Sheila Dillon finds out why sourdough bread is undergoing a major revival. It is the world's oldest leaven bread dating back to Ancient Egypt and it is now experiencing a renaissance. Baker Dan de Gustibus explains how the bread is made from a sourdough starter, a mixture of flour and water which is left to ferment until wild yeasts and bacteria start breeding. But there are many myths around this sourdough starter - bakers compete over who can trace back the oldest lineage. Yeast technologist Dr Bill Simpson debunks these myths to explain the truth behind how sourdough works.And food historian Erica Peters explains why she thinks the famous San Francisco sourdough isn't linked to the Californian Gold Rush, despite its claims.Presenter by Sheila Dillon and produced by Emma Weatherill.