# Convexity, flatness, stiction and food release



## blokey (Jul 2, 2022)

I recently got into an argument online on food release, the other person insist that the only way to increase the food release without resort to thing like S grind is increase the thickness of the knife, and the slight convexity on the kitchen knife is not intentional but rather a flaw of the manufacturing equipment. His point is the flat V grind is better than convex because it cuts better and the slight convexity on kitchen won't affect stiction and food release, and a microbevel will do more for food release. which really contrary to my own experience. This got me curious, is there any serious study on this subject matter? any physical model we can use to determine if the slight convexity actually does affect food release?


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## M1k3 (Jul 2, 2022)

No serious study that I'm aware of.

That said, 2 flat planes have a higher surface contact patch and therefore higher suction/friction/drag/whateverprofessorIansuggests. 2 curved planes have a small contact patch and lower suction/friction/drag/whateverprofessorIansuggests.


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## HumbleHomeCook (Jul 2, 2022)

Even axes aren't true "V's".


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## tostadas (Jul 2, 2022)

M1k3 said:


> No serious study that I'm aware of.
> 
> That said, 2 flat planes have a higher surface contact patch and therefore higher coefficient of friction/drag. 2 curved planes have a small contact patch and lower coefficient of friction/drag.


Science hat on. Coefficient of friction is not affected by surface area. It's based on the smooth/roughness of the surface. Friction or drag force is equal to coefficient of friction times the perpendicular force between the two objects.

That being said, I do believe convexity helps with sticking, but I'm not sure the actual mechanics behind it. But I'd say it's not simply "friction" in the technical sense.


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## esoo (Jul 2, 2022)

Slice a cucumber with a flat grind vs a convex knife and you'll discover the advantage - on a convex knife the slices will come off easier. 

IMO, the advantage of any grind decreases as the size of the product decreases. Slicing a potato, the grind has an effect, but dicing garlic it has little effect.


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## M1k3 (Jul 2, 2022)

tostadas said:


> Science hat on. Coefficient of friction is not affected by surface area. It's based on the smooth/roughness of the surface. Friction or drag force is equal to coefficient of friction times the perpendicular force between the two objects.
> 
> That being said, I do believe convexity helps with sticking, but I'm not sure the actual mechanics behind it. But I'd say it's not simply "friction" in the technical sense.


Corrected my post


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## ian (Jul 2, 2022)

Ya, prob more like a suction cup. Friction doesn’t seem that relevant to why a potato slice sticks to a blade when you hold it vertically and shake it. Pretty hard to stick a (flat) suction cup on a convex surface tho.


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## ian (Jul 2, 2022)

Or maybe a better explanation is something having to do with surface tension? I don’t really know any physics, but watery ingredients are the ones that stick the most. If you have a slice of something that’s stuck onto the knife with water in between, it can’t come straight off perpendicularly because the water will create a sort of suction effect, and maybe somehow surface tension or something keeps it from shearing. If the knife is flat ground, it’s easier for this to happen, while if the slices are flat and the knife is convex it’s harder to get the slice stuck on by water in the first place.

Edit: writing this post makes me realize I don’t really know what surface tension is, beyond my own experimentally informed intuition.


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## MSicardCutlery (Jul 2, 2022)

I've made flatter knives, and more convex ones, and what I found was that food release is better with a thicker knife regardless. My guess is that with a thicker blade, and therefore steeper blade face/cross sectional geometry, the angle of deflection is going to be higher increasing the peeling effect. A wider boat makes a wider wake type idea.


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## ian (Jul 2, 2022)

Wonder if a thicker knife will also create a less clean cut on some ingredients, resulting in less sticking. Or the fact that it’s pushing the slice forcefully to the side stresses it so that at the end of the cut the slice pops a bit, making it more likely to be flung off. Or is that what you meant by the peeling effect?


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## blokey (Jul 2, 2022)

Should we fund a study on that?


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## Se1ryu (Jul 2, 2022)

Maybe the blade finish play role too on food release and stickiness . I tried ashi's AEB-L Gyuto (thin blade, convex edge) and food release is ok. Yanagiba in the other hand if I finish it with 3000 grit rough kasumi finish food will get stuck and create friction, with mirror finish/ polish food just glide thru fish or protein like nothing. Fat content also make a difference and I think most of. People here already know. That come only from my experience and someone maybe experience it differently. 
This is a good thread, let's see what other people think about this.

Maybe someone studied about this


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## MSicardCutlery (Jul 2, 2022)

ian said:


> Wonder if a thicker knife will also create a less clean cut on some ingredients, resulting in less sticking. Or the fact that it’s pushing the slice forcefully to the side stresses it so that at the end of the cut the slice pops a bit, making it more likely to be flung off. Or is that what you meant by the peeling effect?


