Kate Quilton visits Vietnam, where a global surge in demand for pepper has transformed the lives of farmers across the country. She learns the difference between black and white varieties and the secrets of peppercorn quality control, and discovers the remarkable lengths to which producers go to guard their valuable crop from thieves. Jimmy Doherty meets a man whose job entails harvesting different strains of yeast from locations around the world and Matt Tebbutt looks into how manufacturers flavour crisps.