Some metal with your meal?

February 13, 2013 12:04 PM

I often see chefs/cooks sharpening knives right above food or above food prep surfaces. Does that mean that small metal particles are getting into the food? If so, do those people not realize that's happening, or do they think it's innocuous?
posted by Dansaman to food & drink (21 answers total)

They're using honing steels, which don't produce any particles.
posted by MrMoonPie at 12:07 PM on February 13 [12 favorites]

I suspect you are seeing chefs steeling their knives, not sharpening them. Steeling realigns the knife edge and does not abrade away any of the blade material.
posted by letourneau at 12:07 PM on February 13 [1 favorite]

Are they sharpening them or are they using a honing steel? You're supposed to hone the knife right before use, and I don't believe it produces any detritus.
posted by griphus at 12:07 PM on February 13 [2 favorites]

They're steeling, but even if they were sharpening them, the amount of material removed by sharpening is extremely tiny and the particles are *extremely* small and relatively innert (stainless steel is stainless) or harmless (carbon steel is just slightly fancy iron).
posted by pjaust at 12:10 PM on February 13

OK, I have a honing steel (I always called it a knife sharpener) -- the long metal rod dohickey.

Run a tissue or paper towel over the knife blade after sharpening, and it will come away with grey grit. So I have to disagree with the "no particles/detritus" comments.
posted by k5.user at 12:20 PM on February 13 [7 favorites]

Nthed that steeling isn't putting metal in your food--but metal in your meal is not necessarily a bad thing anyways. Cast iron cookware will put a bit of iron in your food, especially if you are cooking something acidic, and that can actually be a benefit to some people who need more iron in their diet.
posted by epanalepsis at 12:20 PM on February 13 [1 favorite]

Didn't you ever do the experiment in chemistry class where you crush up a box of Cheerios put it in a beaker with a magnetic stirrer? Food that's "fortified with iron" literally means they've added iron filings to it.
posted by TungstenChef at 12:34 PM on February 13 [19 favorites]

Seconding k5.user. Whenever I use a steel, it removes small amounts of metal from my knives.
posted by zug at 12:53 PM on February 13

Steeling definitely abrades away some material from the knife. If you run your fingers along the side of the blade or along the steel after honing you'll definitely see a greyish powder on your fingertips.

However, it's a very small amount of material and having trace amounts of iron in your food is not bad for you at all.
posted by Aizkolari at 12:53 PM on February 13

I can attest to having the same experience as k5.user — honing a knife with a honing steel does produce a very small amount of particles.

However, I agree the amount produced is innocuous. Stainless steel is iron, carbon, and chromium, with some of the chromium at the surface as chromium (III) oxide. None of which are particularly toxic, especially at the incredibly small levels generated here.
posted by DevilsAdvocate at 12:53 PM on February 13

There is probably more mercury in the tuna they're cutting with the knife than there are (mostly inert) metal bits from the knife landing in the food.
posted by rtha at 12:59 PM on February 13

If I recall correctly we absorb iron in form of iron salts and through organic molecules that contain iron (I think the term is heme-iron... think hemoglobin). I don't think iron in its pure or oxidized form can be absorbed by the body so the additional iron from steeling won't do any good. In fact I'd think that pure iron that's not oxidized would likely act similar to free radicals and be detrimental since it'll want to oxidize quite aggressively. That said I imagine the amount of iron created by steeling would be too small to have a noticeable effect unless maybe there is long time continuous exposure. IANAD
posted by Hairy Lobster at 1:04 PM on February 13

Forgot to add: I imagine any non-oxidized iron shavings from steeling would quickly oxidize during prepping and cooking of the food anyhow. It's probably unlikely that a lot of pure non-oxidized iron would enter the body.
posted by Hairy Lobster at 1:07 PM on February 13

Food that's "fortified with iron" literally means they've added iron filings to it.

No, it's usually something like iron sulphate.
posted by Chocolate Pickle at 1:32 PM on February 13 [4 favorites]

Regardless of where and over what the knife is being sharpened and/or steeled, you will get detritus from the knife in your food, for one simple reason: your food is being cut with that knife. Every knife will leave particles of metal (or ceramic!) in your food - even if it is never sharpened... that's why knives go dull - they lose their edge... and where do you imagine they lose it in? Food!

The question is how much and is it a health hazard. Metals from cookware get into your body - using metal skillets gets iron in your body. Cooking and boiling water in copper cookware will get copper into your body and so on. Upper class ancient Romans got a lot of lead in their bodies because of lead in their kitchen and dining utensils - to deleterious results. You get a lot of stuff in your body from modern cookware, including knives. How hazardous it is, we can't tell. Some may even be beneficial for those what lack f.ex. iron and copper in their diet. Don't discount the water that comes from pipes lined with metal or plastic or whatever.

