How does leftover wine change after a few days?

Hello there! I'm Dr. Vinifera, but you can call me Vinny. Ask me your toughest wine questions, from the fine points of etiquette to the science of winemaking. And don't worry, I'm no wine snob—you can also ask me those "dumb questions" you're too embarrased to ask your wine geek friends! I hope you find my answers educational, empowering and even amusing. And don't forget to check out my most asked questions and my full archives for all my Q&A classics.
Dear Dr. Vinny,
When I don’t finish a bottle of wine, I pour the rest into an empty half-bottle to drink over the next few days. Is there any change to the taste or quality of the wine over that time?
—Kok, Kuantan, Malaysia
Dear Kok,
First, congratulations on using an excellent method for saving leftover wine for later! That’s the way I save leftover wine as well. As for how the wine changes, that depends a lot on the type of wine, but after a few days you might notice the flavors start to fade and flatten, and perhaps take on nutty, bruised apple notes as the wine is exposed to oxygen over time. Younger, bolder wines will last longer than older, more delicate ones. Wine is always evolving, and those changes speed up as soon as you open a bottle and expose it to more oxygen. One way to slow down those changes is to keep the wine refrigerated after you transfer it to a smaller bottle—the colder temperature will slow the oxidation process.
—Dr. Vinny