You grate a bowl of potatoes, turn round to get the eggs, and by the time you look back the whole lot has gone grey, or a dull pinkish brown. It is not spoiled. It is the potato reacting with the air, and it is harmless.
I grate a lot of raw potato for placki ziemniaczane, Polish potato pancakes, so here is what is happening and how to slow it down.
The short answer
Grating tears open the potato cells. Inside, two things are normally kept apart: some colourless compounds and the enzymes that react with them. Once they meet each other and the air, the enzymes turn those compounds into dark pigments. That is the grey. It is the same reaction that browns a cut apple.
What is going on
In a whole potato the compounds sit in one part of the cell and the enzymes in another, so nothing happens. A grater breaks every cell it touches. The enzymes use oxygen from the air to join the compounds together into bigger and bigger clusters, and those clusters absorb light, so you see them as grey or brown. The finer you grate, the more cells you break, which is why finely grated potato goes grey fastest.
How to slow it down
- Work quickly. The reaction needs time. Grate, mix and get them into the pan without a pause.
- Add a little acid. The browning enzymes work very slowly in acid, so a squeeze of lemon through the grated potato slows it right down.
- Keep the air off. Chilling slows the enzymes down, and grated potato sitting in cold water keeps its colour, because the water keeps the air off. For placki, though, the best way is to mix the grated potato with the other ingredients as soon as it is grated, and fry it straight away. In my experience the onion in the batter slows it a little as well.
- Cook it. Heat destroys the enzymes, so once the potato is in the pan the greying stops. Frying also hides any colour that has already formed.
Does it matter?
Not for taste or safety. In placki the colour mostly disappears under a golden crust. It matters more for things that stay pale, like dumplings, so for those it is worth working fast or using a little lemon.
Boiled potatoes that go grey
That is a different thing. Sometimes a cooked potato develops a bluish grey patch inside after it cools. That comes from iron in the potato, one of its natural compounds and oxygen joining together. A little lemon juice or cream of tartar in the cooking water, added once the potatoes are about half cooked, keeps it down.
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