One of the most useful tips I ever received from a parent at preschool was that Ambrosia apples do not go brown, after you cut them. My daughter has long since graduated from preschool, but I still wonder about them apples.
When most apples get cut, dropped or chewed, the damaged cells release polyphenol oxidase (PPO), a kind of enzyme that adds oxygen to polyphenols (also known as phenolic compounds). After further reactions, this process eventually forms the pigment melanin, which makes things look brown. This pigment may be a form of protection for the damaged cells.
Some sites say that the enzyme responsible is tyrosinase, but this is one kind of PPO that works on tyrosine, which is a type of polyphenol and an aromatic amino acid. Some explanations suggest the browning of an apple is the same as the rusting of a metal. PPOs often do contain metal atoms, particularly copper, and they do involve oxygen, but the browning is not an iron oxide, like you would see in rust.
Ambrosia apples were discovered by the Mennell family in the Similkameen Valley, of British Columbia. A chance seedling grew up in an orchard that previously grew Red Delicious and Golden Delicious apples and then was replanted with Jonagold. They do not seem to have an explanation for the non-brownness of their apples, but here are some possibilities:
- Inhibitors: Ambrosia apples might contain reaction inhibitors that impede the PPO/phenolic reaction. Lemon juice keeps apples from going brown by lowering the pH (that is to say that lemon juice increases acidity). But Ambrosias are described as being low in acidity. Ambrosias could contain other chemicals that keep the reaction from occurring.
- Phenols: Ambrosias might be low in phenolic substrates. Phenolic compounds include the antioxidant molecules, which is why some people believe that you should eat apples, as well as other fruits and vegetables. Many pigments are also phenolic and those include some of the pigments in skin colour. Ambrosias have a combination of yellow and red pigments, so maybe the lower red pigment in the apple suggests lower phenol. If Ambrosias are low in these phenolic substances, the producers might not be so keen to talk about it.
- Cell Walls: Ambrosias might have stronger cell walls. This would limit cell damage and reduce the mixing of PPO and phenolics. Ambrosia apples are crispy and this might reflect the robustness of their cell walls.
- Enzymes: Ambrosias might have lower levels of PPO. The levels can also vary with growing conditions and maturity. If the formation of melanin does have some protective value, then you might expect PPO to provide some advantage and be common in nature.
- Genes: The genes that code for PPO have been identified and a company is working on turning them off to produce genetically modified apples that do not have PPO at all. They are convinced that non-browning apples would be a boon for all humanity.