Reconstruction of the biomass history from carbon and nitrogen substrate consumption, ammonia release and proton transfer during solid cultures of Geotrichum candidum and Penicillium camembertii

Geotrichum candidum and Penicillium camembertii were cultivated on the surface of a gelified medium, simulating the composition of the aqueous phase of a Camembert cheese. The relation of their growth with substrate consumption (carbon or nitrogen), metabolite production (ammonia), or proton transfer (deduced from pH by means of the buffer capacity of the medium) was examined. The coefficients associated with cellular biosynthesis and resulting from cellular maintenance were determined. From these coefficients and the considered substrate utilization or metabolite production kinetics, the growth kinetics were reconstructed until the end of growth. The model allowed analysis of biosynthesis and cellular maintenance contributions to the considered kinetics. At the end of growth, almost all the peptone was used for G. candidum biosynthesis, while most of the lactic acid (62%) was used for cellular maintenance. P camembertii metabolized fewer amino acids as carbon sources, resulting in use of peptone for maintenance (12%), and lactic acid (80%) for cell biosynthesis. For both microorganisms, ammonia production was growth-associated, since this production resulted from the deamination of carbon- and nitrogen-source amino acids, in close relation with peptone consumption.