It smells like a mixture of overripe Gorgonzola and putrefying mushrooms. Its texture is that of slugs stuffed with mozzarella. It is natto, or rotten soya beans, and is one of the world’s most challenging foods, as loved and loathed in Japan as black puddings are in Britain or rotten herrings in Sweden. And in the past few days it has become a cult, as natto-mania has swept Japan, emptying supermarket shelves and leaving soya bean fermenters scrambling to catch up with the unprecedented surge in demand. Earlier this month the television programme Revealed! Encyclopaedia of Living recommended two portions...