What you see: Apple flesh that eventually turns brown after being sliced or bitten
What it is: Enzymatic browning!
Eat or toss: Eat! When that tray of sliced apples left on the counter turns brown, it may look unappetizing, but it doesn’t indicate that bacteria or anything else that could be harmful is present.
When you bite or cut into an apple you’re breaking open tiny apple cells. Enzymes and compounds that the cell had kept separate from each other suddenly mix together. Add some oxygen in the air to the equation and the resulting chemical reaction leaves us with this brown color.
It looks icky, but the apple knows what it’s doing. The compounds are defensive; if you were a microbe or an insect, they might be enough to stop you in your tracks. As Harold McGee puts it in On Food and Cooking, “The brown pigments that we see are essentially masses of spent weapons.”
So, nothing about that brown means that any threatening lifeforms or chemicals have contaminated your apple. But, like all produce, apples’ shelf life shortens once their outer skin is broken. Treat them as you would any prepared food and don’t leave them out for more than a couple hours.
How to prevent browning
Even if the color change is harmless, we can’t deny how unappealing brown apples are. So, if you’re like my husband and can’t stand being in the same room as a browned apple, there’s hope. The easiest option, and the one we go to most often, is to just coat your apples with some lemon juice. The enzymatic browning can only happen at a pH between 5 and 7, so lemon juice, which comes in at a pH of 2, will stop it in its tracks. As an antioxidant, the lemon juice will also suck up all the oxygen for itself. At home, we use this trick so often that apples that haven’t been given the lemon treatment sometimes taste like they’re missing something.
If you want to up the pH without the lemon flavor, some recommend crushing vitamin C tablets into some water and dipping the apple slices in the solution. Temperature can be your ally too: Putting the apples in the fridge will slow down the enzymatic process. Heat will reconfigure the enzyme, preventing this type of browning.
Another approach to hold off the browning is to insulate the apples from oxygen. An easy way to do this is to store them in a bowl of cold water. You can also coat them with sugar, honey or syrup. In that case, you’re creating a barrier between the oxygen in the air and the apple.
Finally, this is just a cosmetic fix, but it’s worked in my house—sprinkle the apples with cinnamon. It’s delicious and makes it harder to notice that the apples have browned a bit.
Other foods and future apples
For a little perspective, consider that while that brown color might be a turnoff on your apples, it’s actually something we seek out in other foods. A similar reaction turns your coffee, tea and chocolate into those rich and satisfying shades of brown.
For those who still want to avoid apple browning, keep an eye on this story. Scientists have genetically engineered an apple that doesn't brown. It's not currently available in stores though, and only time will tell whether farmers and consumers will embrace it.
On Food and Cooking: The Science and Lore of the Kitchen. Harold McGee. p. 269.
Why Cut Apples Turn Brown - Scientific American
Why do apples turn brown when you cut them? - UCSB Science Line
Enzymatic Browning, Polyphenol Oxidase Activity, and Polyphenols in Four Apple Cultivars: Dynamics during Fruit Development - Hort Science
What Factors Affect the Oxidation of Apples? - Science & Health Education Partnership, UCSF
Pre-Treating Helps Prevent Browning - Alabama Cooperative Extension
Why do apples turn brown? - Education.com
Why does your apple turn brown and how can you stop it? - io9
Why apples turn brown - Decoding Delicious
Lemon Juice Keeps the Brown Away - Washington Post
Apple brown? No need to frown!