Not quite. Think of an acute triangle for a moment, pointing upwards, now picture a vertical line intersecting one of the top two lines, that will represent an object in motion making contact with that triangle. The angle formed between that vertical line and the line forming that side of the triangle, though some trigonometric voodoo that I've forgotten since I took physics will give you the the amount of force being directed downwards and the amount of force being directed outwards. The same thing can apply to something with a curved/ogived outline, a boat, a bullet, you name it. So on a thicker knife the angle of the radius that creates the convex section of the edge will be steeper (assuming the height of the convexity is equal between two blades of different thicknesses), the amount of force directed outward is greater. There's greater deflection pressure to pull the food out and away from the primary bevel angle, which is more acute than that of the convexed area.

But also definitely, thicker knives will break harder foods like carrots. Those will pop off and fly for a ways.


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## M1k3 (Jul 2, 2022)

ian said:


> Wonder if a thicker knife will also create a less clean cut on some ingredients, resulting in less sticking. Or the fact that it’s pushing the slice forcefully to the side stresses it so that at the end of the cut the slice pops a bit, making it more likely to be flung off. Or is that what you meant by the peeling effect?


Remember them fat knives that start splitting the ingredient before the edge even gets there?


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## refcast (Jul 3, 2022)

Google Scholar







scholar.google.com





Scientific articles must be paid for . . . Or the authors asked for a free reading sample I guess. Or maybe sci hub

Oh here's one. . . Still doesn't answer food release though, only cutting force. The math is mathy . . . But seems to be calculus and not something more obscure






eNUFTIR: Modeling of cutting of food products







dspace.nuft.edu.ua


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## ian (Jul 3, 2022)

refcast said:


> Scientific articles must be paid for . . . Or the authors asked for a free reading sample I guess. Or maybe sci hub
> 
> Oh here's one. . . Still doesn't answer food release though, only cutting force. The math is mathy . . . But seems to be calculus and not something more obscure
> 
> ...



Damn that article is hard to understand…. I get that English isn’t their first language, but man. I’m not completely convinced it’s not AI generated.


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## refcast (Jul 3, 2022)

@ian

Yeah I looked only briefly st the article, and only briefly trying to find something on it. . . I don't think that much mathematical modeling helps me much hobby wise, or how I practice stuff


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## jwthaparc (Jul 4, 2022)

blokey said:


> I recently got into an argument online on food release, the other person insist that the only way to increase the food release without resort to thing like S grind is increase the thickness of the knife, and the slight convexity on the kitchen knife is not intentional but rather a flaw of the manufacturing equipment. His point is the flat V grind is better than convex because it cuts better and the slight convexity on kitchen won't affect stiction and food release, and a microbevel will do more for food release. which really contrary to my own experience. This got me curious, is there any serious study on this subject matter? any physical model we can use to determine if the slight convexity actually does affect food release?


What is his arguement to support that "thickness helps with food release"?


That's absurd

Edit: by thickness, what does he mean exactly spine? Directly behind the edge? Halfway up the knife? In any regard. Convexity will definitely help. Exactly where the convexity starts and stops I suspect plays a big role, and likely isn't considered enough. 

Maybe someone should do a test, and check convexing the primary bevels of a few knives in different areas, and to different degrees and see where the difference can be found.


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## blokey (Jul 4, 2022)

jwthaparc said:


> What is his arguement to support that "thickness helps with food release"?
> 
> 
> That's absurd
> ...


I think he means by spine, which is not technically wrong but also weird considering convexity use some of the same principle but more smooth cutting experience. (at least according to my personal experience)


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## Jovidah (Jul 4, 2022)

Blade polish has a significant impact. Polish a knife towards a mirrorish finnish and it'll start sticking like hell.


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## Kippington (Jul 4, 2022)

I've put a *lot* of thought into this subject over the last 5 years, but my god... it's difficult to explain in words. Arguments go into crazy circles that tend to go nowhere fast. I try not to get into it these days, hoping that my old threads here on this forum will explain most of it.

The following quote is interesting though:


blokey said:


> ...the slight convexity on the kitchen knife is not intentional but rather a flaw of the manufacturing equipment...


Has anyone here got a Victorinox Fibrox/Classic Chef's Knife to check the grind? I believe they are an example of a flat grind on a mass manufactured knife. I might be wrong though...


