We develop a method for predicting the moisture content of the fermenting solids in an intermittently mixed packed-bed bioreactor on the basis of measurements of the inlet and outlet gas stream temperatures and the assumption that both the gas streams are saturated with water vapor. This method can be used to estimate when an intermittently mixed packed bed should be mixed and how much water should be added during the mixing event. The predictions were experimentally verified for the growth of Aspergillus niger on wheat bran in a thin packed bed. We show that the application of a controller based on our estimation method would have led to adequate moisture control, with significant errors in moisture content prediction only occurring after the last mixing and water-addition event. Our method therefore represents a simple and practical method that can be incorporated into control schemes for intermittently mixed solid-state fermentation bioreactors with forced aeration. (c) 2006 Elsevier Ltd. All rights reserved.