The iron knife shines better after cutting the sour fruits like lemon, raw mango. why?

Dear Student,

Sour fruits like lemons, raw mangoes generally contains organic acids. These fruits usually posses citric acids in them. And citric acid is  basially a weak acid. Thereby, it acts as a chelating agent on traces of metals like iron. So, when iron comes in contact with this type of acids, they react with them. This gives the iron knife more shine on reaction with those acids.

Regards