Emulsions Theory, Rheology and Stability to Inversion

An understanding of the mode of action of the component ingredients of fat spreads during processing requires a fundamental knowledge of the theory, rheology and stability—especially to inversion—of emulsions. 

Liquid emulsions are inherently unstable to a varying degree. It is important to understand, therefore, the mechanisms that contribute to emulsion stability. Before the solidification step, instability of an emulsion can arise due to either phase separation or phase inversion (Mulder and Walstra, 1974). It is evident that the likelihood of phase inversion will increase as the fraction of dispersed phase increases. The vast majority of literature references are concerned with the stability to phase separation as coalescence or creaming in oil-in-water emulsions (Hailing, 1981 Jaynes, 1983). In addition, a method for determining the stability of water-in-oil emulsions to inversion has not been reported. It is usually assumed that certain aspects of oil-in-water emulsion theory apply in reverse to water-in-oil emulsions. 

Flexible macromolecules, such as proteins, and small molecules, such as surfactants, are amphipathic and may form a layer at the oil-water interface. These molecules may also partly stabilize emulsions by forming a physical barrier to close contact, thereby reducing the attractive van der Waals forces to ineffective levels (Dalgleish, 1989). Repulsion can arise in either of two ways and physico-chemical calculations are available for both mechanisms in oil-in-water systems. Either the approaching protein-coated particles will tend to compress or alternatively interpenetrate the adsorbed protein layer on adjacent particles. The optimum structure of the stabilizing protein will be dealt with in the section on protein as an ingredient. 

In a few instances, emulsions can be stabilized by solid particles as shown by S. U. Pickering in 1907 (see Petrowski, 1976). Examples are mustard seeds in traditional mayonnaise (Anon., 1968) or crystals of high melting-point triglycerides in margarine or butter (Precht and Buchheim, 1980 Heertje et al., 1987). The solid particles are considered to act as a barrier, which prevents the coalescence of droplets. Bancroft (1913) observed that the phase that wets the solid more easily will become the continuous phase. 

Traditional mayonnaise is an 80% oil-in-water emulsion, which may rely on mustard seeds for solid particle stabilization. Two constituents of egg yolk, lecithin and cholesterol, are surfactants, which promote the formation of oil-in-water and water-in-oil emulsions, respectively. The ratio of lecithin to cholesterol in egg yolk favors the water-in-oil type but the final emulsion type formed is due to the action of mustard seed, which favors an oil-in-water emulsion (Petrowski, 1976). 

---