Lactic acid production by immobilized Rhizopus oryzae in a rotating fibrous bed bioreactor

Filamentous fungi, Rhizopus oryzae, can produce the optically pure L(+)-lactic acid from complex carbohydrates present in agricultural residues and plant biomass without prior treatment.; In this research, fungal morphology was controlled by immobilization. Contrary to free cell culture in the stirred tank bioreactor, the fermentation carried out in a Rotating Fibrous Bed bioreactor (RFBB) resulted in good control of morphology, and improved oxygen transfer and lactic acid production from glucose. The improved oxygen transfer obtained in the RFBB not only increased lactic acid production rate, but also limited undesirable ethanol production and allowed the bioreactor to be operated for long-term production.; To minimize the production cost, the feasibility of using low-value substrates derived from agricultural residues and plant biomass was studied. It was found that R. oryzae was capable of utilizing both starchy materials present in agricultural residues and pentose sugars which were abundant in plant biomass. Lactic acid yields obtained from these substrates were comparable to the yield from glucose. However, the production rate obtained from fermentation of pentose and insoluble starch was lower than that obtained from fermentations of glucose and soluble starch because of the complicated pentose metabolism and poor oxygen transfer in the cultivation with insoluble starch.; Process engineering techniques were also used to improve lactic acid production in the RFBB. Previous study reported the critical demand of oxygen for lactic acid production. In this research, it was found that increasing oxygen transfer rate led to the increase in lactic acid productivity. Although high oxygen transfer rate was maintained, ethanol production and the estimation of the critical biofilm thickness indicated an anoxic condition in the overgrown immobilized fungal cells on the rotating fibrous matrix. In this study, growth of immobilized cells was controlled by shaving-off the mycelia with rotational shear rate and by limiting the concentration of the nitrogen source in the medium. (Abstract shortened by UMI.)...