Growth and sporulation behaviour of Penicillium roquefortii in solid substrate fermentation: effect of the hydric parameters of the medium

Both growth and sporulation increase linearly according to the initial water content of the solid substrate when Penicillium roquefortii is cultivated on buckwheat seeds. This indicates that water is the limiting factor for fungal development since neither carbon nor nitrogen sources were exhausted during these experiments. This feature validates the concept of available water for fungal growth, defined as initial water content of substrate minus its residual water content when vegetative growth stops. An efficient methodology, based on drawing a regression line of mycelium dry weight production as a function of the initial water content of substrate is presented; it allows estimation of both available water and water content of mycelium. Results show that growth stops when the residual water in the substrate is close to 0.52 g H₂O/g initial dry matter, corresponding to a water activity (a_w) close to 0.96, and that the initial water content in mycelium is near 76%. Thus, both a_w and water content of the substrate have to be taken into account during the course of solid-state cultivations.Correspondence to: C. Larroche