Kinetics

The free energy change associated with emulsion formation determines whether or not an emulsion is thermodynamically stable, but it does not give any indication of the rate at which the properties of an emulsion change with time, the type of changes which occur, or the physical mechanism(s) responsible for these changes. Information about the time dependence of emulsion stability is particularly important to food scientists who need to create food products which retain their desirable properties for a sufficiently long time under a variety of different environmental conditions. For this reason, food scientists are usually more interested in the kinetic stability of emulsions, rather than their thermodynamic stability.

The importance of kinetic effects can be highlighted by comparing the long-term stability of emulsions with the same composition but with different droplet sizes. An emulsion which contains small droplets usually has a longer shelf life (greater kinetic stability) than one which contains large droplets, even though it is more thermodynamically unstable (because it has a larger interfacial area, A^).

Despite the fact that food emulsions exist in a thermodynamically unstable state, many of them remain kinetically stable (metastable) for months or even years. What is the origin of this kinetic stability? Conceptually, the kinetic stability of an emulsion can be attributed to an activation energy (AG*), which must be overcome before it can reach its most thermodynamically favorable state (Figure 7.2). An emulsion which is kinetically stable must have an activation energy which is significantly larger than the thermal energy of the system (kT). For most emulsions, an activation energy of about 20 kT is sufficient to provide long-term stability (Friberg 1997). In reality, emulsions have a number of different metastable states, and each of these has its own activation energy. Thus, an emulsion may move from one metastable state to another before finally reaching the most thermodynamically stable state. A change from one of these metastable states to another may be sufficient to have a deleterious effect on food quality.

The kinetic stability of emulsions can only be understood with reference to their dynamic nature. The droplets in an emulsion are in a continual state of motion and frequently collide into one another, due to their Brownian motion, gravity, or applied external forces. Whether droplets move apart, remain loosely associated with each other, or fuse together after a collision depends on the nature of the interactions between them. The kinetic stability of emulsions is therefore largely determined by the dynamics and interactions of the droplets

Kinetically Stable

Kinetically Stable

Separated Phases

FIGURE 7.2 Emulsions are thermodynamically unstable systems, but may exist in a metastable state and therefore be kinetically stable.

Separated Phases

FIGURE 7.2 Emulsions are thermodynamically unstable systems, but may exist in a metastable state and therefore be kinetically stable.

they contain. Consequently, a great deal of this chapter will be concerned with the nature of the interactions between droplets and the factors which determine their movement in emulsions.

Earlier it was mentioned that if pure oil and pure water are agitated together, a temporary emulsion is formed which rapidly reverts back to its individual components. This is because there is a very low activation energy between the emulsified and unemulsified states. To create an emulsion which is kinetically stable for a reasonably long period of time, it is necessary to have either an emulsifier or a thickening agent present that produces an activation energy that is sufficiently large to prevent instability. An emulsifier adsorbs to the surface of freshly formed droplets and forms a protective membrane which prevents them from merging together, while a thickening agent increases the viscosity of the continuous phase so that droplets collide with one another less frequently (Chapter 4). The role of emulsifiers and thickening agents in emulsion stability will therefore be another common theme of this chapter.

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