cutting plantains with carbon steel

Hello all. I was cutting some plantain chips with tanaka blue #2 and it turned a dark purple black color like squid ink. It came off with a good sponge scrubbing so I don't think it was patina. Has anyone else seen this?

Plantain peels are rich in tannic acids which dissolve iron sulfates into solution.
You can see the same sort of reaction if you soak patinated iron in anything rich in tannin and sulfur, such as over-extracted tea.

People used to make ink with this reaction using the tannins in oak leaf galls: http://en.wikipedia.org/wiki/Iron_gall_ink

I've got some groovy colors from avocado's on white steel.

So... should I have left it on? On one hand tannic acid patina would stop any other reactions, on the other it definitely was discoloring food. Maybe the discoloration would stop eventually though.

Haha daveb I also was cutting avocado. It was a mexican dinner.

Couldn't tell you for sure, but my best friend is Trinidadian, and I remember as a kid, seeing his dad cut up countless plantains. He always used old carbon steel, and just simply washed and wiped the knives down afterwards. As far as I can recall, he pretty much "just left it". Maybe it gets more stable over time(?)

Definitely going to try this haha.

My friend's reaction: " I can't believe you got such a fast scientific response about this at 1 AM."

Me: "You don't know KKF"

Lefty said:

Couldn't tell you for sure, but my best friend is Trinidadian, and I remember as a kid, seeing his dad cut up countless plantains. He always used old carbon steel, and just simply washed and wiped the knives down afterwards. As far as I can recall, he pretty much "just left it". Maybe it gets more stable over time(?)

Click to expand...

Yep, reaction of certain iron compounds with tannic acid results in an easily soluble product, but this reaction can also form stable, insoluble compounds. Here we'd be talking about a form of passivation that involves the reaction between iron compounds and tannic acid to form ferric tannates. Tannin rich extracts, such as fermented unripe persimmon juice, has been historically used to protect iron, and is still used today in various rust conversion products.

http://en.wikipedia.org/wiki/Rust_converter

Different foods cause very unique pantina which can cover a broad range of colors and hues.If you don't like what is happening or the result of a food, forced, or natural pantina, then you can remove it with some Flitz or Baking soda.Barkeepers Friend has worked for me also.

Am I alone here? Every time I use BKF it takes the rust off, but then almost immediately rusts again, but usually worse(I'm talking like 10 minutes after removal and complete and total drying of steel), or it makes the knife insanely reactive to where it almost insta-rusts the first couple times using it post-BKF.

I've found I only use it when absolutely necessary these days, a baking soda slurry is gold 95% of the time in my personal experience.

BKF is okay for me. My go to now is 3k then 5k automotive sandpaper. I understand patina, I was more interested in the discoloration of food which this knife hasnt done before.