Apples change color after being peeled due to a combination of chemical reactions and exposure to air. Here’s a breakdown of the processes involved:

1. Phenolic compounds: Apples contain phenolic compounds, which are responsible for their color. These include chlorophyll, which is green, and anthocyanins, which can be red, purple, or blue, depending on the variety of apple.
2. Enzymatic browning: When you peel an apple, you break through the skin, exposing the fruit’s flesh to oxygen. Enzymes such as polyphenol oxidase (PPO) start to break down the phenolic compounds, leading to a chemical reaction that results in browning. This is the same process that occurs when you cut a piece of apple and it turns brown.
3. Oxidation: The oxygen in the air reacts with the breakdown products of the phenolic compounds, causing them to change color. The brown color is a result of melanin, a polymer formed by the oxidation of phenolic compounds.
4. Asymmetrical cell death: After peeling, the cells on the exposed side of the apple die asymmetrically. This can lead to a change in the structure of the cell walls, causing the apple to wrinkle and the color to darken.
5. Water loss: The exposed apple flesh is also more prone to water loss, which can affect the texture and color of the fruit.

The exact color change and degree of browning can vary depending on the type of apple, its ripeness, and the conditions (such as temperature and humidity) after it’s been peeled. For example, some apples may turn a light brown, while others may turn a deep brown or even redden further due to the breakdown of pigments. The enzymes responsible for browning can be inhibited by substances like lemon juice or vinegar, which is why many recipes for apple dishes recommend adding a little acidity to delay browning.