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## jwthaparc (Jul 4, 2022)

Kippington said:


> I've put a *lot* of thought into this subject over the last 5 years, but my god... it's difficult to explain in words. Arguments go into crazy circles that tend to go nowhere fast. I try not to get into it these days, hoping that my old threads here on this forum will explain most of it.
> 
> The following quote is interesting though:
> 
> Has anyone here got a Victorinox Fibrox/Classic Chef's Knife to check the grind? I believe they are an example of a flat grind on a mass manufactured knife. I might be wrong though...


Hmm I wasn't even considering the entirely mass produced kitchen knives into this conversation. It could be a "manufacturing defect" but I feel like more than likely if they really wanted a truly flat grind, they could easily pull it off.


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## Barmoley (Jul 4, 2022)

Kippington said:


> A1
> Has anyone here got a Victorinox Fibrox/Classic Chef's Knife to check the grind? I believe they are an example of a flat grind on a mass manufactured knife. I might be wrong though...


Many mass produced knives are fully flat including victorinox as far as I can remember when I had one a while back. Spyderco Carter are as well at least the first release of AS core. There is no problem creating full flat grinds.

I think when they say that to increase food release you need a thicker knife I think they mean that you can do a lot more with geometry with a thick stock. You can do extreme convex, concave, wide bevels versions, etc. On a thin stock knife there isn't much you can do.

I think we here care too much about food release because we are heavily influenced by pro cooks doing tons of prep. In that scenario good food release can be very helpful.

The outdoor, folder guys who start looking into kitchen knives don't understand food release at all on the other hand. They also don't understand thinning in general. Due to this they tend to think that really thin full flat chef knives are the best. The reason is that they go through food the best and are the easiest to keep sharp and to have the same grind for the life of the knife. Convex grinds cause all sorts of problems for them. They are also used to mostly cutting dry hard stuff so they don't understand that cutting food in volume and fast is different.

Empirically full flat grind is not the best for a chef knife and food release matters. Very thin blades on chef knives are also not the best. A lot depends on preference, technique and the ingredients being cut most of the time.


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## Jovidah (Jul 4, 2022)

From what I've seen online most people into outdoor and folder knives seem more obsessed with polishing their bevels into a mirror shine than how it actually cuts.


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## Barmoley (Jul 4, 2022)

Jovidah said:


> From what I've seen online most people into outdoor and folder knives seem more obsessed with polishing their bevels into a mirror shine than how it actually cuts.


There is that too of course, but many do know what they are talking about and many think people here don't understand that thinner knives just cut better.

A lot of it has to do with what the two groups cut and how, but also with the size of the knives and just the amount of steel we are dealing with in chef knives. Something that is best in a 4" narrow blade that has to cut wood well might not be that great in a 10" gyuto that mostly cuts all sorts of veggies.


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## jwthaparc (Jul 4, 2022)

I can say there used to be a big disconnect between the pocket knife community, blade stock thickness, and thickness behind the edge. You can still go to pocket knife reviews, and most just worry about how thick the spine is, and ignore the behind the edge thinness.

There has been a movement towards importance of behind the edge thickness taking priority more lately. Before this it seemed there was an idea that just because a knife had thin blade stock that it would slice well. Discounting that something that is thicker at the spine, but thinner behind the edge could potentially cut much better.


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## Ruso (Jul 4, 2022)

I would go even further and say at some not so distant past the focus of folders/fixies was into thick stocks, the thicker the marrier. I am glad to see that in few past years this trend have changed.


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## Barmoley (Jul 5, 2022)

Ruso said:


> I would go even further and say at some not so distant past the focus of folders/fixies was into thick stocks, the thicker the marrier. I am glad to see that in few past years this trend have changed.


I agree even though I feel it is not changing enough. Still a bunch of thick blades on small knives and folders as well. Even when tougher steels are used still blades are very thick on many. The reason is usually to make the knife tougher, but folders weak point is usually the handle to blade attachment and not the blade itself. It is getting better though just slowly.


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## Brian Weekley (Jul 5, 2022)

Like a naughty schoolboy


Well this certainly answered my curiosities regarding Bidinger KnIves B grind. I actually have one on the way, and couldn’t be more excited. @captaincaed is that the 240 Magnacut Dan had posted recently in his IG feed? Man that looks nice. Handle looks very smart and comfortable Marc, I almost...




www.kitchenknifeforums.com





My totally real life but unscientific potato slap down test suggests that a flat grind performs better when it comes to potato stiction. Check out my two top performers. My high flat grind Rader, and another high flat grind Gyuto by Alex Horn. Rader thick, Horn thin. Two top performers. The primary conclusion … expensive, rare flat knives out perform less rare cheaper blades. Tall and thin are preferred to taller and slightly more shapely. 

