Diffusion of solutes in dairy systems as affected by system microstructure and physicochemical characteristics of solutes

Diffusion phenomena are essential to control the quality of dairy products, and are still misunderstood for solutes other than salt and water. The aim of this thesis was then to investigate how solute diffusion is influenced by the microstructure of different dairy systems and by the physicochemical characteristics of solutes. These dairy systems presented the same casein concentration but their microstructure was modulated by different technological treatments such as heat-treatment and rennet-induced coagulation of protein milk concentrates. An automated image analysis procedure was developed to determine microstructural parameters of dairy systems from Transmission Electron Microscopy (TEM) images, obtained using cryo-preparation methods. Model solutes such as dextrans, proteins and peptides were used for their properties of shape (flexible/rigid) and charge (neutral/negative/positive) in a wide range of molecular weights (from 4 to 2000 kDa). Their diffusion coefficients were determined by the Fluorescence Recovery After Photobleaching (FRAP) technique. The diffusion phenomena were explained by microstructural parameters of dairy systems, such as pore size and interface area between solutes and the surrounding matrix. However, the influence of the physicochemical characteristics of solutes on diffusion was much more pronounced than that of the system microstructure. Moreover, we observed that the shape of solutes influenced more their diffusion than the electrostatic interactions due to their charge. On this basis, mathematical models could be developed to predict diffusion behavior from the microstructural parameters of dairy systems and the physicochemical characteristics of solutes