Why do some potatoes leave a white mess on cutting boards?

What you see: Your potato has left a lot of white stuff on your cutting board.
What it is: Potato starch!
Eat or toss: Please do not eat your cutting board. Otherwise, nothing wrong here—the potatoes are fine. 

What’s all that white stuff coming out of those sliced potatoes?

Check out Starch Wars! AKA, the aftermath of a potato chopping session on my cutting board. The white smears here are no phantom menace, just potato starch and water that spilled forth when my knife cut into a potato, slicing open potato cells in the process.

Nora Olsen, a professor and potato extension specialist at the University of Idaho, said that some potatoes have higher water content and/or higher starch content, all of which can cause them to leave a bit more behind on the cutting board. It has nothing to do the potato’s fitness for eating. 

Potatoes are full of water

While a potato might seem dry, the tubers actually contain a lot of water. Potatoes are hard because of the pressure of that water (turgidity), which is stored in little pockets (known as vacuoles) inside their cells. Olsen said that the liquid inside a potato contains salts, sugars and other elements that give it a lower freezing point than pure water. Kind of like an anti-freeze. Cool! Like, really, really cool!

The type of potato and its field experience (literally, its experience in the field) can affect how much water a potato might release onto a cutting board. How and for how long it’s stored can also affect how the potato distributes that water in its tissues.

(While we’re talking about potato storage, quick PSA: Don’t store whole, raw potatoes in the fridge! They might develop black areas inside like this.)

Potato starch content varies

A potato’s starch content can also vary depending on the variety and how it’s stored. Most of the dry matter in a potato is starch, which is produced and stashed inside cells in little pods called amyloplasts. Potatoes differ in the type, amount and size of starch granules they carry. For example, the large starch granules in Russett potatoes give them a dryer texture than the small starch granules in their waxier, moister cousins, red potatoes.

So, like all of us, the potato’s variety and life experience impact its water and starch make up, which can further determine whether you might end up with abstract art in the midst of your dinner prep. 

Potato starch can do lots of tricks

In my experience, the potato starch washes away cleanly (can’t say the same for the white stuff that oozed out of this butternut squash though!). Olsen points out that potato starch is incredibly useful stuff. These days you’ll find it in everything from plastics to gluten-free baked goods. While it would be hard to salvage anything useful from the white mess on this cutting board, water left over after boiling potatoes contains lots of starch and can be frozen and defrosted as needed to thicken soups.