Lactic acid has been widely used in food and pharmaceutical industries. Recently, the worldwide demand has been increasing due to many new industrial applications. Lactic acid bacteria have been used in lactic acid production because of their high growth rate and product yield. However, the limitations including costly substrates and complicated product recovery make bacterial fermentation economically unattractive. In contrast, filamentous fungi such as Rhizopus oryzae can directly produce optically pure L(+)-lactic acid from carbohydrates present in agricultural residues and plant biomass; therefore, can overcome the problems in bacterial fermentation.
However, change and diversity of fungal morphology during fermentation cause many problems in reactor control and operation, and affect lactic acid production. In this research, fungal morphology was controlled by immobilization in a Rotating Fibrous Bed Bioreactor (RFBB). It was found that RFBB provided good morphological control and improved oxygen transfer resulting in increased lactic acid production, limited undesirable ethanol production, and stable long-term production in the RFBB.
Lactic acid production cost can be minimized by using low-value substrates derived from agricultural residues and plant biomass. The results showed that R. oryzae was capable of utilizing both starchy materials present in agricultural residues and pentose sugars derived from hemicellulose to produce lactic acid.
Process engineering techniques were used to improve lactic acid production. It was found that overgrown immobilized cells in the RFBB caused oxygen limitation and lowered lactic acid production. Oxygen limitation was prevented by increasing oxygen transfer rate using high aeration rate or supplying oxygen-enriched air. Controlling cell growth and biofilm thickness by shaving-off the fungal mycelia under high shear rates and limiting the nitrogen source in the medium was also studied.
To achieve controlled growth and immobilization of productive cells for stable long-term operation, spore germination and immobilization at the initial phase of the fermentation were studied. The effect of rotational speed on spore immobilization on different fibrous matrices was investigated. The mechanisms of spore immobilization on different fibrous matrices were elucidated.
The knowledge gained in these process engineering techniques is important to the development of the RFBB and its scale-up for lactic acid production from sugars.