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Shel_B

Potatoes Stick to Knife Blade

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A few nights ago I was dicing potatoes for a batch of potato leek soup.  I was using, as I often do, Yukon Gold potatoes.

 

Whenever I cut into the potatoes, especially when cutting a whole potato in half, the potato would stick to the knife blade.  Not a big deal, but somewhat annoying.

 

Why does this happen, and how can the problem be eliminated or mitigated?  I generally use a standard-style chef's knife (8-inch Victorinox), but the problem is the same for any similar knife I use.  Might the knife blade have something to do with the problem?  Could the problem be solved with a different blade design?  I have heard that using a blade with dimples or ridges will make the problem go away, but have not found anything (yet) to substantiate that idea.


 ... Shel


 

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Santoku helps w this but it isn't perfect.

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The smooth and moist surface of the potato is such that it forms an adhesion to the blade.  Knives with cullens or divots cut in the blade decrease the surface of the knife against the potato so there is less resistance but the potato will still stick above the dished out parts of the knife.  The result makes the knife seem sharper but does not do much for your problem. When I cut a potato, I hold the potato in on both sides of the cut with one hand and pull the knife out after the cut.  It is a little slower but keeps the potato together if you plan to cut it again in another direction as for fries.


Edited by Norm Matthews (log)
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Shel_B,   I've recommended the Santoku that gfweb speaks of to loads of people.   Any more I mention to them to check out the Dollar Tree stores as they carry several sizes of them.  They are easier to use than the French Chef's knife with the hard veggies.

But if a Potato is wet along with the blade of your French Chef's knife the first time you cut through the whole potato, then it's probably going to stick with the combination of starch and a polished surface.

 

Here is some information with images:

 

http://learnfromthechefs.com/santoku-knife/

 

Also, and more expensive, are some of the "Damascus" blade knifes which are forged with a texturing hammer so the knife blade

has numerous indentations over it's surface where air will flow in as cutting takes place.  There are all kinds of methods, designs,

and reasons for this treatment.  In my own opinion these types of knives need to be given special attention when cleaning.

 

DSCF4638.jpg

 

 

Source of Image:

 

http://www.bushcraftuk.com/forum/showthread.php?t=69313

 

 

 

But for a dollar and a visit to a Dollar Tree Store (or some "dollar" value store)  you may find one of these and try it provided you haven't done something like this already.  Consider how often we process potatoes in our cooking. 

 

LOL,  I'm sure that's why they always seem to be out of those French Fry cutters they sell at Harbor Freight.  I'd hate to have to cut a load of French Fries with a knife of any kind.   

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The problem is not adhesion.

 

In General Physics 101, there is a classic experiment you can perform. if you take two flat pieces of glass, put a small drop of water in between, and you will find that you can't pull them apart.

 

What is forcing the glass together is atmospheric pressure at over 14 lbs per sq. in. if the panes of glass are 5" square, you will need over 350 lbs of power to pull them apart.

 

Using two suction cups, you can "walk" up a skyscraper's curtain wall. 

 

dcarch

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The problem is not adhesion.

 

In General Physics 101, there is a classic experiment you can perform. if you take two flat pieces of glass, put a small drop of water in between, and you will find that you can't pull them apart.

 

What is forcing the glass together is atmospheric pressure at over 14 lbs per sq. in. if the panes of glass are 5" square, you will need over 350 lbs of power to pull them apart.

 

Using two suction cups, you can "walk" up a skyscraper's curtain wall. 

 

dcarch

More going on than air pressure with the panes of glass.

 

Haven't we been down this road before?

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More going on than air pressure with the panes of glass.

 

Haven't we been down this road before?

 

Remind me.

 

I am sure there are other possible forces, for instance, all matters produce gravitational force, but that is extremely weak.

 

dcarch

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  1. Potato starch is partly water soluable.  You knife is coated with a thin layer of starch and the pieces of spud stick to the starch. 

 

So, you do what countless cooks the world over do, dip you knife in water every few slices to wash the starch off.

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  1. Potato starch is partly water soluable.  You knife is coated with a thin layer of starch and the pieces of spud stick to the starch. 

