Whenever a recipe calls for making an emulsion, the directions exhort to add the oil slowly. I understand what an emulsion is, but I’m not sure why it’s necessary to add the oil slowly. Can someone explain?
The emulsion only works when the emulsifying agent is able to surround and separate little “bubbles” of the oil in your water-based solution. If you pour in a lot of oil at once, you have to whisk harder in order to get those little bubbles because they keep reconnecting with other little bubbles before they can be fully separated and emulsified. If you only add a little oil at a time, you have less to fight.
I think it also has something to do with making sure you don’t add too much oil on accident.
Because you are trying to squeeze in molecules between each other, so you need to have the whisk work on as little as possible, because the oil is thick and dense and you need to separate as much of it from itself to wedge it between the molecules of the other concoction in the bowl.
If you dump a lot at once or fast, sure the whisk can go ahead and whisk the whole batch, but it can’t possibly beat up the oil as if it were being poured slowly. You are trying to get that oil wedged in between the other ingredients, so the whisk needs its best chance and time to get at that thick oil!
More importantly, an emulsion can “break” and separate out. Once that happens, there’s no way to reemulsify the existing ingredients. Instead, you have to create a brand new emulsion and readd the broken emulsion slowly to the new one.
Alton Brown did a good job of explaining this on the “Mayo Clinic” episode of Good Eats (link: http://www.youtube.com/watch?v=6bKya9uMHYs). He starts explaining this around the 7:30 mark.