Yeast

Why yeast causes dough to rise remained a mystery for 6,000 years and was only understood 141 years ago, when its carbon-dioxide-generating effect was discovered by Louis Pasteur. Natural microscopic yeasts are fungi and are all around us in the air. When the type called brewer’s sugar fungus or brewer’s yeast settle in an appropriate environment, they give off carbon dioxide and cause fermentation, delivering a distinct flavour and – in liquid solutions of grape or grain – alcohol.

Brewer’s yeast is the perfect raising agent for strong, high-gluten bread flours, because it generates gas slowly and over hours, or even days. This is trapped as bubbles by the unique elastic and plastic qualities of the gluten, causing the dough to rise and producing leavened (as opposed to unleavened) bread. Putting raised dough in a hot oven makes the gas bubbles expand further and the dough cooks around them, delivering light-textured loaves.