The subject matter milk and its emulsion is complex and broad. It can be tackled from a chemical, physical, sensory, more or less scientific and in-depth point of view.
I want to treat it by observing it from the bartender’s point of view and trying to give some practical advice extrapolating it from my own experience.
I’ll start immediately with a question: how to choose the milk jug?
Ceramic milk jug has a much slower thermal reaction than metal.
It is also much more fragile. Therefore it’s better to opt for the metal one.
Milk should be heated just one time, so you need to use a milk jug that has the right size to emulsify the exact amount of milk you need at that time.
Considering that emulsified milk increases in volume and you need to have the space to make it twirl, I suggest you to use the following milk jugs:
Milk: fat and proteins
Milk protein creates foam, not fat. Proteins have the ability to forge air and they are efficient at temperatures below 35-40°. The greases work as adhesive and create a dense and persistent foam, but they start to melt at 35-40°. You can imagine them like crazy little pebbles moving inside the milk.
Skimmed or semi-skimmed milk allows you to create the foam but will be less dense and persistent than the one created with whole milk.
During milk emulsion the steam condenses and becomes water which is added to the milk itself. We are talking about a percentage that can vary from 7 to 12% depending on the type of machine and, above all, the level of cleanliness of the steam lance.
Heating milk several times means, in essence, diluting it with water. Proteins already used, moreover, do not allow to create a perfect emulsion.
Warm milk is a fertile environment for bacteria, so it is better to avoid putting it directly in the fridge!
During the emulsion of the milk one hand must hold the milk jug from the handle and the other hand in support laterally as a temperature control function. Holding the milk jug from the body and not from the handle does not allow to heat the milk beyond a certain limit. The hand placed underneath the bottom of the milk jug senses the temperature of the steam and not of the milk, with the consequence of having a distorted perception.
Cleaning the steam lance is essential
Slamming the milk jug on the counter after emulsion will help to break the bubbles on the surface. Usually two smacks are enough. If more than two smacks were necessary, that would indicate a poor emulsion technique.
Use always one hand on top of the milk jug to prevent milk spill during this action.