What is an Emulsion?
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*An emulsion is the mixing of two unmixable liquids - one water-based and one oil-based – by physical agitation.*
Unless there is the presence of a protein, which stabilizes the suspension of the molecules, the mixture is temporary and will split in two.
Vinaigrettes are unstable emulsions (i.e. oil and vinegar) and dressings are stable emulsions due to the presence of protein (i.e. mayonnaise).