Oil and vinegar: those two could never get along! As a child, I was always fascinated by how our homemade oil and vinegar salad dressing separated into two parts. I would have to shake it furiously to get them to mix. It was as though oil and vinegar had a terrible argument and would never speak to each other again. What is it that makes this famous salad dressing separate in this way?
The Chemistry Behind It
It all relies on the chemistry behind these two unhappy partners. Vinegar is a mixture of water and acetic acid, sometimes including other acids or alcohols. Both water and acetic acid are polar molecules. Polar molecules have one slightly positive end and one slightly negative end. Stronger atoms pull electrons away from weaker atoms, creating an uneven distribution of positive/negative charge. This uneven sharing of electrons makes the molecule polar. Polar molecules love attaching themselves to other polar molecules. This means they easily integrate and mix with water molecules.
However, oil is a completely different story. The fatty acids that make up oil generally share electrons well, as the electron distribution is fairly even, making it a “non-polar” molecule. Non-polar molecules love to be around other non-polar molecules but hate integrating with polar molecules. In this instance, opposites definitely do not attract!
As a result, when the polar vinegar and non-polar oil come in contact, they each stay within their respective groups – like the cliques at most high-schools. When we do shake them together in the bottle of salad dressing, they reluctantly leave their cliques and somewhat interact, but get away from each other at the first chance.
Put a Stop to the Battle!
Now, how can we stop this annoying habit? We can use an emulsifier. Emulsifiers have a polar end and a non-polar end. They make the oil and vinegar come together and cooperate. Common kitchen emulsifiers are tomato paste, mustard, egg yolks and honey.
Next time a crisp salad, garnished with oil and vinegar calls your name, just remember, a teaspoon of mustard will sort your dressing problems out!