Summer, spring, winter and fall: there is no better dish than a piping hot plate of spaghetti with pummarola, the rich Italian sauce made from tomatoes, basil, garlic and olive oil. But how do you make the sauce if you have no fresh tomatoes? You open a tin of tomatoes, of course. But sometimes we forget that there was once no such thing as canned food. Someone had to invent the process that we now take for granted when we purchase anything from tinned asparagus to pet food from the supermarket.