Why did my potatoes go grey or black after cooking?
Two harmless discolourations with two different causes — iron reacting with a potato's own acids, and simple oxidation — plus the one kind of dark patch that means throw it away.
- Difficulty
- beginner
- Time
- 5 min
- Read
- 2 min
Short answer
Grey or blue-black patches that appear an hour or two after cooking are after-cooking darkening: iron in the potato reacting with chlorogenic acid. It is harmless, it is worse in some varieties and in potassium-poor growing conditions, and a splash of lemon juice or vinegar in the cooking water prevents it. Grey that appears while raw potatoes sit cut is oxidation, also harmless. Sunken dark rotten patches with a smell are a different matter and go in the bin.
None of the common potato discolourations are a food safety problem, which is worth saying first because they look alarming. The useful thing is knowing which one you have, because two of the three are preventable.
Step by step
- Note when the colour appeared.After cooking, over one to a few hours, especially in mash held warm or in boiled potatoes left in the pan: that is after-cooking darkening. While sitting raw in water or on a board: that is enzymatic browning. Present before you cooked it, in a sunken soft patch: that is rot or bruising.
- For after-cooking darkening, acidify the water.A tablespoon of lemon juice or white vinegar per large pan of water is enough. The acid binds the iron so it cannot form the dark iron–chlorogenic acid complex. Cream of tartar does the same job and is what commercial kitchens use.
- Cool or serve promptly.The reaction needs time and air. Potatoes eaten straight from the pan rarely show it. If you are cooking ahead, cool them quickly and cover, rather than leaving them warm and exposed.
- For raw grey, keep cut potatoes under water.Cut surfaces oxidise on contact with air. Full immersion in cold water stops it completely and holds for several hours in the fridge. Drain and dry thoroughly before cooking, because surface water is the enemy of every browning method.
- Reject the ones that are actually spoiled.Soft sunken brown or black areas, a musty or sweet-rotten smell, weeping liquid, or a hollow dark centre are decay. Cut generously past a small firm bruise; bin anything soft, smelly or extensively affected.
- Change variety or supplier if it is constant.Susceptibility to after-cooking darkening is varietal and is also driven by growing conditions — low potassium in the soil makes it much worse. If one variety greys every single time and another never does, believe the pattern.
Tips
- A cast iron or unlined aluminium pan can add to the grey. A stainless pan is the neutral choice for pale foods.
- Mash discolours faster than whole potatoes because you have exposed vastly more surface to air.
- Green patches under the skin are a different thing again — that is light exposure, and green means solanine, so cut it away generously or discard.
Questions people ask
Is it safe to eat grey potatoes?
Yes. After-cooking darkening and oxidation are both cosmetic. Only spoilage — softness, smell, weeping — and green flesh are reasons not to eat a potato.
Why does it happen more with mashed potato?
More surface area exposed to air, more mechanical mixing to spread the reactants around, and mash is usually the thing that gets held warm the longest.