Food Structure and Health
Structured Emulsions and interfaces
- Describe the interfacial and molecular interaction processes responsible for the formation of the salivary mucous film, and identify structural changes resulting from interaction with food components.
- Identify key colloidal and interfacial mechanisms in vitro that underpin the complex rheological properties of emulsions.
- Define the key interfacial mechanisms that control the digestion of emulsions and subsequent transport of lipids and lipid soluble nutrients.
The rising incidence of obesity demands that rational strategies be developed to reduce dietary fat and energy intake. We are facilitating the development of such strategies by investigating in vitro some key interfacial and colloidal mechanisms underlying the physicochemical behaviour of emulsified fats in the GI tract. This knowledge will enable new strategies to be developed to control dietary fat intake, for example by manipulation of interfacial behaviour to affect fat digestion.
Our research is focussed around the following key questions:
- What is the physical structure of the salivary film, and how does it change upon interactions with food structures, affecting the digestibility of food?
- What are the critical, interfacially-mediated processes that enhance the rheology of protein-stabilised emulsions?
- What are the key interfacial mechanisms that control lipolysis?
Saliva contains many proteins and components that we have already shown form complex, structured films on surfaces. This is in part due to formation of an incredibly strong, viscoelastic interfacial film, which completely dominates the physical properties of the salivary film as a whole. This complex structure is driven by molecular scale interactions and is thought to be responsible for the lubricating properties of saliva. Interactions of saliva with food during processing in the mouth will affect its behaviour as it passes into the stomach and interacts with digestive secretions. The physical properties of the lubricative layer it forms around food structures may indeed affect the rate and extent of digestive processes including phase behaviour in the gastric compartment. It also plays an important role in how we perceive food structures as components such as proteins and emulsifiers can affect the lubrication properties of saliva. There is a need for an accurate physico-chemical model of saliva, as current artificial and simulated saliva does not mimic the complex surface properties of human saliva, and human saliva is physically very unstable and difficult to handle. Such a model could facilitate the development of more accurate physicochemical simulations of oral and gastric processing that will compliment current digestion models.
The physico-chemical properties of interfaces can have a profound effect on rheology and microstructure of emulsions. Specifically, proteins behave differently to surfactants and emulsifiers at interfaces by forming a visco-elastic, heterogeneous, interfacial layer, which can change the interactions between emulsion droplets, thus changing the rheology of the emulsion as a whole. Only by understanding these fundamental processes properly can we reach a more holistic description of the environmental responses of protein-stabilised emulsified foods. Our approach has always been to understand how interactions at a molecular scale can be used to manipulate macroscopic events, resulting in marked changes in bulk rheology and "consistency". We have shown recently how protein-stabilised emulsion droplets result in emulsions that are more viscoelastic than those stabilised by surfactants. This explains why changing only the interfacial composition of emulsion droplets controls the sensory perception of fat content. This is purely an interfacial effect, hence the potential applications are powerful and widespread, but the mechanisms are not entirely clear. Such knowledge is required if we are to tailor interfacial design to optimise functionality. We are investigating new or modified mathematical models describing interactions between emulsion droplets, and the subsequent changes in emulsion structure and rheology. This will enhance our understanding of how emulsion structure not only influences sensory properties, but also how changes in environment in the GI tract could influence macroscopic changes in emulsion structure and hence influence digestion processes. These models will be used to design experimental systems to optimise the interfacial impact on emulsion rheology and to examine the impact of interfacial design on the sensory perception of fat content. This knowledge will also provide information on interactions of emulsions with other structures such as the mucus layer, the salivary film, other food components, and also during lipolysis and so enable us to fully understand the entire process of emulsion breakdown and digestion, and its subsequent impact on human diet and health.
Lipid digestion is a complex, multistage process which is focussed on events at the interface between lipid and aqueous phases in the gut lumen. Initially, lipase and colipase have to adsorb onto the lipid surface to initiate lipid hydrolysis (lipolysis); this process is dependent on the interfacial composition. Lipolysis then takes place at the interface where reaction products accumulate. Finally, these lipolysis products are transported from the interface to the micellar phase, probably through the action of bile salts and other bio-surfactants present in the gut lumen. This is crucial to avoid product inhibition of lipase. The specific mechanisms involved in this transport, including the role of the initial interfacial composition and bio-surfactants, such as bile salts and phospholipids, are not entirely clear. Understanding the interfacial processes that control lipolysis are pivotal to engineering the interfaces of emulsified fats to optimise lipid digestion and transport of lipid-soluble nutrients. For example, slowing lipid digestion can promote satiety by increasing the concentration of lipolysis products in the distal ileum, which induces a gastrointestinal satiety feedback mechanism known as the ileal brake. The aim of our research is to determine how interfacial structures control lipase activity by creating and characterising interfaces with desired composition and/or physical properties. Another aspect of lipid digestion which is poorly understood is the selective transport of lipids and lipid-soluble nutrients from the oil phase into the luminal micellar phase. This is thought to be mediated by the displacement of lipolysis products from the interface by bile salts and other bio-surfactants, and their formation into so called mixed micelles, to facilitate uptake by the gut epithelium. We plan to combine in-vitro lipid digestion with MS-based lipidomics approaches to determine at what stages of digestion certain lipophilic compounds are transported from the oil phase interface, the role of bile salts and other bio-surfactants and the trigger mechanisms for transport (internal concentration, interfacial packing). This knowledge will enable us to develop rational approaches for controlling lipolysis to reduce hyperlipidaemia and for improved delivery of lipid-soluble nutrients. The knowledge obtained will also be applied to studying release from novel delivery devices such as multiple emulsions, where triggered release mechanisms are unknown.