Behaviour of protein-stabilised emulsions under various physiological conditions

Emulsion forms a major part of many processed food formulations. During the past few decades, the physico-chemical properties of oil-in-water emulsions under various food processing conditions have been extensively studied. However, over the recent years, interest has turned to understanding the behaviour of emulsions during consumption, i.e. physiological processing. In general, on ingestion, an emulsion is exposed to a relatively narrow range of physical (e.g. shear and temperature) and biochemical (e.g. dilution, pH, pepsin, pancreatin, mucins and bile salts) environments as it passes through the mouth into the stomach and then the intestines. There is currently limited knowledge of the physico-chemical and structural changes, which an emulsion may undergo when it passes through the physiologically active regime. A better understanding of the gastro-intestinal processing of emulsions would allow manipulation of physico-chemical and interfacial properties to modulate lipid ingestion, improve bioavailability of lipid soluble nutrients and reduce absorption of saturated fats, cholesterol and trans fats.Food emulsions are commonly stabilised by proteins, as they are not only excellent emulsifiers but also provide nutritional benefits to the product. The effects of digestion conditions on interfacial protein structures are complicated because of potential breakdown of these structures by proteolytic enzymes of the gastrointestinal tract. Studies dealing directly with the behaviour of protein-based emulsions under digestion conditions are very limited. This paper provides an overview of the behaviour of oil-in-water emulsions stabilised with globular proteins, namely lactoferrin and β-lactoglobulin. Recent advances in understanding the interactions between interfacial proteins on oil droplets and various physiological materials (e.g. enzymes and bile salts) in in vitro digestion systems are considered. Major emphasis is placed on the recent work carried out in our laboratory at Massey University on the behaviour of milk protein based emulsions (lactoferrin or β-lactoglobulin) during their passage through the gastro-intestinal tract.

Graphical AbstractSchematic illustration of the possible changes in emulsions as they pass through the in vitro physiological model (reproduced with the permission of Elsevier Inc. [2]).Figure optionsDownload as PowerPoint slideResearch Highlights
► The behaviour of protein-stabilised emulsions under oral conditions depends largely on initial charge on the emulsion droplets, and salivary mucins play a crucial role in droplet flocculation.
► Under gastric conditions, rather than the droplet charge, it is the nature of the protein interfacial layer and its susceptibility to pepsin that drives the emulsion destabilisation process. Globular proteins after adsorption onto the droplet surface appear to be more susceptible to pepsin than in their native state.
► Under the intestinal conditions, initial droplet charge and composition of protein adsorbed layers play an insignificant role, as most of the adsorbed protein/peptide layer is further hydrolysed by trypsin and chymotrypsin. Emulsions appear to be stabilised by the adsorption of bile salts under these conditions.
► Pancreatic lipase cleaves triglycerides to form 2-monglycerides and fatty acids; some of these digestion products are surface active and complete with bile salts and peptides for the droplet surface.