Explanation of Lemon Juice-Invisible Ink?

Question

A cute homegrown spy-trick we all know: lemon juice invisible ink. But there is no scholary article available in the internet about this phenomenon. Lemon juice—and the juice of most fruits, for that matter—contains carbon compounds. These compounds are pretty much colorless at room temperature. But heat can break down these compounds, releasing the carbon. If the carbon comes in contact with the air, a process called oxidation occurs, and the substance turns light or dark brown. Question: What is the mechanism going on here?

Answer 1

In the example you gave (lemon juice on paper) the mechanism is reversible loss of a proton to make ascorbate in aqueous solutions this is ongoing, radical donation to the ascorbate (light), repeating this process to get dehydroascorbic acid, which is brown. Remove the light source or the water and this process is much slower or non-existent. In biological processes, the mechanism is different.