ABSTRACT The fungi Geotrichum candidum and Penicillium camembertii play an important role in the ripening of Camembert-type cheeses. However, the monitoring of fungal biomass appears difficult due to their mycelial growth. The development of indirect methods to monitor growth may be therefore highly relevant. An unstructured model was therefore proposed to predict microbial growth based on carbon or nitrogen substrate consumption, ammonia or carbon dioxide production and proton transfer. The model was developed and validated in liquid culture, since an on-line monitoring of biomass concentration, deduced from turbidimetric measurement, was available. The model is based on a partial association of substrates consumption or metabolites release with growth; the contributions of the considered kinetics to biosynthesis and cellular maintenance can be then deduced from the model. In solid culture the prediction of microbial growth deduced from CO2 evolution may be especially useful since it constitutes an on-line and non-destructive method to monitor fungal growth in solid culture. Stimulation of ammonia and flavour gas emission by the addition of glutamic acid and methionine (1 g l-1 each) to the medium showed that such a gaseous emission took place at the end of the growth phase of G. candidum, while no significant emission of ammonia and sulphur gas was observed during the growth of P. camembertii. For the media and strains used, ammonia and volatile sulphur compounds emissions showed unequivocally the growth of Geotrichum candidum. These results may be helpful in view of the differentiation of each fungal population in case of mixed cultures as it was the case in the curd during ripening
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