You know the smell of salad dressing, right? Though vinegar is acetic acid, acetic acid doesn’t have much smell but the compound on the way from alcohol does and that is acetaldehyde. The process comes when you mix oxygen and alcohol in the presence of a bacteria known as acetic acid bacteria.
There is a simple test that you can perform to figure out what you’re smelling when you smell salad dressing in your wine. Place a little wine on your tongue and inhale over it. If the air going into your lungs causes you to cough then it is acetic acid. It also leaves a little scratchiness on the back of your throat. If it doesn’t cause you to cough then hold the glass up to your eye. If the vapor from the wine glass causes a cold sensation on your eye then it is acetaldehyde. When either are strong enough that it gets in the way of smelling and enjoying the wine you should exchange the bottle. Many times at wine bars they may have a wine open too long and this might happen if you had a glass from that bottle.