Rader 







Horn


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## esoo (Jul 5, 2022)

Brian Weekley said:


> Like a naughty schoolboy
> 
> 
> Well this certainly answered my curiosities regarding Bidinger KnIves B grind. I actually have one on the way, and couldn’t be more excited. @captaincaed is that the 240 Magnacut Dan had posted recently in his IG feed? Man that looks nice. Handle looks very smart and comfortable Marc, I almost...
> ...



That Radar looks hollow ground to me - what happens if you put a ruler against it?


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## Racheski (Jul 5, 2022)

I believe it is important to discuss how moisture affects food release across different geometries, because in my experience stiction between the knife and wet ingredients causes most of the issues (and scientific articles theoretically support this). Is the capillary force between the knife and the food significantly affected by the blade geometry? From my limited investigation, the answer is probably complicated...if there are tiny amounts of water in the knife-food interface, there is relatively large capillary pressure which causes attraction, but if enough force is applied and enough water enters the interface those capillary bridges break and water acts as a lubricant (Unexpected friction behaviours due to capillary and adhesion effects). This experimental set up did not simulate cutting food, but some of the conclusions are relevant.

We really need a tailored experiment to answer this question IMO, so until then I remain skeptical about any claims regarding food release and blade geometry.


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## Brian Weekley (Jul 5, 2022)

esoo said:


> That Radar looks hollow ground to me - what happens if you put a ruler against it?



High flat at the heel.


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## tostadas (Jul 5, 2022)

I agree that the question is more complicated than it seems. Additional things that may not be considered include 

to what degree the particular object deforms when cut
moisture at the knife-food interface, and expanding on that, if the moisture changes due to pressure while cutting. For example when you squeeze a potato, there's a lot of liquid you can get to drip out, but if it just sits undisturbed, it can be relatively "dry"
Elasticity of the object being cut, whether it wants to naturally fall away, or try to hold itself together.


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## Kippington (Jul 5, 2022)

Brian Weekley said:


> High flat at the heel.
> 
> View attachment 187237


Your image makes it look like a concave/hollow grind, to be honest. Might just be the photo though, and not the actual knife.
Added a red line for reference:


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## Brian Weekley (Jul 5, 2022)

It’s high flat … don’t believe your lying eyes. My iPhone doesn’t take the best pictures … however whatever it is, it was a top performer in the great potato slapdown.


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## stringer (Jul 5, 2022)

Let me preface this by saying that I think food release is one of the most irrelevant attributes to whether or not I like a knife. But if I did care about food release I would want something thick at the spine. Thin behind the edge. Convexed on one side and high flat grind on the other. I have not handled too many fancy s-grinds but that might work too depending on what you're cutting. But a symmetrical full flat grind kind of sucks. There's a random vic at work. I have sharpened it a couple of times but I haven't touched the factory grind. Full flat grind. Mirror polish. Too thin at the spine. Too thick behind the edge. The geometry is horrible. Everything sticks to it. I still use it for fun sometimes. I just move faster to keep the product siding off and I do more draw/pull type cuts than I normally would. All knives have pretty good food release at the tip. Less surface area to stick. 

I know for a fact that the food release properties will be greatly improved if I relieve the bevel shoulders and add some microconvexity to the right face because I have done it before. But I probably won't because I'm not even sure whose knife it is.


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## Jovidah (Jul 6, 2022)

Yeah I'm kinda with stringer on this... I'm not sure I actually _care_ about food release. For most things that I cut it will eventually get pushed off the blade anyway, and moving stuff from the blade to the pan actually saves me the step of picking up the dough scraper.

I only ever remotely cared about it for stuff like cubing potatoes and making brunoise, where it tends to interrupt your flow if you constantly have to push stuff off your knife and realligning it to make the next cut.
Until I discovered the wonderful world of draw cuts... then I basically stopped caring about food-release overnight.


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## McMan (Jul 6, 2022)

I think it’s hard to talk about “food release” as a distinct property that either a knife has or doesn’t. Bad technique changes how a knife acts. Good technique can help—“good” here meaning experimenting and identifying where a knife excels vs. is limited and trying to make adjustments to coax performance and avoid shortcomings.

Take a laser for example. With such a thin knife, there’s little room for the grind to operate because the knife is so thin. Slow cutting guarantees sticking. Fast cutting often increases food release. Very, very fast cutting gets sticking again for some things, but continues to have food release for some things.

So, in this example, the same knife lacks and has food release, depending speed of cut, and then has or doesn’t have food release depending on speed of cut + type of product.

I’m not even talking about technique here. Just speed. Technique adds more complexity—push/pull, chop, what part of the blade used, speed, angles, etc.

