Biochemical engineering of solid-state fermentation

Important progress has been made recently in improving our understanding of SSF. We have developed a model-based scale-up strategy for packed-bed reactors. This strategy has been experimentally validated for the fungusC. minitans, an effective biological agent for pest control in many food crops, and is currently extended toA. oryzae, an important food fermentation organism. Mixed fermenters allow improved simultaneous control of temperature and moisture content. We have successfully cultivated several fungi in continuously mixed fermenters. For slow-growing fungi wall cooling offers sufficient perspectives for scale-up; for fast growers evaporative cooling and water addition are indispensable. A model was used to estimate dry weight, intracellular water and extracellular water in the solid substrate from on-line measurements, and to calculate the required water addition. Behaviour of solid-state fermentation at the particle level is still poorly understood. We present experimental evidence for oxygen concentration gradients in fungal mats growing on solid substrates and show the consequences. Aerial hyphae turned out to be very important for respiration ofA. oryzae in SSF. Gradients in water content and glucose concentration in solid substrates were determined using NMR.