Correlative effect of dissolved oxygen and key enzyme inhibitors responsible for l-lactate production by immobilized Rhizopus oryzae NRRL395 cultivated in a static bed bioreactor

Abstract

In typical fermentation by Rhizopus oryzae, ethanol was found as the major byproduct. The simplest way to limit ethanol was to supply high dissolved oxygen (DO) during fermentation. Our previous work showed metabolic fluxes shifted toward lactate production with lower ethanol formation in the fermentation containing enzyme inhibitors including 1,2-diazole, 2,2,2-trifluoroethanol, 4-methylpyrazole, and 3-hydroxypyruvate. In this study, we further investigated the dynamic responses of the living R. oryzae cultivated in the static bed bioreactor to the effects of enzyme inhibitors and DO levels together. Either increasing DO level or adding inhibitor alone limited ethanol and promoted lactate production while cell biomass remained unchanged. Lactate yield was increased by 24% with the increasing DO from 40% to 80% during the fermentation. Among 4 inhibitors studied, 2,2,2-trifluoroethanol gave the maximum improvement in fermentation efficiency. Approximately, 32% increasing lactate yield was obtained in the fermentation at 40% DO with 0.1mM 2,2,2-trifluoroethanol. Together, inhibitor and high DO level better controlled metabolic flux toward lactate production. Dramatic increasing lactate yield of 54% was obtained in the fermentation containing 2,2,2-trifluoroethanol at 80% DO control with 100% improved productivity. Findings from this work exhibited the simple yet effective way to improve lactate fermentation by R. oryzae.