First assessment of diffusion coefficients in model cheese by Fluorescence Recovery after Photobleaching (FRAP) analysis
Résumé
In cheese technology, mass transfer of solutes like salt, moisture and metabolites, is very important for the final quality of cheese, through the control of the brining and ripening process. Numerous studies have reported salt and water transfer in cheese, but very few papers have dealt with the mass transfer properties of other solutes in cheese. Most of the reported diffusion coefficients have been obtained by macroscopic and destructive methods. The objective of the study was to develop, for the first time, the FRAP technique that allows in situ measurements of diffusion properties at the microscopic scale inside cheese. The effect of the matrix microstructure on mass transfer properties of small solutes was also studied.
A model matrix based on ultrafiltrated milk, mimicking soft-type cheese, was used. Its structure was modified by adding gelatine and analysed by confocal microscopy and rheological measurements.
Two different sizes of FITC-dextran molecules (4 and 20 kDa) were chosen as models of small migrant solutes. Diffusion coefficients were estimated with a new modelling approach which allows to take into account diffusion of the molecules during the bleach phase.
The two FITC-dextrans were able to migrate in the model cheese network, but their mobility is reduced compared to water: diffusion coefficient values were equal to 62 9 µm²/s for the 4kDa and 26 3 µm²/s for the 20kDa dextran.
The composition of the matrix has a great influence on the mass transfer properties. The diffusion coefficients of the dextrans were reduced by a factor 3 in the model cheese with gelatin. Structural analysis showed that gelatin led to a matrix with a more “solid-like” rheological behaviour, with a more heterogeneous microstructure than the UF model cheese.
This study shows the power and the potentiality of the FRAP technique to study mass transfer properties of fluorescent solutes in complex food matrices such as cheeses