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Viscoelastic Properties

As a semi-solid, mayonnaise has an extremely high viscosity and because of this, its flow properties have been studied extensively. Shear stress is an important term when discussing liquids and solids at any viscosity and is defined as the force per unit area that is required to drag one layer of substance passed another layer. Rheology is the study of science that deals with the flow and deformation of matter and is an umbrella for a few of mayonnaise’s properties. One of which includes yield stress which can be defined as a minimum shear stress required to initiate flow. So, with all of these terms now defined, one can look at the specific properties that mayonnaise possesses. Mayonnaise has a high shear stress with a typical yield stress around 100 [[Pascal (unit)|Pascals). For reference, ketchup has a yield stress of about 15 Pascals. With such a high yield stress, mayonnaise is able to resist low forces and even return to its original conformation. A simple equation can be written to explain the relationship between these terms:

τ = τ₀ + kγⁿ

In this equation, τ is equal to the shear stress, τ₀ is equal to the yield stress, γ is the shear rate and k & n are model parameters that influence the shape and curvature of the stress/rate curve. Mayonnaise happens to be a Bingham fluid where k is equal to the plasticity constant and n is equal to 1. This equation is in the form of y=mx+b and thus produces a straight line. In more laymen’s terms, the yield stress is the tipping point for conformational change in the mayonnaise after initial force is applied and it is held constant.

Freezing Mayonnaise

Mayonnaise is an oil in water emulsion which is stable at room temperature because it reaches phase equilibrium. At freezing temperatures, the structures inside of mayonnaise undergo crystallization which is bad news depending on the type of emulsion. Butter is a water in oil emulsion with the water phase having a higher melting pointfreezing point than oil. In the freezer, the small water droplets that are suspended in the continuous oil phase freeze in their specific locations while the rest of the oil stays in place. When the butter is needed, one only needs to take it out of the freezer and thaw it for the general properties of butter return as the water droplets melt in their respective locations. Mayonnaise is different; when oil in water emulsions are frozen, the continuous phase is water which freezes and allows the oil droplets to flocculate. This whole process causes a phase separation between the water and the oil. Once ice crystals start to form in the continuous water phase, they begin to inhibit the lecithin and phospholipids from working. These two emulsifying agents lose their functionality due to [[dehydration|dehydration]: water is crystallizing with itself and is leaving these agents “out to dry.” The oil droplets begin to flocculate without the emulsifying agents keeping them separated. At water’s freezing point, this is enough to destabilize the mayonnaise emulsion but at even lower temperatures, the fused oil droplets reach their freezing points and fat crystal nucleation begins. Mayonnaise therefore, should not be stored at frozen temperatures due to their thermodynamic instability. Freezing of the water phase in mayonnaise is catastrophic to the stability and shelf-life of the entire product.