So, here’s my thinking… Food release is not an inherent attribute of a knife. Rather, it is knife plus technique at a minimum. Or probably even sometimes knife plus technique plus product.

I’m not trying to say “there’s no such thing as food release”. Some knives certainly have a much better baseline than others -- and I think this is mostly due to what @stringer describes ("workhorse grind" -- thin spine, asymmetrical convex, distal taper, thin edge, room for the grind to operate on the blade face). Or other purpose-driven grinds (S, hook, etc.).

Trying to measure food release in relation to grind, or technique, or product density, or how wet something is, just points out how we should probably be thinking about how multiple factors inter-relate. But grind/geometry is an important starting point--and often an obvious hint.


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## Jovidah (Jul 6, 2022)

Don't leave out blade finish. It's huge. I turned a knife from being hafl decent in release to sticky as hell simply by polishing it up into a mirror finish.
They're selling cheap ass teflon-coated knives for a reason...


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## M1k3 (Jul 6, 2022)

As long as food doesn't stick like there's a suction cup on the knife, I'm good. 

That said, I would not hesitate to use a Kippington Fish Hook grind.


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## captaincaed (Jul 6, 2022)

blokey said:


> His point is the flat V grind is better than convex because it cuts better and the slight convexity on kitchen won't affect stiction and food release, and a microbevel will do more for food release.


Long forgotten but required reading on the subject. 

I'll double down on what Mike said, preventing the suction cup especially on the back is most important to me. Allows speed work without catching and popping. Makes the blade feel les predictable, and that's when I cut myself. For that reason I don't like flat or subtle concave on the back side.


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## captaincaed (Jul 6, 2022)

Kippington said:


> Your image makes it look like a concave/hollow grind, to be honest. Might just be the photo though, and not the actual knife.
> Added a red line for reference:


Rader's knives have a faux bolster, makes these pics hard.

FWIW mine is subtly convex where the cutting happens. Efficient cutter with little to no sticking issues. He's got many grind styles up his sleeve from what I've seen.


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## ian (Jul 6, 2022)

One comment: I find that these threads often degenerate to “all these other factors are important”. While that’s true, it sometimes seems like a way to avoid the original question, which is still perfectly relevant. I also don’t think that we necessarily need super controlled novel studies to know that convexity can help with stuff sticking. I’m sure most of us have slightly convexed a flat ground knife and noticed an immediate improvement…


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## captaincaed (Jul 6, 2022)

For me the #2 factor is a good patina. Like a cast iron pan. Duh.


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## ModRQC (Jul 6, 2022)

Motion/Momentum > Thickness/Geometry > Finish ... is in my sense the reality of this thing.

Cutting out Motion/Momentum (even if I believe it IS the most crucial aspect of release) from the equation, as @ian says, doesn't make the main subject less relevant. In fact, it makes it more relevant - if a certain consensus is reached with the two other criteria, anyone can benefit from the knowledge no matter how they use the knife. 



ian said:


> I’m sure most of us have slightly convexed a flat ground knife and noticed an immediate improvement…



Or thinned a knife making if flat(ter) and notice immediate worsening. Or went into mirror polish to find the same... etc etc.


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## Matt Jacobs (Jul 6, 2022)

I find full carbon convex knives perform the best overall in my kitchen. The carbon once a patina has formed helps as does the convexity for food release. That being said I have 3 different S-grinds. I prefer food release over most other aspects as long as its a good knife. I have used some lasers and they make you go wow until you cut a potato or other sticky food. It slows me down, makes me use 2 hands and gets food all over the board, counter, floor. I did an onion test the other day with an off the shelf henckels and a Nordquist S-grind. Maybe not fair as the Nordquist is a far superior knife, but look at the difference of where the food landed on board. The S-grind gives me more room to prep.


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## mengwong (Jul 6, 2022)

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McMan said:


> So, in this example, the same knife lacks and has food release, depending speed of cut, and then has or doesn’t have food release depending on speed of cut + type of product





Fantality said:


> My ex-head chef's "chef's knife" i mentioned above didn't even need to have the cucumber fall off at the shinogi line. The cucumber just stayed in-tact, in the same manner it laid there before it was cut into a hundred pieces.


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## blokey (Jul 7, 2022)

Thanks for everyone participated in the debate! I have learned a lot of valuable things. I just did a test with my ShiHan and Yugiri, both very thin behind edge with some substantial back, ShiHan is more convex while Yugiri is on the flatter side, similar height. Both actually did good in pushing the giant carrot slice away so food release is no concern here, but ShiHan cuts noticeably smoother while Yugiri suffers from some stiction. This is just my personal experience tho.
ShiHan Choil




Yugiri Choil




Spine are of similar thickness through out.


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