Grated potato goes grey, then pink, then brown, and it starts within minutes.
It is oxidation — enzymes reacting with air — and it is cosmetic rather than dangerous.
Here is what actually stops it, best first.
1. Grate the onion in as you go
The traditional fix and the most effective. Onion contains sulphur compounds that inhibit
the browning enzyme, so alternating — a handful of potato, a little onion, more potato —
keeps the mixture pale from the start.
Adding all the onion at the end works far less well, which is why the order matters
more than the quantity.
2. Have the pan hot before you start grating
The most reliable method is simply not giving the mixture time. Fry in batches as you go
rather than grating everything first. Deruny are a dish you cook while you prepare,
not after.
3. Add a little acid
A teaspoon of lemon juice or a splash of vinegar per kilo slows the reaction noticeably.
Not enough to taste, and it buys several minutes of working time.
Third rather than first because it helps less than the two above.
4. Cover the surface with film
If you must hold grated potato briefly, press cling film directly onto it —
no air contact. A holding measure, not a solution.
What not to do: soak it in water
The intuitive move, and it backfires. Water does prevent browning, but it also washes away
the starch that binds the pancakes together. You end up with pale deruny that fall apart —
worse than grey ones that hold.
And a note on the food processor
Faster, but it produces a lot of surface area at once and warms the potato slightly,
both of which accelerate the reaction. A hand grater and small batches keep the colour better.
If it went grey anyway
Cook it. Oxidation is a colour change, not spoilage — the mixture fries perfectly well,
just to a duller pancake. What you should not do is leave grated potato standing for hours;
by then the concern is no longer colour.
See also:
What is deruny ·
How long do peeled potatoes last in the fridge ·
Can you freeze potatoes ·
Why potatoes darken after boiling