The best way to buy food is undoubtedly fresh from the farm but it's hard to imagine our lives without the convenience of tinned and frozen foods. And, of course, those that come in a Tetra-pak. It's the cardboard container that's more complicated than that sounds, and which revolutionised the transport and sales of everything from milk and fruit juice to soups and even wine. It's so much part of our kitchen landscape that it's easy to forget just how ground-breaking it was when it first appeared.