Why does recipes for blackcurrant jam tell you to boil the fruit in copper or glass pans not iron pans?

This is because iron can react with the acids in the blackcurrants, causing the jam to turn a dark, muddy colour and develop an unpleasant metallic taste. Copper and glass are both non-reactive materials, so they do not react with the acids in the fruit and the jam will retain its bright colour and flavour.

In addition, iron pans are more likely to cause scorching than copper or glass pans, which can further degrade the colour and flavour of the jam.