 

So, you do what countless cooks the world over do, dip you knife in water every few slices to wash the starch off.

 

 

Not true.

 

Starch water is not an adhesive. It work better if you dip you knife in water, because thicker layer of water acts as a lubricant.

 

It is so easy to find out the power of atmospheric pressure. Get two flat panes of glass with a thin layer of water in between and see if you can pull it apart.

 

dcarch

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 because thicker layer of water acts as a lubricant.

 

And compare that to,

The problem is not adhesion.

 

In General Physics 101, there is a classic experiment you can perform. if you take two flat pieces of glass, put a small drop of water in between, and you will find that you can't pull them apart.

 

 

So which theory is it now?

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Starch is certainly part of the story. Potatoes stick much more than other watery veg. Around 8 minutes into this video, you can see Heston using water and potato starch to put up wallpaper. Granted, he's using a lot of starch.

 

 

On an off topic note, anyone know how to embed YouTube videos on the forum? (EDIT: Thanks Martin!)


Edited by btbyrd (log)
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With a knife you can rapidly flick your wrist to the side and the slice will usually pop off.

I like to use a Chinese cleaver because it's easy to prevent sticking by cutting with the tip.

Martin Yan demonstrates in the following video starting at about 3:25......

 


~Martin :)

"Unsupervised, rebellious, radical agrarian experimenter, minimalist penny-pincher, self-reliant homesteader, and adventurous cook. Crotchety, cantankerous, terse, curmudgeon, non-conformist, and contrarian who questions everything!"

The best thing about a vegetable garden is all the meat you can hunt and trap out of it! 

 

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Remind me.

 

I am sure there are other possible forces, for instance, all matters produce gravitational force, but that is extremely weak.

 

dcarch

http://forums.egullet.org/topic/142365-nonstick-coatings-on-knives/?hl=+dcarch%20+glass%20+water%20+pressure

 

Cohesion and surface tension.

 

If air pressure was the important force then water between the glass would be unnecessary.

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“---If air pressure was the important force then water between the glass would be unnecessary.—“

 

That is very true. The water helps because flat glass plates are not really flat. However, if you are to use optic flat glass, so flat that when you put the two plates together you can produce Newton’s Rings, than water may not be needed.

 

“---Starch is certainly part of the story. Potatoes stick much more than other watery veg. Around 8 minutes into this video, you can see Heston using water and potato starch to put up wallpaper. Granted, he's using a lot of starch.----“

 

Starch in water has no adhesion, until you cook it, then it become glue.

 

“--So which theory is it now?----“

 

The two conditions are for different theories. Very thin layer (molecule thinness) of water seals the very thin unevenness of “flat” glass, thicker water layer becomes a lubricant, just like when you are skiing, you in fact are skiing on water, not on ice (that’s again another physics theory).

 

You can slide the glass plates apart much easier than pulling them apart because according to the law of physics, you need no force to work perpendicular against the other force (How bicycle works). 

 

dcarch

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Starch is certainly part of the story. Potatoes stick much more than other watery veg. Around 8 minutes into this video, you can see Heston using water and potato starch to put up wallpaper. Granted, he's using a lot of starch.

 

 

On an off topic note, anyone know how to embed YouTube videos on the forum? (EDIT: Thanks Martin!)

The video uses cooked starch which gelatinizes. Uncooked starch performed completely differently.


PS: I am a guy.

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Starch in water has no adhesion, until you cook it, then it become glue.

 

 

 

 

Slice a spud a few times with a knife, and let the knife sit out on the counter for a few minutes.  You will see a thin crust of starch forming on the knife.  When still wet, it wipes off fairly easily, when dry it has to be scrubbed off, or you can scratch it off with a fingernail. 

 

A spud is mostly water and starch.  Slice it, and some residue will adhere to your knife, and this residue dries very quickly.

 

You just have to wash it (the starch) off..... 

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"---Slice it, and some residue will adhere to your knife, and this residue dries very quickly.---"

 

If you are a dare devil, I would advise walking up a skyscraper with air pressure generated by suction cups, not with two half sliced potatoes. :-)

 

Atmospheric pressure acting on two flat plates, such as with a flat knife blade and flat slice of potato, or any non-starchy food, is a well known phenomenon.

