Slice an apple and watch it brown. Here is the chemistry happening right on the cutting board.
The Enzyme Meets Air
Cutting an apple exposes its flesh to oxygen, activating an enzyme called polyphenol oxidase. The University of Wisconsin-Madison Extension says that enzyme kicks off the entire browning reaction.
Melanin Does the Rest
Once activated, the enzyme speeds a reaction that builds melanin, a brown pigment. It is the same chemistry behind a bruised banana, and scientists call it enzymatic browning.
Lemon Juice Fights Back
Lemon juice carries vitamin C, an antioxidant that grabs oxygen first. Its acidity also calms the enzyme, which is why UW-Madison tests found lemon-treated slices stayed pale.
It Happens to Other Fruits Too
Pears, bananas, and eggplants brown through the same enzyme reaction. One enzyme, many victims, which is why a cut pear browns just as fast on the counter.
Some Apples Resist Longer
Apple varieties carry different amounts of the enzyme and its chemical fuel. That is why one sliced apple can look fresh for an hour while another browns in minutes.
A Fun Test for Home
Slice two pieces from one apple and squeeze lemon juice over just one. Check back in ten minutes: the difference between the pale slice and the brown one is dramatic.
Sources: UW-Madison Extension, Science Buddies