What causes cut fries to turn black?

Cut fries turning black is primarily caused by them being out of water for too long. When cut potatoes are exposed to air, the enzymatic browning process can occur due to the reaction of polyphenol oxidase enzymes with oxygen in the air. This leads to oxidation, causing the fries to develop a black or dark brown coloration.

Keeping cut potatoes submerged in water helps prevent this oxidation process, as it limits their exposure to air. Additionally, soaking can help to remove excess starch, which can improve the texture and frying efficiency of the potatoes.

While factors like excessive frying temperature or improper storage can impact the quality and taste of fries, they do not directly lead to the blackening that results from oxidation when fries are left exposed to air. Similarly, too much salt doesn't cause the cut fries to darken; rather, it's a flavoring issue that affects taste more than appearance.