Abstract Geotrichum candidum and Penicillium camem- bertii were cultivated on the surface of a gelified medi- um, simulating the composition of the aqueous phase of a Camembert cheese. The relation of their growth with sub- strate consumption (carbon or nitrogen), metabolite pro- duction (ammonia), or proton transfer (deduced from pH by means of the buffer capacity of the medium) was ex- amined. The coefficients associated with cellular biosyn- thesis and resulting from cellular maintenance were de- termined. From these coefficients and the considered sub- strate 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 deami- nation of carbon- and nitrogen-source amino acids, in close relation with peptone consumption. Introduction The ripening of Camembert cheese is under tight control of the growth of two fungal populations: Penicillium camembertii and Geotrichum candidum (Fox et al. 1993; Gripon 1993; Amrane and Prigent 1997a). Therefore, the concentration of viable biomass is undoubtedly one of the key factors for the monitoring and control of cheese ripening. For this purpose, counting cfu appears to be the most useful method. However, this method is convenient only for use with cells associated in regular clusters, which is obviously not the case for filamentous micro- organisms like P. camembertii and G. candidum (Lange et al. 1995; Molimard et al. 1995). In the absence of a simple way of measuring viable biomass, total biomass determination appears to be a useful alternative. Total biomass can be deduced from the dry weight measurement of the thin layer of biomass obtained at the surface of solid cultures (Amrane and Prigent 1997a). This method has been validated on geli- fied media, but its transposition to the real system (curd), which is obviously rather more complex (Gaucheron et al. 1999), would be very tedious (Molimard et al. 1994). Therefore, the development of indirect methods for bio- mass measurement are of particular interest. With this aim in mind, some results have been previously obtained in liquid cultures: ● A turbidimetric device for the on-line monitoring of growth of filamentous microorganisms has been suc- cessfully tested (Amrane and Prigent 1998). ● A linear relationship between growth of G. candidum and P. camembertii and the amount of protons trans- ferred has been previously shown (Amrane et al. 1999). ● A model for the reconstruction of the biomass history from carbon dioxide emission has also been recently validated (Couriol et al. 2001). This model is based on a partial association between growth and carbon dioxide production. Growth kinetics are then recon- structed after identification of the coefficients for growth- and non-growth-associated production, re- spectively. To our knowledge, no similar results are available in solid cultures. For this reason, and due to the complexity of the curd during ripening, to gain a better understand- ing of the fundamental phenomena, we studied pure cultures of each of the two fungi (P. camembertii and G. candidum) on the surface of a gelified medium simu- M. Aldarf · A. Amrane ( ✉ ) · Y. Prigent Laboratoire des Procédés de Séparation (UPRES EA 3191, UC INRA), Université de Rennes 1, Campus de Beaulieu, Bât. 10A, CS 74205, 35042 Rennes Cedex, France e-mail: abdeltif.amrane@univ-rennes1.fr Tel.: +33-02-99282952, Fax: +33-02-99282957 Appl Microbiol Biotechnol (2002) 58:823–829 DOI 10.1007/s00253-002-0955-2 ORIGINAL PAPER M. Aldarf · A. Amrane · Y. Prigent 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 Received: 5 November 2001 / Revised: 22 January 2002 / Accepted: 25 January 2002 / Published online: 7 March 2002 © Springer-Verlag 2002