Peter Jaskiewicz

One day, you decide to pack an apple for lunch. Instead of having it whole, you cut the apple into slices and pack them into a plastic bag. Come lunchtime, you notice the apple slices are now a brownish tint. They also have a different taste compared to how it was when you cut it.

You would think this apple went bad after being left alone. However, the apple is still edible despite this change. What happened was an example of enzymatic browning, or a chain of biochemical reactions.

When you cut an apple, oxygen is introduced into the plant tissue or the inside of an apple. This triggers an enzyme known as polyphenol oxidase (PPO) to oxidize, or expose to oxygen, polyphenols in the apple’s flesh, resulting in new chemicals known as o-quinones to react with amino acids to produce brown-colored melanins, or pigments, into the apple. Different apple varieties can brown at different rates due to different amounts of the initial enzyme and polyphenols. Biting apples can also expose apples to oxygen, leading to browning.

Enzymatic browning is not unique to just apples. Pears, bananas, and eggplants can brown quickly when cut. It is also the cause of the dark color of prunes, coffee, black tea, and cocoa.

Apple Slices

It is possible to prevent or at least slow down the browning. Refrigerating slices can slow the browning. You can also coat them with lemon or pineapple juice to slow the reaction. Honey, caramel, sugar syrup can block the exposure to oxygen, but it will give them a bit of added sugar.

Even if you do not attempt to prevent enzymatic browning, do not worry. They are still safe to eat. The taste might be different after browning, but it should not cause any serious problems down the line. However, if the apple has signs of soft spots or bruising, wrinkled skin, holes and brown blemishes, liquid oozing from the skin, a mushy texture, or a mealy or bland and grainy taste, it might have gone bad. Furthermore, keep an eye on your apples before eating them. You may never know when they go bad.