Influence of Casein and Milk Phospholipid Emulsifiers on the Digestion and Self-Assembled Structures of Milk Lipids

Interfacial compositions of fat globules modulate the digestion behaviour of milk triglycerides in the gastrointestinal tract, thereby affecting lipid metabolism and delivery of nutrients. In this study, we aim to understand the impact of emulsifiers on lipid digestibility and the self-assembled liquid crystal structures formed by anhydrous milk fat (AMF) during digestion. AMF was emulsified with casein and milk phospholipids, and digestion was performed in both gastric and small intestinal conditions to account for changes at the oil/water interface following enzymatic digestion in the gastric phase. Small angle X-ray scattering was used to characterise the self-assembled structures of the digestion products, while coherent anti-Stokes Raman scattering microscopy was utilised to probe changes in lipid distribution at the single droplet level during digestion. Our findings confirmed that emulsifiers play a key role in the digestion of AMF. Milk phospholipids exhibited a protective effect on milk triglycerides against pancreatic lipase digestion by slowing digestion, but this effect was slightly negated in emulsions pre-digested under gastric conditions. The overall types of liquid crystal structures formed after digestion of casein- and milk phospholipids-emulsified AMF were comparable to commercial bovine milk irrespective of gastric pre-treatment. However, emulsification of AMF with milk phospholipids resulted in changes in the microstructures of the liquid crystal phases, suggesting potential interactions between the digested products of the fat globules and milk phospholipids. This study highlights the importance of emulsifiers in regulating lipid digestion behaviour and lipid self-assembly during digestion.