Solid-state fermentation of stale bread by an edible fungus in a semi-continuous plug-flow bioreactor

Abstract

Stale bread, one of the major food wastes, holds great potential to produce new food products by solid-state fermentation using the edible filamentous fungus Neurospora intermedia. Yet, this process is limited to small-scale batch production. In this study, a plug-flow bioreactor was developed and successfully operated semi-continuously without addition of external inoculum. Two critical process parameters namely residence time and inoculum-to-substrate ratio (ISR) were studied in two experimental set-ups, 48 h and ISR 10:65 or 24 h and 20:55. The fermentation performance was assessed by the CO2 evolution rate and starch degradation in the substrate. Both experiments resulted in a relatively stable CO2 evolution rate up to 10 days and the starch content was reduced from 65 % in bread to 40.6 % and 43.2 %, respectively, in the product. Performance of the plug-flow bioreactor was compared to batch fermentation in tray bioreactors using the same ratio of inoculum. No significant differences of the final starch content were observed between the bioreactors, which indicate improved productivity by the semi-continuous process compared to the batch. The fungal biomass yield was calculated to range from 0.12 to 0.5 mol Cbiomass/mole Csubstrate. A material balance of the process revealed that 220 g dry bread and 200 ml water were required to produce 400 g fermented product with a fungal biomass content of 15 % in 24 h.