Why Apples Turn Brown Once Cut?
Apples are one of many fruits that contain a large amount of an enzyme called polyphenol oxidase. As its name suggests, it is capable of oxidizing polyphenols, molecules that play a variety of roles in plants, from protecting against infections to giving them their pigments. Polyphenol oxidase and the polyphenols themselves are stored in separate areas of the plant's cells, but when the cells are damaged say, when an apple is sliced open, or dropped and bruised the cells are ruptured, and the enzyme comes into contact with its prey or substrate. With the help of oxygen, which is in the air around the damaged cells, the polyphenol oxidase initiates a series of chemical reactions, transforming the polyphenols and eventually producing melanins brown pigments.
The general name for this process is enzymatic browning, and the problem is that it doesn't just change the appearance of produce; it also alters flavor, scent, and nutritional value and usually not in a good way. (Although it's worth mentioning that it does help produce desirable effects in some of our foods, like teas, cocoa, and dried fruits like raisins.)
How to Prevent Brown Apples?
Preventing browning, then, becomes a question of how to put a stop to enzymatic browning. There are scores and scores of scholarly articles out there that have examined this in depth after all, there's a pretty big financial incentive for the produce industry to find a solution, since as much as 50% of some types of produce is lost annually due to the process.
Most solutions involve blocking the oxygen in one way or another, reversing the oxidation reaction, changing the pH of the environment, or halting the reaction through exposure to either high or low temperatures.
Often, on the industrial scale, these methods are used in concert to maximize their effect say, by combining the powers of ascorbic and citric acids. But that's not practical for the home cook who just wants to stave off browning for a short period. You know, so the apple slices don't turn the color of a muddy sponge while they sit on your cocktail party's cheese tray.
Here's the short version: The best way to prevent browning is to soak the cut fruit in a saltwater solution (half a teaspoon of kosher salt per cup of water) for 10 minutes, then drain and store until ready to use. The mild salt flavor can be rinsed off with tap water before serving. The best part is that even after rinsing, the browning is still just as effectively reduced.
Using Water, Lemon Juice, and Citric Acid to Prevent Apples From Browning
One of the easiest things you can do to prevent browning is to submerge the cut fruit in plain water, which reduces the amount of air, and therefore oxygen, that can get to it. Since a lot of fruits float, it helps to either lay a clean paper towel on top, which, once wet, will push them under, or to put them in zipper-lock bags with the air pressed out. I did the latter, since it's easier to see the apple slices in the photos that way.
A lot of people will tell you to squeeze some lemon juice into the water first, which acidulates it; lemon contains ascorbic acid, which not only lowers pH (as does the citric acid also found in lemons) but can also reverse the oxidation reaction through a process chemists call reduction. In my tests, I used three tablespoons fresh lemon juice (more or less equivalent to the juice of an average lemon) per quart of water.
Then, just for the heck of it, I also played around with citric acid in two different (very strong) solutions, sprinkling dry crystals of it directly onto the cut apple surfaces. The concentrations of the acid in these samples were too strong for the apples to actually be edible, but it's interesting to see what higher levels of the acid can do.