Characterization of the growth and sporulation behavior of Penicillium roquefortii in solid substrate fermentation by material and bioenergetic balances

When Penicillium roquefortii is grown on two kinds of buckwheat, exhibiting a low [0.85 g water/g dry matter (DM), buckwheat A] and a high [1.5 g water/g initial dry matter (IDM), buckwheat B] water content, a marked difference in the mode of development of the fungus is observed. Material balances for buckwheat A show that growth does not stop because of nutrient exhaustion. Analysis of water balance shows that active growth proceeds with a permanent limitation by the turgor potential which disappears when the water activity of the substrate is close to 0.96, thus arresting growth. This limitation causes intensive water excretion from the system due to the lowering of the water activity of the substrate. The water content of the mycelium thus decreases from 79% at the beginning of the cultivation to 74% when the growth stops. This is linked to a substantial oxidative metabolism and a high sporulation efficiency, close to 0.85. The spores obtained have a low dry weight and a reduced nitrogen content. In the case of buckwheat B, the active growth is shown to stop because of available mineral nitrogen depletion. No significant decrease in the water activity of the substrate is found during the protein synthesis, and the turgor potential remains high at the end of this period. Culture proceeds with new wall synthesis; the sporulation efficiency remains high and the spores obtained exhibit a high dry weight and a high nitrogen content. The bioenergetic balances show that the P/O ratio varies with the kind of substrate used; its value is close to 1.56 for the low water medium and to 2.21 for the high one. The ATP yield Z is always close to 1, and fungal development occurs with limitations of both anabolism and catabolism on buckwheat B and only of anabolism on buckwheat A.