An emulsion is a macroscopic dispersion of two liquids, one of which forms the continuous part of the system (Friberg, 1976). Proteins as emulsifiers form a thick film (skin) around the surface of emulsified droplets. Such a film is usually of anisotropic character and its viscoelastic properties mainly determine the extent of emulsion stability against coalescence. The emulsion stability of food systems is, in general, a function of oil volume fraction, oil type, temperature, pH, type and amount of emulsifier(s), and the presence of counter ions and legends (e.g. salts, sugars, and carbohydrates).
Lobo and Wasan (1989) examined the stability of soybean oil/water emulsions, stabilized by a protein and a nonionic emulsifier, along with the interfacial rheological properties and dynamic thin film behavior. The protein was found to cause a large increase in the interfacial viscosity that inhibits film drainage and, hence, stabilizes the emulsions. On the other hand, emulsions containing the nonionic emulsifier, which does not affect the interfacial viscosity appreciably, were also found to be stable. Draining thin films formed from such a nonionic micellar solution was found to exhibit sharp thickness transitions, which is indicative of the stratification phenomenon. With such observations, they proposed a mechanism of emulsion stability, that is, due to the presence of long-range structural forces in thin films over the distance of the order of 1000 Å and the formation of colloid crystal-like structures within the film.
Das and Kinsella (1990) studied the adsorption behavior of heat-treated β-lactoglobulin at the oil/water emulsion interface and the coalescence stability of the emulsion droplets. They found that heating of β-lactoglobulin increased its surface hydrophobicity probably due to uncoiling. The film strength increased with decreased stability of the compact structure of β-lactoglobulin. Adsorbed proteins, in the multilayer film, were loosely bound and showed no significant contribution to coalescence stability. Surface hydrophobicity and film strength showed strong correlation with coalescence stability. The results indicated that conformational instability of β-lactoglobulin was more important than film thickness in controlling droplet coalescence.
Kiosseoglou and Mourlidis (1991) examined the influence of minor surface-active lipids occurring in virgin olive oil. They found that such lipids enhance the interfacial area per milliliter of oil created during the emulsification by bovine serum albumin (BSA). These components adsorb at the newly formed oil/water interface, bring about a marked decrease in the interfacial tension value and aid, together with adsorbed protein molecules, in the formation and stabilization of emulsion oil droplets.
Lee, McCarthy, and Dungan (1998) examined emulsion droplet formation and destabilization of both Tween 20- and β-lactoglobulin-stabilized emulsions, using Nuclear Magnetic Resonance (NMR) technique. The effect of surfactant type, surfactant concentration, pH, and ionic strength on droplet sizes of a 40-wt% octane/water emulsion was studied. They found that addition of the low-molecular-weight Tween 20 forms finer emulsion droplets than does addition of the protein, and that the Tween 20 emulsion is sensitive to surfactant concentration below a threshold ‘saturation’ concentration, unlike the case of protein-stabilized emulsion.
It has been observed in our laboratory that conductivity curves obtained for egg white-stabilized oil/water emulsions followed neither Langmuir nor Fruendlich behavior, which are usually observed when using the conductivity method for determining emulsion stability (Kato et al., 1985 and Suttiprasit et al., 1993). This paper develops a new and more general model to fit such new observed behavior. Accordingly, a new definition for emulsion stability is proposed. It is worth mentioning here that the words emulsifying activity or emulsion stability used in this text refer to emulsifying ability and short-term stability against coalescence, respectively.