Efficient Production of L-(+)-Lactic Acid Using Mycelial Cotton-like Flocs of Rhizopus oryzae in an Air-Lift Bioreactor

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Abstract
L‐(+) ‐Lactic acid production was enhanced in a culture of Rhizopusoryzae 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.

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