The effect of wall cooling in tray-type solid-state fermenters was investigated by cultivation of Aspergillus niger ATCC 10 864 on wheat bran. Temperature, moisture, pH and glucoamylase activity in the bed were monitored. Application of the heat exchanger plate reduced the overall water loss but increased the heterogeneity of moisture distribution in the bed. Moisture content increased markedly close to the heat transfer plate while the upper regions of the bed dried out. The thermal conductivity of the bed was measured to be 0.19 W m(-1) K-1. At these conditions and depending on the air flow rate, 58-82% of the metabolic heat was removed via conduction towards the plate. A mathematical model was developed to describe the behaviour of the system. Model predictions were in good agreement with the experimental results. Diffusion coefficients of water vapour and oxygen in the bed were estimated to be around 1.3 x 10(-5) m(2) s(-1) and 1 x 10(-5) m(2) s(-1) respectively. Based on these values the mathematical model describes the way oxygen and water vapour diffusion influences biomass growth and moisture distribution in the bed. (C) 2004 Society of Chemical Industry.