim$ 6 but under low pHs. Undissociated lactic acid can be regenerated by the elution of methanol, ethanol, or hot water after the fermentation.
Degree
Ph.D.
Advisors
Tsao, Purdue University.
Subject Area
Chemical engineering
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