The best way to keep apple slices from turning brown is to keep your apple whole for as long as possible and only cut it right before you're ready to eat it. And really, there's nothing necessarily wrong with browned apple slices; they're still totally safe to eat. But if you're really on a quest to keep your apple slices looking fresh for as long as possible, and keeping the apple whole isn't a viable option, there are a few things you can try.
First, you need to understand what causes apple slices to change color in the first place. As Harold McGee explains in On Food and Cooking: The Science and Lore of the Kitchen, apples contain a specific protective plant enzyme that reacts to oxygen. So when an apple is cut or damaged and the flesh is exposed to air, the enzyme kicks into gear and turns the fruit brown. That's why most of the preventative measure include some sort of acid, which slows down the speed at which this plant enzyme reacts to oxygen, thereby preventing browning, or a compound that'll also interact with the fruit.
Add Lemon Juice
"The single handiest method for the cook is to coat cut surfaces with lemon juice," writes McGee, adding that "the browning enzymes work very slowly in acidic conditions." The main downside to this technique when it comes to apples is that the lemon juice can, and probably will, make the apple slices taste lemony. But they'll look fresh, if that's what matters to you.
Use Honey and Water...
If you're looking for a slightly sweeter way to keep apple slices from turning brown than lemon juice, consider dunking them in a mix of honey and water. According to the editors at Cook's Illustrated, soaking apple slices in a cup of water with two tablespoons of honey kept the fruit looking fresh for up to 24 hours. This method works because there's a compound in honey that also inhibits the browning enzyme in fruits.
Or Pineapple Juice
It sounds kooky, but another sweet way to prevent apple slices from browning is to use pineapple juice. "Mix one tablespoon pineapple juice in one cup water," explain the apple experts at Stemlit Growers, and drop the apple slices into the mixture, letting them soak for five minutes. You can then put them into a plastic bag until you're ready to eat them, and the result is a not-so-brown apple slice that's still sweet to eat.
If you forgot to treat your apple slices, and they turned brown, don't throw them away! You can make your own applesauce with apple slices that look less than perfect. They'll still taste totally delicious when cooked down, so there's no need to let ugly apples go to waste.