Introduction

An emulsion is a mixture of two immiscible liquids in which the dispersed phase (larger particles) exists as small droplets distributed throughout the continuous phase (similar to a matrix).

Types of Emulsion in Cooking

We can find two types of emulsions:

Unstable emulsions, where an aqueous and an oily liquid are mixed, and after whisking, if left to stand for a while, the phases eventually separate. An example would be vinaigrettes.

Stable emulsions involve not only a liquid and a fatty element but also an emulsifying agent that reduces the surface tension between both phases, allowing closer contact. The most common example is mayonnaise, where the emulsifying agent is the lecithin in egg yolk that stabilises the micelles.

To achieve an emulsion, agitation is necessary to provide energy to the liquid with higher surface tension, forming small droplets.

• Fat-in-water emulsion (O/W): milk, cream, mayonnaise – stabilised with water-soluble compounds (phospholipids and proteins which act as emulsifiers).
• Water-in-fat emulsion (W/O): butter, margarines – stabilised with fat-soluble substances such as cholesterol.
• Complex emulsions (multiple phases): ice cream, pastries.

The salt added to mayonnaise helps stabilise the emulsion by maintaining repulsive forces between micelles.

Emulsion Destabilisation

It is very common in practice for emulsions to break down, which can happen for several reasons:

• Sedimentation. Due to gravity, the phases tend to separate based on their density. This is common in vinaigrettes.
• Flocculation. Occurs when micelles come close together to form aggregates, which move as a unit. This is caused by inappropriate electrostatic charges.
• Coalescence. Involves the breaking of the film surrounding the micelles, which then merge and cause the emulsion to break.

When making mayonnaise, if too little oil is used, the result may be fluid because the micelles have too much space to move around.

Emulsifiers are substances that contain both polar and non-polar groups. The polar groups orient towards the aqueous phase, while the non-polar ones orient towards the oil phase.

Examples of emulsifying agents include lecithin (which has a polar head and a non-polar tail). It incorporates oil into the micelle, with the outer part remaining polar. Cholesterol is used in water/fat emulsions, and mustard has a similar effect in vinaigrettes.

Proteins and starch are stabilising agents in emulsions; they form barriers between dispersed droplets and aid in maintaining stability.

It is very important to have a sufficient quantity of emulsifying agent, and several factors influence emulsion stability:

• Acids: Ingredients such as lemon juice or vinegar help lower the pH and increase the number of negative charges. As they also add water, the mayonnaise becomes more fluid.
• Temperature: A low temperature increases the viscosity of mayonnaise by hindering micelle mobility. This is why it is important to use eggs at room temperature.
• Salt: Increases emulsion stability because Cl^– ions bind to phosphate groups, increasing negative charge and thus repulsion between micelles.