Oil and Water Mixtures

The Theory

An emulsion is a system with two or more non-miscible liquids. One liquid phase acts as the dispersing agent, in which the other liquid phase is scattered as fine droplets.
Most emulsions consist of water and oil/fat.

Depending on the relative amount of each phase, two ways of dispersion are possible:
If water acts as the dispersing agent, the emulsion is an O/W-emulsion, and its behaviour is mainly determined by the characteristics of water (e. g. milk, mayonnaise). If oil acts as the dispersing agent, it is considered a W/O-emulsion. The oil determines its properties (e. g. butter, medicinal ointments).

How Stable Are Emulsions?

An emulsion can separate. For example, if too much oil is added to mayonnaise during its preparation, the layer of egg yolk around the oil droplets breaks up and the mayonnaise clots. After some time, liquid emulsions with big dispersed droplets (>100 nm) start to separate of their own accord.

The stability of an emulsion can be increased by adding small amounts of an emulsifier. Emulsifiers are substances that enclose the oil droplets in order to prevent separation. Examples are lecithin of the egg yolk in mayonnaise or egg white in pastry. Without emulsifiers, oil would float on the surfaces like a blob of fat on soup. By homogenising (i.e. e. pressing through very thin nozzles), it is possible to get an extremely fine dispersion of oil droplets in water, which do not separate any more. Milk is an example of such a fine dispersion.

Emulsified fats are more easily digested by the body, as the emulsification increases the surface of the fats, providing more space for digestive enzymes to work on.

Soap as an Example of an Emulsifier

If you have got greasy hands, you clean them with soap. The soap dissolves the oil and can easily be washed away with water. Soap has the chemical property needed to enfold oil in fine droplets. Water can then wash around these droplets, mixing the water and the oil. There are no longer two big drops of water here and oil there, but countless small oil droplets that float in the water. The two liquids have become an emulsion and soap served as the emulsifier.

The Production of Butter

Fat droplets in cream are surrounded by aqueous membranes. When butter is churned, the membranes are broken open by mechanical forces, and the fat droplets accumulate into bigger spheres. Now the fat of the butter encloses the water, and a phase conversion from an oil in water emulsion (milk) to a water in oil emulsion (butter) has occurred. In addition, if water droplets smaller than 10 mm were formed, they are now able to impede microbial growth during storage.

Why Does Whipping Cream Become Firm?

Two facts are important to know about whipping cream. First, fat globules in whipping cream are covered by a protein coat. This coating enables the fat to disperse in the whipping cream instead of floating on top of it.
When the whipping cream is whipped, the cover becomes disrupted in some places. In those places the fat globules are "sticky" and therefore they stick together.
Additionally, another crucial process takes place: Air bubbles are stirred into the cream during whipping, which also adhere to the sticky places. As a result, long compounds consisting of air bubbles and fat globules form. That is what makes the whipping cream firm.

In other words, a phase conversion of the emulsion occurs during whipping: In the liquid whipping cream, the fat is finely dispersed in the aqueous phase (oil in water emulsion). In the whipped cream, a water in oil emulsion is formed. There, water and air are dispersed in the cream’s fat.