Why do Cooked Potatoes Turn Dark?

I boiled a pot of "Syracuse Potatoes." Basically boiling little potatoes in water with about 1 cup of salt per pound. Forms a salty crust on skin, inside is creamy. Unfortunately, I stupidly left salty water remainder in stainless steel pot overnight. By the next day, the water had turned a dark color, almost a dark purple or gray. The pot was not stained but there is some pitting. Exactly what chemical reaction caused the water to turn such a color? I have hard water, did that contribute to the reaction? Should I be concerned?Thank you!
- Matt (age 26)
Seattle, WA

Mmmm, I love Syracuse potatoes. The effect you noticed is called "after-cooking darkening," and it can happen with any kind of cooked potatoes in any kind of pan. Typically, it happens if you boil (or fry) potatoes and then store them—they will sometimes turn black or purple over time if they are exposed to air. This is very annoying to caterers or anyone selling precooked potatoes.

According to the American Journal of Potato Research:

After-cooking darkening is caused by the oxidation of the ferri-chlorogenic acid in the boiled or fried potatoes.

The severity of the darkening is dependent on the ratio of chlorogenic acid to citric acid concentrations in the potato tubers. Higher ratio normally results in darker tubers. The concentration of the chlorogenic and citric acids is genetically controlled and influenced by environmental conditions.

Since the potatoes release molecules into the cooking water when you boil them, the same effect could cause the cooking water to turn dark over time. It has no effect on the taste or safety of potatoes, so I wouldn't be concerned about your pan either.

Rebecca H.

(published on 02/25/2016)