 

dcarch


Edited by dcarch (log)

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I'm not a smart fella, physics is in the distant past, don't know anything about air pressure except my tires like it and liquids and starches are too much like chemistry.

 

I do know that with a thin, stamped knife like a Victoinox, the blade will be relatively straight from the spine down to the primary bevel or edge.   There will be little to no bevel to "push away" food from the blade.  Thus food will stick to the blade.   Knife geeks even refer to this quality as "sticktion".  It’s a function of the bevel, finish, and other voodoo.   Some foods stick more than others and potatoes are probably the worst.  Suspect it has to do with moisture content and density but…(don't forget the not smart part)

 

Shel_B can mitigate food sticking by using draw (pull) cuts through the food rather than push cutting or rocking.  This is slower and if it's unfamiliar it will increase the chance of dna ending up in the food.  It may be worthwhile for obtaining precision cuts.  Alternatively she can substitue cauliflower for potatoes.   Kidding.

 

Sticktion can be reduced by upping the game on the knife used.  An entry level Japanese Chef or Gyuto will have an aysymetric edge and a secondary bevel,  both will serve to push away food from the blade.  A Suisin Western is an example of this type of knife and can be had for around $100.00.   Gratins, dimples, holes, et al serve well to sell knives.

 

http://korin.com/Susin-Inox-Gyutou?sc=27&category=280068


Edited by daveb (log)
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"-

 

Atmospheric pressure acting on two flat plates, such as with a flat knife blade and flat slice of potato, or any non-starchy food, is a well known phenomenon.

 

dcarch

 

 

Yup, tomatoes and pickles will stick on to a knife, but not as badly as potatotes.  What's a tomato? Water mostly, yet they don't stick on to a knife as tenaciously as a potato slice.

 

A potato is mostly starch and water, unlike the tomato, or pickle, or fruit, etc, which has no, or very little starch. 

 

 

Which brings us right back to ShelB's original question as to why potatoes stick so badly to a knife and how to avoid it, or at least to mitigate it.

 

 

Here's something just about everyone can relate to:

 

You are adding a cornstarch slurry to a soup/sauce/pudding.  You stir the starch with a cold liquid in  a separate bowl  in order to pour it into your boiling liquid.  Yet you know that if you stir the cornstarch/cold liquid smooth, the starch will settle down at the bottom of the bowl with the liquid swimming on top in a very short period of time.  When you grasp the whisk/fork/impliment  to agitate the slurry again, you find it "welded" to the bottom of the bowl.  A few brief motions with the impliment will free it from the bed of starch.

 

When you slice a potato you are smearing a thin layer of water and starch on your knife. It sticks.

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There are many forces in nature, as I said, gravitational force is one of them for all matters, then there is electrostatic, and ferromagnetic, etc., but atmospheric is the strongest in the case of flat surfaces.

 

Surface tension is a force, but not that strong a force compared to 14 lbs/sq of atmospheric pressure. If you add detergent to water which will supposedly destroy water surface tension, and use that in between two plates of glass, you still will not be able to pull the glass apart. Water surface tension is strong enough to form a bead. If that is so strong, it will try to form a sphere in between the glass and push the plates apart, not pulling them together. Which may be interesting to try using mercury, which is 9 to 10 times stronger in surface tension. 

 

Try using alcohol, which has much lower surface tension, between the glass plates, you will experience the same difficulty in pulling them apart.

 

It is really very simple, think suction cups and cutting food with a shine knife surface.

 

dcarch


Edited by dcarch (log)

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so a three inch diameter slice of potato stuck to a knife has 14.7 psi pushing on one side, holding it up to the knife.

 

hmmm, 3 inch diameter, = 7+ square inches of area, x 14.7 lbs/sq-in = about 104 pounds of force sticking the potato to the blade.

 

my bet is, put the potato on the floor, cover it with a cutting board, stand a 104 lb cook on the cutting board....

and you'll get mashed potato.

 

something else is responsible.

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