A novel honeycomb matrix for cell immobilization to enhance lactic acid production by Rhizopus oryzae

A new support matrix inspired by honeycomb was developed for cell immobilization to control fungal morphology and enhance mass transfer in bioreactor for lactic acid production by Rhizopus oryzae. The immobilization matrix composed of asterisk-shaped fibrous matrices in a honeycomb configuration provided high surface areas for cell attachment and biofilm growth. More than 90% of inoculated spores were adsorbed onto the matrices within 6–8 h and after 10 h there was no suspended cell in the fermentation broth, indicating a 100% immobilization efficiency. Compared to free-cell fermentation, lactic acid production increased ∼70% (49.5 g/L vs. 29.3 g/L) and fermentation time reduced 33% (48 h vs. 72 h) in shake-flasks with 80 g/L initial glucose. The immobilized-cell fermentation was evaluated for its long-term performance in a bubble-column bioreactor operated in a repeated batch mode for nine cycles in 36 days. The highest lactic acid production was 68.8 g/L, corresponding to a volumetric productivity of 0.72 g/L h and 93.4% (w/w) lactic acid yield from consumed glucose. The overall yield and productivity were 77.6% and 0.57 g/L h, respectively. The fermentation can be improved by increasing aeration and mixing in the bubble-column bioreactor.