Why does vitamin C decrease when cut fruit is exposed to air?

Vitamin C levels decrease in cut fruit exposed to air primarily because the vitamin is highly susceptible to oxidation. Its chemical structure allows it to readily donate electrons, which makes it an effective antioxidant but also causes it to be easily destroyed when it reacts with oxygen.1

When fruit is cut, the cellular structure is disrupted, exposing the interior to air and activating various enzymes that accelerate this degradation. Specifically, enzymes such as ascorbic acid oxidase directly catalyze the destruction of vitamin C. Additionally, polyphenoloxidase contributes to both the browning of the fruit and the loss of the vitamin. Because these reactions are sensitive to environmental factors, vitamin C is difficult to preserve in foods once the protective integrity of the fruit's skin or peel has been compromised.21