ABSTRACT
The rational design and fabrication of an emulsion-based food product with specic physicochemical and sensory properties relies on understanding the characteristics of the different kinds of molecules* within the product, as well as the nature of the interactions between them (Bishop et al. 2009, Israelachvili 2011). The various molecules within an emulsion may form a variety of different structures depending on their interactions with each other (Figure 2.1). Molecules may be part of a bulk phase (which may be solid or liquid) where they are surrounded by similar types of molecule, for example, oil molecules in an oil phase. Molecules may be part of a regular solution where they are surrounded by a mixture of similar and dissimilar molecules that are randomly organized, for example, sugar molecules dissolved in water. Molecules may be part of an ordered solution where they are preferentially surrounded by dissimilar molecules that tend to have a specic organization, such as mineral ions dissolved in water that are surrounded by a shell of highly organized water molecules. Molecules may accumulate at an interface between two phases, such as surfactants at an oil-water interface. Molecules may associate with similar or dissimilar molecules and form molecular clusters dispersed within a bulk phase, such as surfactant micelles in water. Molecules may be incorporated into a 3D network that extends throughout the system and gives it some solid-like characteristics, such as gelatin molecules within a hydrogel. Finally, molecules may be part of complex biological structures, such as phospholipid molecules within cell membranes. The overall physicochemical properties of food emulsions ultimately depend on the nature, properties, and interactions of the structures formed by the molecules they contain, for example, separate phases, interfaces, clusters, and networks. The structural organization of a particular set of molecules is determined by the forces that act between them, as well as the environmental conditions, such as temperature and pressure. From a thermodynamic viewpoint, a certain arrangement of molecules may have the lowest free energy since it is the best comprise that maximizes the number of favorable interactions, minimizes the number of unfavorable interactions, and maximizes the various entropy contributions of the system. Nevertheless, foods are rarely in their most thermodynamically stable state and, therefore, the structural organization of the molecules is often governed by kinetic factors that prevent them from reaching the arrangement with the lowest free energy (Section 1.2.1). For this reason, the structural organization of the molecules in foods is often determined by their previous history, that is, the temperatures, pressures, gravitational forces, and applied mechanical forces that they experienced during their lifetime. To understand, predict, and control the behavior of food emulsions, it is important to be aware of the origin and nature of the forces responsible for holding the molecules together, the factors that impact these forces, and how these forces lead to the various types of structures formed in food emulsions. Only then will it be possible to rationally design foods that have internal structures that are known to be advantageous to food quality. The purpose of this chapter is to give a brief overview of the major types of molecular forces and entropy effects important in materials, and to show how these factors inuence the conformation and structural organization of molecules.
