An emulsion is a mixture that results when one liquid is added to another and mixed but does not dissolve into it. An example of an emulsion is a vinaigrette dressing where the oil is added to the vinegar and stirred. Although the oil is finely dispersed, it does not dissolve into the vinegar. Ice cream is an oil-in-water emulsion, meaning that the milkfat is finely dispersed into the water.
Because water molecules prefer to be surrounded by other water molecules rather than oil molecules, emulsions are inherently unstable, meaning that, after a while, the oil and water will separate: if you leave a vinaigrette dressing, it will separate into an oil layer and a water layer.
So what does all this have to do with ice cream making you ask? Well, we want a stable emulsion in our ice cream so that the milkfat and water stay together and do not separate. If our ice cream emulsion becomes too unstable, the milkfat will separate from the water and form large clumps of fat that will be noticeable upon eating.
So how do we stabilise an emulsion and prevent milkfat and water from separating? We use surface active molecules.
WHAT ARE SURFACE ACTIVE MOLECULES?
Emulsifiers and protein are surface active molecules that act to keep two liquids that do not mix naturally, in our case milkfat and water, from separating.
So emulsifiers are added to ice cream to prevent the milkfat and water from separating? Well, it is actually the milk protein that emulsifies an ice cream mix and not the added emulsifiers. Emulsifiers are not needed in ice cream to stabilise the fat emulsion due to an excess of protein and other amphiphilic molecules (Goff and Jordan, 1989a).
If it is the protein that acts to emulsify the ice cream mix by preventing the water and fat from separating, why are emulsifiers used in ice cream? Emulsifiers are used in ice cream because they contribute greatly to smooth and creamy texture by promoting fat destabilisation.
FAT DESTABILISATION
So, we know that emulsifiers contribute greatly to smooth and creamy texture by promoting fat destabilisation. Fat destabilisation refers to the process of clustering and clumping (known as partial coalescence) of the fat in an ice cream mix when it is churned in a machine.
Because it is the proteins that stabilise the fat emulsion in an ice cream mix, emulsifiers are actually added to ice cream to reduce the stability of this emulsion and encourage some of the fat globules to come together, or partially coalesce (Goff and Hartel (2013)).
When a mix is churned in an ice cream machine, air bubbles that are beaten into the mix are stabilised by this partially coalesced fat, giving a smooth texture to the ice cream. If emulsifiers were not added, the air bubbles would not be properly stabilised and the ice cream would not have the same smooth texture (Goff and Hartel (2013)). This beneficial fat destabilisation is enhanced by the emulsifiers in common use (Goff and Jordan, 1989b)
THE BALANCED MIX
The balance between protein and emulsifier is critical for ice cream making because it controls the stability of the emulsion and of the air bubbles. If an ice cream mix contains too much emulsifier, the formation of objectionable butter particles can occur. However, if there is too much protein, the emulsion may be too stable so that not enough fat is destabilised. This produces an unstable foam, and the ice cream is coarse and wet (Goff and Hartel (2013)).