Model-based bioreactor selection for large-scale solid-state cultivation of Coniothyrium minitans spores on oats

Non-mixed and mixed SSF reactors were evaluated for their applicability in large-scale spore production of the biocontrol fungus Coniothyrium minitans. The major problem to overcome in large-scale SSF is heat accumulation. Testing various cooling strategies in large-scale bioreactors would be very expensive and time consuming, therefore lab experiments in combination with mathematical simulations were used instead. The metabolic heat production rate, estimated from the oxygen consumption rate of C. minitans on oats in Erlenmeyer flasks, was about 500 Watt per m3 bed. Conductive cooling in packed-bed reactors was insufficient to cool large reactor volumes (radius > 0.2 m). The poor thermal conductivity of the bed (λb = 0.1 W m−2 K−1) resulted in steep radial temperature profiles. Adequate temperature control could be achieved with forced aeration, but concomitant water losses lead to significant shrinkage of the oats (30%) and critically low water activities, even though the bed was assumed to be aerated with water saturated air. Mixed systems, however, allowed heat removal without the need of evaporative cooling. Simulations showed that large volumes could be cooled via the wall at low mixing intensities and small temperature driving forces. Experimental studies showed no detrimental effect of mixing on spore production by C. minitans. The spore production yield in a continuously mixed scraped-drum reactor (0.2 rpm) was 5 × 1012 spores per kg dry oats after 450 hours. Based on the scale-up potential of the mixed system and the absence of detrimental mixing effects it is believed that a mixed bioreactor is superior to a non-mixed system for large-scale production of C. minitans spores.