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.
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)).
Egg yolks have traditionally been used as a natural emulsifier in ice cream making. They act similarly to added emulsifiers, presumably due to the lecithin-protein complexes primarily found in the yolk, although they are not as efficient at promoting partial coalescence of the fat globules as the synthetic glycerol or sorbitan esters.
Emulsifiers used in ice cream manufacture today are of two main types: the mono- and di-glycerides and the sorbitan esters. Typical concentrations in use are 0.1-0.2% mono- and di-glycerides and 0.02-0.04 polysorbate 80 (Goff and Hartel (2013)).
Mono- and di-glycerides are derived from the partial hydrolysis of fats of animal or vegetable origin. To make the sorbitan esters water-soluble, polyoxyethylene groups are attached to the sorbital molecule. Polysorbate 80, polyoxyethylene sorbitan monooleate, is the most common of these sorbitan esters (Goff and Hartel (2013)).
How much egg yolk should I use in my ice cream mix at home?
Goff and Hartel (2013) state that about 0.6-1% egg yolk, or 2-3% whole eggs, is needed to produce noticeable effects in ice cream, and 1.4% egg yolk solids is required in French vanilla or custard ice cream.
My own experiments show that 3.77-4.00% to be optimum for homemade ice cream. Above about 4%, a noticeable egg note starts to form and the ice cream becomes too rich.
To sum up
So, we know that an ice cream mix is an oil-in-water emulsion and that it is the proteins in this mix that stabilise this emulsion and help prevent the fat and water from separating. Emulsifiers are added to ice cream to actually destabiise the emulsion and encourage the fat globules to partially coalesce.
This partially coalesced fat is beneficial as it stabilises the air bubbles that are incorporated into a mix during the churning process, giving the ice cream a smooth and creamy texture.
The balance between protein and emulsifier is critical. If too much emulsifier is added, too much fat will coalesce and form large butter particles that are detectable on eating. However, too much protein may prevent enough fat from partially coalescing, producing and ice cream that is coarse and wet.
Emulsifiers are added to ice cream to:
- promote the partial coalescence (coming together) of fat particles;
- This partial coalescence contributes significantly to smooth and creamy texture due its effect of stabilising more numerous air bubbles during the churning process;
- produce a dry and stiff ice cream;
- increase resistance to rapid meltdown.
Between 3.77 and 4.00% egg yolk is optimum for homemade ice cream. Above 4% and the ice cream will become very rich and a noticeable egg note will start to develop.
Will will look at partial coalescence of fat globules and how this contributes significantly to a smooth and creamy texture by stabilising numerous air bubbles in the next post.
Hope that helps. Feel free to be part of the ice cream science community and contribute any questions or comments.
All the best,
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Goff, University of Guelph Dairy Science and Technology, https://www.uoguelph.ca/foodscience/ice-cream
Goff, H.D., J.E. Kinsella, and W.K. Jordan. 1989a. Influence of various milk protein isolates on ice cream emulsion stability. J. Dairy Sci. 72: 385 – 397
Goff, H.D. and W.K. Jordan. 1989b. Action of emulsifiers in promoting fat destabilization during the manufacture of ice cream. J. Dairy Sci. 72: 18 – 29.
Goff, H. D. and Hartel R. W. (2013) Ice Cream. Seventh Edition. New York: Springer