We make emulsions to reduce the sensation of fat on the palate, to achieve smooth, creamy textures, and for optimal preservation. When it comes to chocolate, emulsifying the mixture will produce a chocolate mousse or ganache that has a true taste of chocolate and a very creamy texture.
An emulsion is a mixture of two liquids that do not combine naturally, such as water and oil. We just think back to those salad dressing science experiments from school. If you just lightly shake the liquid oil mixture, you will see that the oil is floating at the top of the water! When you rapidly mix the sald dressing, it seems to completely combine with the liquids emulsifying to disperse one of them into the other in the form of minuscule droplets.
There are two types of emulsion. For the first type, we incorporate oil into water. When we make mayonnaise, the oil, in what is known as the "oily phase," is progressively incorporated into the mixture of egg yolk, vinegar, and mustard, known together as the "aqueous phase." For the second type, we incorporate water into oil. For a ganache, we incorporate the chocolate, the "oily phase," into liquid such as cream, milk, fruit juice, or coffee (the "aqueous phase").
When you make a mayonnaise, you gradually add the oil to the yolks and mustard until you have a shiny, elastic texture. If you continue to add oil (that last drop that is the one too many), the emulsion becomes saturated in fat and separates. The texture is no longer homogenous. At this point the mayonnaise looks ruined: professionals say that the mixture has separated or split. In fact, to remedy the situation, all you need to do is add a little liquid (water, lemon juice, or vinegar) and beat hard, and once more the homemade mayonnaise is restored to its full glory. This is quite unlike what happens with ganache. We start off with a saturated fat base, and then progressively pour the liquid (milk or whipping cream) into the chocolate. This is why, as soon as we pour a little liquid over the chocolate, the mass rapidly thickens. And this is why you should never add water in with your chocolate to melt it. Adding water leads, usually, to separation.
In the initial phase of the emulsion, the ganache will not look very attractive, and that is normal. There is no reason to panic and add liquid faster. Instead, you should take your time to make the emulsion, ensuring you have the elastic texture that means the emulsion is well on its way to forming, and stir energetically while you gradually add the liquid, just as you would gradually add oil to a mayonnaise. Mixing energetically with small circular movements in the very center of the mixture ensures that the globules of fat and water droplets are organized regularly and homogenously. This homogeneity stabilizes the emulsion and reduces the impression of fat on the palate. It results in smooth, creamy, melting textures.