Bottom line: yes, you get that stuff on your food. Is it bad: impossible to generalize - sometimes yes, sometimes no.
posted by VikingSword at 1:36 PM on February 13 [1 favorite]

Ever used a cast iron skillet or eaten something prepared on a cast iron surface? You're getting iron from that and I'm confident that there's more iron content in a cast iron skillet than in the gray powder made from honing a blade.
posted by cooker girl at 1:37 PM on February 13

If I recall correctly we absorb iron in form of iron salts and through organic molecules that contain iron (I think the term is heme-iron... think hemoglobin). I don't think iron in its pure or oxidized form can be absorbed by the body so the additional iron from steeling won't do any good. In fact I'd think that pure iron that's not oxidized would likely act similar to free radicals and be detrimental since it'll want to oxidize quite aggressively. That said I imagine the amount of iron created by steeling would be too small to have a noticeable effect unless maybe there is long time continuous exposure. IANAD

I don't think you're recalling correctly. Elemental iron will quickly be converted to iron salts (specifically iron chloride) by stomach acid, which is absorbable and doesn't act anything remotely like a free radical.

No, it's usually something like iron sulphate.
Maybe with other foods, but Straight Dope has my back that cereal contains iron filings.

IANAD, but I had delusions of being a pre-med for a while and slogged through more than my fair share of biochemistry and physiology before realizing that I was much happier as an archaeologist. Your "patients" don't complain nearly as much when they're 50,000 years old. :-p
posted by TungstenChef at 1:54 PM on February 13 [6 favorites]

Honing knife edges does sometimes break off microscopic particles of steel or iron alloy from the burr of the edge (the burr is the very edge of the sharp part of the knife that can bend over from true sharp through either use or from sharpening - by grinding on a grinder or on sharpening stones).

But, many honing steels are slightly or intentionally magnetic so many of the particles stick to the hone until cleaned or wiped.

Microscopic iron or steel particles are generally considered safe to eat, either because they're biologically inactive or because they're a natural part of dirt, which we often eat with the rest of our food.

This discussion of iron bioavailability is chock full of links to other articles about iron in the diet and whether or not it is dangerous. Like most sensible folks this author's conclusion is that you should avoid food additives that you don't know you need. I don't have time to do all the homework, so i can't go further for you, but I would temper this analysis with the following:

- The amount of metal that may be coming off a honing steel into your food is probably quite microscopic, so I wouldn't assume high dosages. I think it's very probable that honed knives are putting far less iron into your food than cereals fortified with (powdered) iron filings.
- It's strongly likely that the metallic/elemental iron (if any) that gets into your food is rusted. The additives food companies add are usually coated or in alloy with something else to keep them from rusting. This may well change the equation. The reason I say this is that the nature of the particles you're going to break off the edge of the knife is that they'll have a very high surface area, which means a very quick oxidation.
- It's probable that it's biologically expensive for the body to metabolize elemental iron or oxidized iron (the rust) into nutritive iron (usually for hemoglobin, but possibly other uses), so it may be bioinactive because it's too much work to work with it.
- Whether the iron is oxidized or not strongly affects the biochemical cost of your body's metabolism working on it, so if you do find evidence to back up worrying about elemental iron, keep in mind that rust will work a completely different way.
- I just spoke with a professional cooking instructor who says that sure, some folks might hone over food, but it's not ergonomically sensible - most people tend to take the hone and knife to the side so they can hold the whole works lower and work at a more natural position.
posted by kalessin at 3:12 PM on February 13

Oh, also, depending on the kind of honing steel you have, you may get more or less filings from a knife (also depending on how much pressure you put on the steel with the knife while honing).

Hones come in two general kinds: the file steel and the smooth steel. The file steel is the kind I most often see, where small ridges run the length of the steel. Those can take more metal off the edge of the knife than the smooth steel, which is, well, smooth.
posted by kalessin at 5:21 PM on February 13

I wonder if there could be a flavour difference in some foods between using ceramic or steel knives?
posted by anonymisc at 6:23 PM on February 13

Iron supplementation using an Iron Fish. I saw this somewhere else a while back. A group of people with iron poor diet and aversion to medicine/supplements of the normal pill form were given cast iron fish to add to their cookpots to increase iron levels.
posted by zengargoyle at 8:56 PM on February 13 [1 favorite]