DIFFUSION OF CALCIUM AND INORGANIC PHOSPHATE AT THE SURFACE OF A SOLID MODEL MEDIUM IN RELATION WITH GROWTH OFGEOTRICHUM CANDIDUMANDPENICILLIUM CAMEMBERTII

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ABSTRACT Solid-state cultures of Geotrichum candidum or Penicillium camembertii were carried out on peptones and lactate to simulate the aqueous phase of Camembert cheeses. Phosphate and calcium were also added into the medium to examine their migration in the gel. Their consumption by the cells induced concentration gradients that were maximum during exponential growth and became nil at the end of growth; however, significant amount of both components always remained near the top of the gel, demonstrating the absence of mass transfer limitation. A significant part of phosphate and calcium, more than 50% for cultures carried out at 14C (i.e., close to the ripening temperature), precipitate at the surface of the gel under its dicalcic form, because of medium alkalinization. Precipitation rate was too low to induce concentration gradients; however, phosphocalcic precipitation continued during stationary state, following alkalinization resulting from energy substrate assimilation, lactate and peptides, leading to ammonium production after amino acids deamination. PRACTICAL APPLICATIONS Among other things and to simulate the aqueous phase of Camembert cheeses, the solid model medium considered in this work contained phosphate and calcium, which precipitate at the surface of the gel. It was shown that dry weight of the superficial layer can be corrected from phosphocalcic precipitate to deduce total biomass concentrations. The procedure for biomass determination would be helpfully applied to the real medium, lactic curd during ripening. On the considered medium, the precipitate was found to be mainly dicalcic phosphate. However, because of the complexity of the calcium phosphate chemistry, additional analytical works have to be carried out to more precisely examine the structure of the phosphocalcic precipitate and its evolution in relation with the pH and the temperature, and subsequently in relation with the complex casein network.