# Question of the Week

## Why don't potato peelers need sharpening?

Sun, 13th Mar 2011

Part of the shows Why did a Laser Make My Nuts Glow? and Why don't potato peelers need sharpening?

### Question

Why don't potato peelers need sharpening?

We put this to Tony Atkins, Emeritus Professor of Engineering at the University of Reading...

Diana -   It turns out that it doesn’t need sharpening, simply because it doesn’t need to be sharp...

Tony -    There is something called the critical crack opening displacement which means how much have you got to stretch the end of the crack before the crack will carry on propagating?  If you're using a knife to get rid of the skin or the peel, then you have to wedge open the material at the end of the blade, at least as much as this property called the crack opening displacement.  I was surprised to discover this magic displacement that you have to achieve for potato is actually much bigger than you would have to do for meat or cheese.  Now the implication of that is, that to cut meat certainly and cheese, you really need something very sharp whereas with potato, because this property value is big anyway, you can say that, “Well, why bother to have something which is sharp?”

Diana -   A potato peeler doesn’t need sharpening because it still works well even when it’s blunt.  But there's also something about the angle of cutting which makes using a less sharp tool even easier.

Tony -   What is much more interesting about cutting is the whole business of why, if you take a knife, however sharp it is, you can cut, but it is so much easier if you introduce some horizontal reciprocating motion.

That turns out to be a very, very interesting problem that I've solved and it goes like this: If you're say, cutting something that requires work to do it, which is force times displacement, and you say, “Okay, well if I put a bit of work in sideways, clearly I won't require as much work pressing down.”

That is true, but when you do the sums, you get a strange non-linear coupling between the forces.  Meaning that the slightest horizontal movement reduces the vertical force considerably and that's why it’s so noticeable.  What this also goes on to is that it means that the overall forces required to cut are less – if you have this slice and push it together, and that means that you don't damage the surfaces that you cut.

That can be commercially very important if you think of buying salads or something containing melon in a supermarket.  In the normal way of cutting them, you damage lots of cells adjacent to the surface that you've cut and these weep out liquid.  So this shelf life is improved if you do the cutting properly.

#### Make a comment

There is an invisible metal mesh built into the top layer of potatoes that sharpens the blade as you peel the skin off.
it vanishes once the skin is removed !

Here you can see it revealed after I had performed the ' Reveal-Da-Thrill-Of-Da-Peel ' incantation !

The Magical-Metal-Mesh in all it's glory !

neilep, Sun, 6th Mar 2011

Differential hardening applied to one side of the blade makes one side harder than the other,
so it wears asymmetrically, creating a new sharp edge as it is used ...

RD, Sun, 6th Mar 2011

I have two propositions:
(a) The potato doesn't dull the blade of the potato peeler, due to the starchy, soft composition of the potato. The potato is not hard enough. In geology, a harder mineral/rock will scratch a less-hard one. Since the potato is not as hard - geologically speaking - as a (presumably) steel peeler, it wont scratch or dull the blade.

(b) According to WolframAlpha, http://www.wolframalpha.com/input/?i=potato+mineral+content
A 100 g Irish potato has about 1.1 g of minerals in it. That's 1.1% of its total mass.

So the potato could have a great enough mineral content for the minerals in it to rub up against the steel blade to keep it sharp upon peeling.
It would be like cutting with a knife into a big chunk of candle wax that has mineral particles spread out inside it. The minerals embedded in the wax could be concentrated enough to scratch the sides of the blade. Lamprey5, Sun, 6th Mar 2011

I suspect it has something to do with the angle you 'cut' (peel) at. JnA, Mon, 7th Mar 2011

I suspect that it does not get dull because there is nothing in the potato to dull the blade.  Thanks for comments.  Joe L. Ogan Joe L. Ogan, Mon, 7th Mar 2011

It's obvious, even to a casual observer (like, for example, Neil).

As the blade dulls, it takes a deeper cut, which also requires the operator to exert a bit more force. The explanation lies in a lot of fancy vector crap analysis that I'd rather not get into at the moment.

Geezer, Mon, 7th Mar 2011

http://www.ebay.co.uk/ RD, Mon, 7th Mar 2011

I don't think that's the link you meant to post RD

BTW, does it come with a cardboard pyramid? Geezer, Mon, 7th Mar 2011

I didn't want to link directly to the peeler product and give it a free advert.
It's on ebay, readers can Google the quote if they want to find it.

The "self sharpening" phenomenon is legit: no pyramid power required. One side of the blade is harder than the other, the softer side wears away faster, as it wears a new angled (sharp) edge is constantly being created  ... RD, Mon, 7th Mar 2011

Almost all the minerals in a potato are in solution. Bored chemist, Mon, 7th Mar 2011

I can't wait to watch the show on march 13th!!!:D justin_ruch1180, Tue, 8th Mar 2011

I didn't want to link directly to the peeler product and give it a free advert.
It's on ebay, readers can Google the quote if they want to find it.

The "self sharpening" phenomenon is legit: no pyramid power required. One side of the blade is harder than the other, the softer side wears away faster, as it wears a new angled (sharp) edge is constantly being created  ...

Ah! Sneaky. I wonder why they don't make razor blades like that. Wait a minute, I think I just figured that out

Geezer, Tue, 8th Mar 2011

Some very good ideas being put forward!

My first inclination is that the starchy, mineral laden 'meat' of the potato has some sort of abrasive property that continually sharpens the soft metal of the peeler as it's used.

My second idea is that the materials that are being peeled (could be carrot, apple, etc.) are actually soft enough that they're not dulling the blade at all, in fact, the blade isn't really cutting anything, it's just being forced like a specialized inclined plane (go simple machines!) under the skin of the fruit/veggie at the correct angle to deflect the skin off of the flesh of the fruit.

I imagine my second answer to be most correct because over my years of peeling, I'f definitely seen some reduced efficiency in the peeler, but it hasn't met with outright failure due to the fact that it's not actually necessary for it to do any 'cutting', it's just being wedged under the skin with enough applied force to peel regardless of the sharpness of the blade.

Love the show! thanks for the shout out this week!

Tay Sharpe Tay, Thu, 10th Mar 2011

I agree with this poster - who could not?  "Cos as tay sharpe said ..."  Go on say it aloud! he cannot be wrong imatfaal, Thu, 10th Mar 2011

Welease the hounds!

Q: What ticks on the wall?

A: Ticky paper. Geezer, Thu, 10th Mar 2011

The potato peeler may be auto sharpening but also it is not used in a slicing motion. The thing is that when you sharpen a knife on a steel, what you are actually doing is creating hundreds of tiny little metal burrs on the knife edge. These burrs have a fine tearing effect on whatever you are slicing with the slicing motion and make a very fine cut. A freshly sharpened knife will cut through a tomato skin with only a tiny slicing motion. But as the blade is used the burrs wear off and the tearing effect is greatly reduced. So you need to define what "sharp" really means when applied to a food cutting knife.
A poatato peeler has no slicing motion so the burrs have much less effect so the lack of them makes little difference.
Get your microscope out and compare the blade of a blunt knife (no burrs) and one freshly sharpened with a steel.
percepts, Tue, 29th Mar 2011

Thank you, percepts; nicely put. Meera, Tue, 29th Mar 2011