Quantitative Microstructure Analysis of Casein Network Dynamics using STED Microscopy with Relation to Macroscopic Gel Properties

Abstract

Casein proteins are the main structural element in many fermented dairy products. Understanding how protein functionality is affected during modern food processing, such as filtration and spray drying, is key to product and process optimization. Advances in Super Resolution Microscopy have provided novel tools to investigate protein microstructures. Herein, Stimulated Emission Depletion (STED) Microscopy has been used to image casein micelle aggregation, gel formation and network coarsening over time. Protein structures have been resolved to > 100 nm. Native skim milk and reconstituted skim milk powder were used to produce gelled systems. Gelation was induced either through rennet addition and acidification with GDL. Images have quantitatively analyzed to monitor the development of casein network over time, and the fractal dimensions of the different systems have been determined. Label free Coherent Anti-Stokes Raman Spectroscopy (CARS) Microscopy was used to verify that the presence of fluorophore did not interfere with the gelation process during STED microscopy. Image data has been directly correlated with dynamic texture analysis.We demonstrate the ability to resolve casein micelle structures on the scale of a single micelle, and extract valuable information that directly relates to the macroscopic properties of a gelled system. This work provides the basis for the use of STED microscopy to analyze other colloidal structures. We present a framework to extract meaningful information that directly relates to the macroscopic properties and functionality of the system.