Efficient production of L-(+)-lactic acid using mycelial cotton-like flocs of Rhizopus oryzae in an air-lift bioreactor.

L-(+)-Lactic acid production was enhanced in a culture of Rhizopus oryzae by induction of a mycelial flocs morphology. By conventional culture the morphology of R. oryzae is that of a pellet-like cake; however, when mineral support and poly(ethylene oxide) are added to the culture, the morphology of R. oryzae takes on a cotton-like appearance. The formation of these cotton-like mycelial flocs was induced by the addition of 5 ppm poly(ethylene oxide) into a 12-14 h culture containing 3 g/L of the mineral support before the formation of the conventional pellet morphology. The cotton-like flocs were also formed in cultures grown in an air-lift bioreactor. This morphology allowed effective mass transfer inside the flocs and effective fluidity of culture broth in an air-lift bioreactor. L-(+)-Lactic acid concentration produced by mycelial flocs in an air-lift bioreactor, with the support and poly(ethylene oxide), was 104.6 g/L with a yield of 0.87 using 120 g/L of glucose as the substrate; for this culture without both, the concentration was 43.2 g/L. These results demonstrate that cotton-like mycelial flocs are the optimal morphology for use in the air-lift bioreactor culture of R. oryzae.