When you bite or cut into an apple, air reaches the inner part of the fruit. Once exposed to oxygen, enzymes in the apple begin converting natural chemicals called polyphenols into ‘melanin’, an iron-containing compound that gives the flesh a brown, rusty colour. The reaction happens quickly, and so a sliced apple can start to turn brown in only a few minutes.
Earlier this year, the US government approved a genetically modified apple variety that can’t produce the browning enzyme. It hasn’t crossed the Atlantic yet, but these new apples can be sliced and diced without losing their fresh appearance.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.