Sweet potatoes turned black after boiling

Why do some sweet potatoes develop black patches after cooking?

Central to this reaction is chlorogenic acid, a naturally occurring compound in sweet potatoes and many other plants. While it’s likely colorless in a raw sweet potato, during the heat of cooking iron easily binds to it. This alone probably won’t result in a color change, but once this new arrangement is exposed to air, it oxidizes to a dark color somewhere on the black, gray, green spectrum. The new color might not be apparent right after you boil the potatoes, but could show up over time.

Why did only some parts of the potato turn black or gray?

Chlorogenic acid levels can vary between types of sweet potatoes and even at different areas within an individual potato. And, it’s good stuff! It’s an antioxidant and helps protect the potato from hungry critters and environmental stressors (like extreme temperatures or too much or not enough water). 

Chlorogenic acid is found throughout a sweet potato, but is most concentrated in the outer layers; this makes sense because that’s where the sweet potato is most likely to encounter threats. A growing sweet potato has to contend with everything from hungry insects and microorganisms to environmental threats like extreme temperatures or too much or not enough water. A stressed sweet potato could even produce more of the stuff. 

Because the ends of a sweet potato are tapered, they contain relatively more skin than other parts of the potato. A higher-skin to flesh ratio also means a higher concentration of chlorogenic acid. That could be why, if you look closely at the image above, you’ll see that the greenish black area is an end portion of one sweet potato, while the rest of the potato pieces have generally retained their normal orange color. 

Do white potatoes do this too?

Yes! This phenomenon is called “After Cooking Darkening” and it occurs in “regular” potatoes as well (here’s more). While sweet potatoes and “potato potatoes” aren’t closely related, they both do have this tendency.

If your water has higher concentrations of iron, is this darkening more likely?

No. Often areas with “hard water” have higher levels of iron in the water, but this darkening reaction involves naturally occurring iron found within the potato itself. 

Can you prevent sweet potatoes from darkening after boiling?

You can make the reaction less likely with a little understanding of the chemistry. Adding an acid, like lemon juice, to the cooking water would, for example, help bind with the iron before it could get tangled up with the chlorogenic acid.