Ordinary Steel Bolo/Knife Leaves Dark Blue Stain on Coconut, Banana etc…

Yesterday, I opened two young coconuts, buko. I used a straight steel bolo – the usual tool for opening young coconut – to break it open. Apparently, using hand saw or axe is absurd. People around gonna laugh out loud.

I noticed a dark blue stain on coconut meat. The place where the steel wedge passed through and stayed for few seconds. It’s a reaction between the steel and the coconut meat. The same color did not appear by scooping out the meat with a stainless steel spoon and by slicing it with a stainless steel knife.

young coconut meat with dark blue stain

When I was teaching on how to make banana products such as banana chips and banana ketchup, I never failed to remind the audience not to use ordinary steel knives. For two reasons: 1) Ordinary steel knives have disagreeable odor. Use it for daily cooking chores then smell it. It’s not just the smell of foods clinging to metal but the smell of steel itself. 2) Using ordinary steel knives leaves a dark blue stain on some food. Never sure what’s the name of specific substance. It might not cause drastic harm but it looks so ugly.

Stainless steel knives are recommended. They do react with foods but not as reactive as steel bolo. Wash every after use to prevent any unwanted reaction due to prolonged exposure.

Example of delayed acid to stainless steel reaction: We ordered an acid resistant stainless steel tables. The chosen supplier gave us a piece of metal as proof of compliance. I soaked it in two strong acids, sulfuric acid and then hydrochloric acid. I observed no reactions so we agreed and the tables were delivered. After few days, my co-staff poured hydrochloric acid on termites nest. The tables were on the same laboratory room. Acid fumes scattered around touching every object including the stainless steel tables. Brown rust appeared on table after a week.


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