Every sourdough – that culture of lactic acid bacteria and yeast in a mixture of flour and water – is unique, like a fingerprint, because in every region there are individual bacteria in the air that give each loaf its very own flavour. One person who knows more about the infinite variety of sourdough cultures than most – whose culture is culture, so to speak – is the Belgian Karl De Smedt, keeper and archivist of Europe’s only sourdough library.