Salt, sugar and sourdough

Bakers control the rise with a few ingredients and conditions. Each one changes the yeast, the gluten or the flavour.

Salt. Salt adds flavour, tightens the gluten net a little and slows fermentation, because it draws water out of yeast cells by osmosis. Bread recipes typically use roughly 2 percent salt compared with the flour mass. Bread made without salt tastes flat and can rise too fast and then slump.

Sugar. A little sugar is food for the yeast, and it helps the crust brown. A lot of sugar slows yeast, again because strong sugar solutions pull water out of the cells. This is why very sweet doughs need more yeast and more time.

Temperature. The warmer the dough, up to about 35 °C, the faster the rise (World 1, lesson 1).

Sourdough is bread raised without packet yeast. Instead it uses a starter: a paste of flour and water that has caught wild yeasts and lactic acid bacteria from the flour and the air. The two live together in one jar. The yeasts make CO₂. The bacteria make lactic acid and some acetic acid.

The acid is what gives sourdough its tang. A ripe starter has a pH of roughly 3.5 to 4.5 (pure water is 7, vinegar is about 2.5 to 3). The acid also changes the way gluten behaves and helps keep the bread from going mouldy. Few other microbes survive in such an acid paste, and the yeasts and bacteria in a starter have settled into a stable partnership.

A starter is like a pet: it needs regular feeding. Bakers throw away most of it and add fresh flour and water, so the microbes always have food and the acid does not build up too far. A starter is kept loosely covered, because it makes gas, and in a clean container. Warmer starters tend to make a milder, lactic tang, and cooler ones a sharper, more acetic one, though the whole picture depends on the culture.

Sourdough bakers often let dough rise slowly in the fridge for many hours. The cold slows the yeast much more than it slows the flavour reactions, so the loaf gets time to develop complex taste.