Why do my potatoes go black after cooking? | Kitchen aide

2 hours ago 1

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.”

Much better, O’Toole says, to drain those cooked spuds, spread them out on a rack and leave to steam dry with a tea towel over them. “If you want to be super-prepared, though, you could try my frozen ready-to-roast potatoes: simply parboil, drain and steam-dry some potatoes, then coat them in melted fat [duck, goose, beef, say], then pop in the freezer for roasties on demand. Otherwise, Hunt would store any leftover cooked potatoes in a sealed container in the fridge, but only once they’ve cooled completely. “You could then turn them into crisp potato cakes to have with smoked trout or a poached egg,” Murphy suggests. “That’s a simple but delicious supper.”

More often than not, however, the discoloration of potatoes happens before cooking, and Murphy wonders if that’s maybe what’s happening chez Jean. Of course, if you’re cooking for a gang, realistically your peeled and cut potatoes will need to sit around for a bit before boiling. Oxygen is the nemesis of the cut potato, Hunt adds, “so I would keep them in cold water, though that will stop them turning brown for only so long.” Again, you’ll want to lower the pH level of that water, and Hunt usually adds a drop of vinegar or half a lemon, should he have one knocking about.

“At the restaurant, we always cook our potatoes with a bit of seaweed,” Murphy adds. “Potatoes are mineral suckers, and by adding seaweed when boiling, you’re helping to replace those lost minerals.” And, that way, you’ll also need less salt and add more flavour, which is a win all round.