Why does a cut apple turn brown?
When you cut an apple, air reaches the inside. A helper chemical in the apple, called an enzyme, reacts with oxygen. That makes new brown-colored stuff. It is a bit like the apple getting a tan.
Transcript
Dr. Nina Byte: Chip asked me why a cut apple turns brown. I have three PhDs, and this is my job now.
Dr. Nina Byte: Step one. This is an apple. When you cut it, air reaches the inside.
Chip Darnell: So the apple is just sitting there, minding its own business, and then boom, oxygen shows up like an uninvited guest at a potluck.
Dr. Nina Byte: Step two. Inside the apple is a helper chemical called an enzyme. Per Wikipedia, it reacts with oxygen in the air.
Chip Darnell: The enzyme is the ride operator! He's got the little lever and a cigarette behind his ear!
Dr. Nina Byte: Step three. That reaction makes new brown-colored stuff. It is a bit like the apple getting a tan.
Chip Darnell: Right, so the operator gets bored, spins everybody too fast, and the whole ride goes brown with regret.
Dr. Nina Byte: No ride. No operator. Air, enzyme, brown. Please.
Chip Darnell: Okay. But why does lemon juice stop it?
Dr. Nina Byte: That is actually the right question. Lemon juice is sour, so it is acidic, and acid slows down the browning enzyme.
Dr. Nina Byte: Per Wikipedia, acids like lemon juice slow enzymatic browning. Chip, that was genuinely smart.
Chip Darnell: So the lemon is the guy who shuts down the Tilt-a-Whirl for being sour. Does he get a fair pass?