Emulsions are formed when tiny droplets of one liquid are suspended within another liquid. A mixture of oil and water is a good example of an emulsion.
It is not uncommon for foods that we eat to contain emulsions of oil and water. To prevent the oil and water from separating (and thus the food spoiling), soap-like chemicals called emulsifiers are added. Many common foods like bread, ice-cream, sauces and biscuits contain emulsifiers.
Emulsifiers have a similar structure to fats and oils. One or two fatty acid groups can be added to a molecule of glycerol. They are made by reacting edible oils with glycerol.
While they form ester links with the glycerol backbone, there are still unused hydroxyl group(s) on the molecule.
One emulsifier that is commonly listed as a food additive is E471.
The two molecules above are a monoglyceride (with two hydroxyl groups remaining) and a diglyceride (with one hydroxyl group remaining). Hydroxyl groups are hydrophilic and dissolve in water, whilst fatty acid chains are hydrophobic and dissolve in oil, forming a stable emulsion.
This results in E471 being a very effective emulsifier. It holds together oil and water emulsions to prevent food from spoiling.