Non stainless knifes...oxidation

Hi guys...i have some japanese carbon knives such as a shiraki ...and i have a problem with the knife oxidation. The soft iron jaket leaves black green oxide on onions and any acidic foods. Now some of you might think of smell or taste alteration but i am thinking of food poisoning. As far as i know from my chemestry lessons, oxides are posionous. Is it ok to prepare food and leaving traces of oxides on it?

I would recommend to allow a patina to form as it will tame the reactivity of the carbon steel and cladding. The behavior you just described is well known. With patina it usually disappears- at least with very most carbon knives. Or just get some stainless-clad knife

I'm not a chemist, doctor, or metallurgist so, take my words with a grain of salt and use your own best judgment.

Steel in reasonable amounts isn't toxic. The greenish hues are probably more a visual problem than a real medical issue for most healthy people. However, between the visual aspects and any taste differences, most people are probably going to feel a bit 'ill' or 'nauses' or ...

Take a lemon or anything similar and develop a patina on your knife. That should eliminate your problem. You could also treat it like a cast iron skillet and hit it with some oil and a flame to seal ("season") it. That works with onions and tomatoes in cast iron but, it does wear off over time with acidic exposure (I fry a lot of bacon and sausage so, the seasoning of my skillet generally isn't an issue).

deskjockey said:

You could also treat it like a cast iron skillet and hit it with some oil and a flame to seal ("season") it. That works with onions and tomatoes in cast iron but, it does wear off over time with acidic exposure (I fry a lot of bacon and sausage so, the seasoning of my skillet generally isn't an issue).

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Definitely do not heat the blade up.

danemonji said:

As far as i know from my chemestry lessons, oxides are posionous. Is it ok to prepare food and leaving traces of oxides on it?

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Some oxides are poisonous, but oxides in general are not. Metal oxides and metal salts can be a problem if the particular metal is toxic (e.g., lead, mercury, copper), and can be a problem at high doses even if the metal is required for normal functioning of the body (e.g., zinc). Unless it's a huge amount, or you suffer from hemochromatosis or a similar condition, iron is fine. It's common for people to not get enough iron in their diet, so some extra iron oxide in food might be a good thing.

The recommended upper limit for iron intake for adults is about 50mg per day. That's 1/20 of a gram, which, if it was to come entirely from a knife, would mean ingestion of the entire knife (by one person) over a little over 10 years.

https://www.nrv.gov.au/nutrients/iron

If iron was that much of a worry I'd have died from drinking well water years ago.

Nice one Sir 👍👍

danemonji said:

Hi guys...i have some japanese carbon knives such as a shiraki ...and i have a problem with the knife oxidation. The soft iron jaket leaves black green oxide on onions and any acidic foods. Now some of you might think of smell or taste alteration but i am thinking of food poisoning. As far as i know from my chemestry lessons, oxides are posionous. Is it ok to prepare food and leaving traces of oxides on it?

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Chemist checking in. What you are seeing is unlikely to be sloughing off of iron oxide from your knife. First, iron oxide is characteristically red-brown. Second, we all know from our experiences sharpening knives that steel doesn't just fall off the knife. What you are seeing is most likely due to reaction of substances in your food with the iron in the knife. There is a lot of free iron in carbon steel, leaving it available to catalyze reactions. Those reactions can occur with the organic compounds in your food, and can occur essentially instantaneously. They are typically oxidation reactions, but they don't have to be. In fact, if the iron itself is oxidizing, then whatever you are seeing is most likely reduced (the opposite of oxidized) unless the reaction system is highly complex.

Bottom line: you aren't seeing iron coming off from your knife into your food, and the black/green color is unlikely to be oxidized (and not an oxide of any kind).

Regarding the presumed toxicity of oxides, I'd love to have a chat with the chemist or toxicologist that gave you that notion. This guy sums it up accurately:

TimoNieminen said:

Some oxides are poisonous, but oxides in general are not. Metal oxides and metal salts can be a problem if the particular metal is toxic (e.g., lead, mercury, copper), and can be a problem at high doses even if the metal is required for normal functioning of the body (e.g., zinc). Unless it's a huge amount, or you suffer from hemochromatosis or a similar condition, iron is fine. It's common for people to not get enough iron in their diet, so some extra iron oxide in food might be a good thing.

The recommended upper limit for iron intake for adults is about 50mg per day. That's 1/20 of a gram, which, if it was to come entirely from a knife, would mean ingestion of the entire knife (by one person) over a little over 10 years.

https://www.nrv.gov.au/nutrients/iron

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