Why Does Bread Rise?

Yeast is a single-celled fungus. Given flour, water and warmth it feeds on simple sugars and produces carbon dioxide gas and small amounts of alcohol as waste products.

That gas has to go somewhere. In a properly mixed dough it collects in thousands of tiny pockets, and as more gas is produced those pockets expand and the whole dough swells. Most of the alcohol burns off in the oven, though it contributes to the aroma of baking bread.

Yeast alone would not be enough. Flour contains two proteins, glutenin and gliadin, which combine with water to form gluten. Kneading lines these proteins up into long linked strands.

The result is a network that is both stretchy and strong, so it can expand around a growing bubble without tearing. This is why bread flour with a higher protein content gives an airier loaf, and why cake flour with less protein produces something softer and tighter.

Kneading develops that gluten network by working the dough, though time can do a lot of the same job. Long slow rises in the fridge produce good structure with far less effort.

The first rise lets the yeast multiply and build flavour. Knocking the dough back redistributes the yeast and the gas, and the second rise gives a finer, more even crumb. Rushing either stage is the most common cause of a disappointing loaf.

Bread does most of its final rising in the first few minutes of baking, an effect bakers call oven spring. Several things happen at once:

• •The gas already trapped in the dough expands as it heats
• •The yeast has a final burst of activity before the heat kills it
• •Water in the dough turns to steam, adding more pressure
• •Gluten and starch then set, locking the risen structure in place

Soda bread and most cakes rise without any yeast, using a chemical reaction instead. Baking soda is an alkali that releases carbon dioxide when it meets an acid such as buttermilk, yogurt or lemon juice.

Baking powder combines the soda with a powdered acid, so it only needs moisture and heat to work. These reactions are fast, which is why soda bread goes into the oven immediately while yeasted bread needs hours. Sourdough sits in between, using wild yeast and bacteria that work slowly and add a sour tang.

A sourdough starter is not just wild yeast. It is a stable partnership between yeasts and lactic acid bacteria, and the bacteria are what produce the sour flavour and the distinctive keeping quality.

The bacteria also lower the pH of the dough, which strengthens the gluten network and slows the staling process, so sourdough keeps noticeably longer than a standard yeasted loaf. The trade off is time. A sourdough rise can take twelve hours or more, because the wild organisms work far more slowly than commercial yeast bred for speed.

Bread rises because yeast produces carbon dioxide and a gluten network traps it, then oven heat expands the gas one final time before the structure sets, which is why both good kneading and enough resting time matter so much.