Improved continuous fumaric acid production with immobilised Rhizopus oryzae by implementation of a revised nitrogen control strategy

A novel fermentation system was employed whereby the mycelial mat of Rhizopus oryzae was attached to a polypropylene tube. Batch operation was used for growth, while continuous operation was employed during the fumaric acid production phase. A clear decrease in respiration, fumaric acid (FA) and ethanol production was observed when zero nitrogen was fed in the production phase, with FA productivity decreasing from an initial 0.7 g L−1 h−1 to 0.3 g L−1 h−1 after 150 h. With the addition of 0.625 mg L−1 h−1 of urea FA productivity dropped to only 0.4 g L−1 h−1 after 150 h and 0.3 g L−1 h−1 after 400 h. Under these conditions it was observed that the ethanol production rate decreased 20 times faster compared with the FA production rate, therefore resulting in high FA yields towards the end of the fermentation (instantaneous 0.96 g g−1 and average 0.81 g g−1 after 400 h). Increasing the urea feed rate to 1.875 mg L−1 h−1 resulted in a clear increase in FA production and respiration rates. This condition also resulted in a 25% increase in biomass after 150 h, while the decline in the ethanol production rate was seven times lower than in the 0.625 mg L−1 h−1 urea fermentation, resulting in lower FA yields.