S41 Mayonnaise - Emulsion

Topic

This mayonnaise is a highly concentrated oil-in-water emulsion, stabilized primarily by emulsifying components of the egg yolk—including lecithin, which is the subject of another model in this set. Due to the high oil fraction, the dispersed droplets are packed very closely together and may deform under compression. As this oil fraction approaches maximum packing, inter-droplet interactions intensify sharply, giving the mayonnaise its characteristic semi-solid consistency.

Droplet size depends on the interplay between the forces generated during mixing and the interfacial tension that tends to hold the droplets together—a relationship described in another model in this set via the Weber number. The Kolmogorov-Hinze model describes how increased energy dissipation can break droplets down into smaller sizes. Efficient transfer of mixing energy to the emulsion generally results in smaller droplets, promoting a fine texture and slowing the rate of gravitational separation.

Viscosity increases sharply with the oil fraction. The Krieger-Dougherty model describes how viscosity can rise dramatically as the droplet concentration approaches the limit of effective packing. Furthermore, mayonnaise exhibits non-Newtonian behavior that can be modeled using equations such as the Herschel-Bulkley model: a yield stress must be overcome before flow begins, and once in motion, the apparent viscosity decreases as the shear rate increases. As the oil fraction moves closer to the packing limit, both viscosity and flow resistance increase.

Stability against gravitational separation also depends on droplet size and the high viscosity of the continuous phase, in accordance with principles related to Stokes' law. Small droplets and a viscous medium significantly reduce the rate of creaming. Mayonnaise can thus remain kinetically stable for long periods, even though—like other emulsions—it does not represent a state of thermodynamic equilibrium. Electrostatic and steric interactions provided by emulsifiers adsorbed at the interfaces also help prevent droplets from coming into contact and coalescing.

Mayonnaise can break (separate) when the formation or stabilization of new droplets is no longer sufficiently rapid. Adding oil too quickly can create regions with a local excess of oil and an interfacial surface area that the available emulsifiers cannot stabilize. Temperature, yolk composition, pH, and the proportions of water and oil also affect emulsion stability. A common way to recover a broken emulsion is to first form a new, stable emulsion using a small aqueous phase and egg yolk (or another emulsifier), and then gradually incorporate the broken mayonnaise.

Finally, even an initially stable emulsion can slowly change over time through processes such as flocculation and coalescence. The rate of these processes depends on the barriers hindering droplet approach, which are determined by electrostatic, steric, and attractive interactions. In a simplified description based on DLVO concepts, increasing the energy barrier against droplet contact reduces the probability of aggregation and coalescence, thereby extending the stability of the mayonnaise.

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