Why Do Sweet Potatoes Turn Black When You Cut Them?

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I used to think they'd gone bad

For a long time I assumed the colour change meant the sweet potato was spoiled, so I threw out a lot of perfectly good ones. Then I found out it isn't spoilage at all. It's enzymatic browning, the same reason a cut apple turns brown. Once I knew that, I stopped wasting them and started working out how to keep the cut ones looking decent.

One side note, since people get confused: durian honey sweet potato isn't a cross between durian and sweet potato. It's just a very sweet kind of honey sweet potato with a texture like durian, no tough strings, and it smells amazing when baked.

Short-term storage (1-2 hours)

The fastest and easiest way is to put the cut sweet potato straight into plain water, with the water just covering the cut surface. The water keeps the air off it so it doesn't change colour, and I change the water once partway through to be safe. Don't soak it for more than 4 hours though, because it soaks up water and turns mushy, and the texture is ruined.

If soaking isn't convenient, I wrap the cut sweet potato tightly in cling film, especially over the cut surface, squeeze out as much air as I can, then put it in a container in the fridge. This keeps the colour and the moisture in, and when I take it out it's still soft and sticky.

Longer storage (2-6 hours)

For longer than that, my first choice is light salt water, because it doesn't change the taste. I mix 1000ml of water with 5g of salt, about as salty as you'd cook with, stir until it dissolves, and soak the cut sweet potato for 10-15 minutes. Then I drain it and put it in the fridge. It stops the browning and keeps the sweetness, so it tastes just like it did before.

The other option is lemon water or vinegar water. I squeeze 1-2 slices of lemon into water, or add 2-3 drops of white vinegar, and soak for 5-10 minutes. The acid slows the browning and the lemon adds a bit of fruity smell. Go easy on the vinegar, because too much covers up the sweet potato's own sweetness.

What not to do

Don't leave cut sweet potato at room temperature or in direct sunlight, because heat makes it go dark faster. Get it into the fridge quickly, 0-4℃ is best.

And don't mistake normal browning for spoilage. If it's only slightly yellow or brown and it isn't mushy or sour-smelling, it's fine to eat. If it's very black, soft and smells off, throw it out. The easiest fix is to only cut what you're about to eat, since fresh-cut tastes best anyway and there's nothing to brown.

My quick summary

For everyday use, plain water soaking or sealing it in the fridge is enough. For longer storage, use light salt water. It's easy, and it means no more wasted sweet potatoes.