Why does a cut apple turn so brown after a while? It starts at the cut surface
You say the apple had been cut up and left at room temperature. That combination is enough to make the flesh darken quickly, and there is a simple way to slow it down.
What you are seeing is ordinary enzymatic browning. Cutting an apple exposes substances in the flesh to oxygen, and the cut surface gradually turns brown. The photograph is consistent with an apple that has been cut and left out for a while.
Because your apple was cut before being left out, and was kept at room temperature, the process happens more quickly. It does not automatically mean the apple has gone off, although its flavour and texture often deteriorate the longer it is left.
To keep the pieces looking lighter next time, put them straight in the fridge and sprinkle a little lemon juice over the cut surfaces. This noticeably slows the browning.
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