The moment a

knife slices through apple—spilling the contents of apple cells along

the surface of the cut, and allowing everything to mix—a reaction

begins. In particular, an enzyme known as polyphenol oxidase that had

been held in check is loosed.

Almost immediately, it begins altering polyphenols, a group of

health-promoting chemicals with antioxidant activity that naturally

occur in apples and other produce.

“As soon as you break the apple cells, you expose (the enzyme) to

polyphenols and oxygen, and then it goes after the polyphenols and

breaks them down. There’s a whole set of chemicals produced, and one

set of the chemicals is colored,” explains Rich Hartel, a professor of food science at UW-Madison.

It is believed that this reaction, called enzymatic browning, is a

form of plant defense against pests and pathogens, says Hartel. Some of

the chemicals created via this reaction have been shown to help ward

off invading organisms.

In addition to apples, enzymatic browning is evident in avocadoes

and potatoes. It’s also what gives raisins, prunes and apple cider

their deep hues. Interestingly, these brownish-colored chemicals belong

to a family of pigments known as melanins, some of which give human

skin its color