Label-free imaging of milk lipid digestion using synchrotron FTIR micro-spectroscopy

Understanding the behaviour of lipids during digestion of food and formulations can enable more rational design of lipid-based materials to enhance the oral delivery of drugs and nutrients. Traditionally, light microscopy and electron microscopy have been used to 'watch' digestion at the droplet level, while small angle X-ray scattering has more recently given structural insights at the ensemble level, however none of these techniques provide information on local changes in chemical composition of a digesting fat droplet. In this study, we used synchrotron Fourier transform infrared microspectroscopy (s-FTIRM) as a chemical imaging technique to map the distributions of milk components and their digestion products in individual fat globules to obtain a better understanding of the interfacial characteristics and chemical specificity behind the structural changes that occur during digestion. The results showed localisation of fatty acids at the core and the oil/water interface of digesting fat globules and the presence of fatty acid-calcium soaps, which exist both as isolated aggregates and are partially accumulated at the interface. This proof-of-concept study highlights the novel application of s-FTIRM as a label-free, chemically-specific imaging tool to resolve the spatial distribution of digestion products, such as free fatty acids and fatty acid-calcium soaps, within and around individual milk fat globules. This approach provides new insights into the localised chemical changes during lipid digestion at the single droplet level.