Where did I see it? Jocasta Innes' The Pauper's Cookbook, actually my mother's copy from about 1976. Does it work? Yes it does. Get to the point, Kelly. What is the wretched tip? (I suspect the clue is in the picture).
Well here goes . . . tinned tomatoes (particularly those in the supermarket value ranges) can taste a little metallic, particularly the juice of the tomato. Usually I compensate for additional sourness (as some of the cheaper tinned tomato brands seem to use tomatoes that are not always that ripe) with about a half teaspoon of sugar to balance the flavour.
However, as Jocasta Innes suggests that "a pinch of instant coffee counteracts the tinny sharpness from tinned tomatoes" I thought I would give it a go, since tinned tomatoes seem to be such a staple of my cooking life, particularly in the winter. Admittedly I had to borrow the instant coffee as I don't drink the stuff myself (I'm a tea drinker). I did get a bit of an odd look from my neighbour when I said that all I needed was about a quarter of a teaspoon. But I guess they are getting used to my odd requests and I am getting used to the funny looks!
So do try this, t works, oh ye of little faith, I promise you!