A drop of lemon and being selective about your choice of cookware could zap any troubles with blackened spuds, as would a highly novel solution from the seaside

Why do some potatoes turn black on cooking, and how do I stop this happening? I usually leave them to cool in the cooking water, but should I plunge them in cold water instead?”
Jean, Hampshire
“We’ve all been there,” sympathises spud queen Poppy O’Toole. “It’s a harmless chemical reaction,” the author of The Potato Book continues, “but it looks rank and only gets worse with the slow cooling process that Jean’s using.” But let’s wind things back for a moment. According to the food science guru Harold McGee, in his bible On Food & Cooking, the darkening of cooked potatoes “is caused by the combination of iron ions, a phenolic substance [chlorogenic acid] and oxygen, which react to form a pigmented complex”. So what’s the solution? Make the pH of the water “distinctly acidic”, which McGee does by adding cream of tartar or lemon juice “after the potatoes are half-cooked”.

Another possible suspect for Jean’s blackening tubers is her cookware: “Reactive metals such as a carbon steel knife or aluminium pan may also be the cause of the issue,” says the Guardian’s Tom Hunt, which is why he recommends using a non-reactive metal (think stainless steel) instead. “Leaving the cooked potatoes in water is also a bad idea,” adds Jess Murphy, chef patron of Kai Galway in Ireland and author of The Kai Cookbook:“They are like little sponges.” Hunt couldn’t agree more: “The potatoes will absorb the water and turn soggy and less fluffy – and never refresh them under cold water or in a cold plunge, either, unless it’s momentary.”

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