Imaging of Fermented Dairy Products

Abstract

The spatial organisation of the structural components in gelled dairy products plays a crucial role in their textural and sensorial attributes as well as on their storage stability. Colloidal protein aggregates, fat globules, bacterial (exopolysaccharides) or added polysaccharides comprise the main structure-forming elements in fermented milk products. Noninvasive characterisation of the microstructure of dairy gels has been challenging due to high water content (up to 90 %) and the presence of milk fat with a low melting point. The structure is usually unstable and changes upon minimal shearing or temperature stress. Thus, microscopy techniques that allow visualisation of multicomponent systems in their hydrated or naturally preserved state are necessary for thorough structural analysis. Confocal laser scanning microscopy (CLSM) is one of the most widely used techniques for microstructural characterisation of dairy products as it allows analysis in undisturbed, fully hydrated state and optical sectioning. Electron microscopy techniques are complementary to CLSM and provide high-resolution imaging of nanoscaled and mesoscaled structures. For conventional scanning electron microscopy (SEM) and transmission electron microscopy (TEM), harsh sample preparation including fixation of fat, extensive cross-linking of proteins and slab dehydration are necessary and may cause artefacts in certain structural elements. Cryo- and environmental-electron microscopy, which allows observation of specimens without need of harsh fixation steps or staining, has been successfully applied to gelled dairy products. In this chapter, widely used techniques for imaging of the protein network, distribution of fat and other components in the structural matrix of fermented dairy products are reviewed.