To make more stable salad dressings (see previous post), you need a chemical which can help to suspend the oil particles in the water-based liquid. Mustard and cayenne pepper are natural emulsifiers which can help to suspend oil in water. Adding these to a vinaigrette will slow the mix from separating after you shake it.
Egg yolks (these contain lecithin) are used to emulsify mayonnaise. It’s easy to test how good an emulsifier these are by shaking them with a little water and oil and seeing how long the mixture takes to settle back into layers.
Milk is a mix of fats and liquids. If you leave fresh milk to sit for a while, the fat will eventually settle out, leaving a cream layer. This can be collected for sale as cream, or used to make butter. To prevent this, milk for drinking is homogenised. This involves forcing the milk at high pressure through a narrow opening. Cream globules are broken into very small droplets. These have larger surface areas, and are more spread out throughout the milk. This and the emulsifiers naturally found in milk (including casein and lecithin) allow the cream to stay evenly distributed throughout the liquid for longer.Thus the milk you buy in the supermarket does not have a cream layer even when left to stand for a few days.