Solid-state culture of Geotrichum candidum and Penicillium camembertii on a glutamate and lactate based medium

Geotrichum candidum and Penicillium camembertii have been cultivated at the surface of a glutamate, lactate-based medium. Glutamate has been chosen since it is a convenient carbon source for both fungi, in addition to a nitrogen source. The surface growth of both fungi induced the diffusion of substrates from the core to the rind. However, significant substrate concentrations (glutamate and lactate) always remained at the top of the gel, showing the absence of diffusional limitation of growth, in addition to the absence of substrate limitation. An absence of diffusional limitation, when the medium contained peptones instead of glutamate, was also indirectly deduced from the comparison of both media. Indeed, peptones are too complex for a possible identification of diffusional limitation from gradients analysis. An inhibitory effect of pH limited growth: at the end of the linear growth (oxygen limitation), inhibitory pH values were observed at the surface of the medium, even if it was not the case for the mean pH values. Since growth was limited by the alkaline pH at the surface of the gel, to account for this inhibition, an additional term has been introduced in the expression for the partial linking between consumption or production and growth. The diffusion coefficients for glutamate, lactate and ammonium have been also considered. Then, the concentration gradients for glutamate, lactate and ammonium have been calculated (second Fick law) and found to match with the experimental gradients.