Résumé
The apparent diffusivity (Da) of water in various intermediate moisture model gels containing 10% (w/w dry weight) agar-agar and 90% (w/w dry weight) humectant (or 70% saccharose and 20% NaCl, proteins or fatty material) was evaluated. The determination of Da was achieved at three water contents (20, 37.5 and 52% wet weight) by using the drying curves (50.degree. C) of plane sheets of the model gels. The migration of water in the model gels occurred by molecular diffusion. Da (50.degree. C) strongly decreases with the water content following a linear relation, in a range of water contents from 20 to 52% (wet weight). Increasing the gel rigidity (by increasing the agar concentration or by adding gelatin) or adding fatty material resulted in a significant decrease in diffusivity. On the other hand, the nature, molar mass and physical state (liquid or solid at the temperature of the measurement) of the humectant only slightly affected the Da value. The apparent activation energy of water diffusion in a model gel of saccharose is independent of the water content and is equal to 21.5 kJ mol.