Final answer:
Cut fries turn black when exposed to oxygen due to the oxidation of phenolic compounds present in potatoes. The process is similar to how apples turn brown. Submerging cut fries in water prior to cooking can help prevent browning.
Step-by-step explanation:
When cut fries turn black, it is a result of chemical reactions that occur when the fries are exposed to air. The browning or blackening of cut fries is primarily due to the oxidation of phenolic compounds, such as tyrosine and catechol, present in the potatoes. When these compounds come into contact with oxygen in the air, an enzyme called polyphenol oxidase starts oxidizing them, leading to the formation of dark pigments called melanins.
This process is similar to how apples turn brown when exposed to air. It is important to note that the browning does not indicate that the fries are unsafe to eat; it is just a visual change in the appearance of the potatoes.
To prevent this browning, it is recommended to keep the cut fries submerged in water until they are ready to be cooked. This helps to minimize their exposure to oxygen and slows down the oxidation process, keeping the fries from turning black.