The Science of Yeast and Gluten

1What yeast is

Yeast is a single-celled fungus. Baker's yeast is the species Saccharomyces cerevisiae. It is sold as fresh (compressed) yeast, active dry yeast, which is first mixed with warm water, and instant dry yeast, which can be mixed straight into the flour.

2Fermentation

Fermentation is the process in which yeast breaks down sugar without using oxygen, releasing energy for itself and producing carbon dioxide and ethanol (alcohol):

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
glucose → ethanol + carbon dioxide

The carbon dioxide makes the dough rise. The small amount of ethanol evaporates in the oven, adding to the smell of baking bread. The sugar comes from the sugar in the recipe and from starch in the flour, which enzymes break down into sugars.

3Conditions yeast needs

• Food: sugar.
• Moisture: dry yeast is dormant (inactive) until it is wet.
• Warmth: yeast works slowly when cold, well at about 25 to 35 °C, and is killed at about 55 to 60 °C. For active dry yeast, use water that feels warm on the wrist, not hot (roughly 32 to 43 °C).
• Time: fermentation needs time; a dough in a cool kitchen simply takes longer to rise.

4Gluten

Wheat flour contains two proteins, glutenin and gliadin. When water is added and the dough is worked, they join to form gluten: a stretchy, elastic network. Kneading lines up and strengthens the network, so it can stretch like the skin of a balloon around each bubble of carbon dioxide instead of letting it escape.

5What happens in the oven

1. In the first minutes the gas bubbles expand in the heat and the yeast makes a last burst of gas, so the loaf rises further (oven spring).
2. At about 55 to 60 °C the yeast dies and fermentation stops.
3. The starch absorbs water and swells (it gelatinises) and the gluten sets, fixing the risen shape as the soft crumb.
4. The surface dries and browns, forming the crust; the browning comes from reactions between sugars and proteins, and from caramelising sugar.

Worked example: the balloon experiment

Experiment: Three bottles each contain 5 g dry yeast, 5 g sugar and 100 ml water, with a balloon stretched over the neck. The water is at 5 °C (bottle A), 35 °C (bottle B) and 70 °C (bottle C). The bottles stand for 30 minutes.

Results and explanation:

• A (5 °C): the balloon barely inflates. The yeast is alive but works very slowly in the cold.
• B (35 °C): the balloon inflates the most. Warmth close to the ideal range lets the yeast ferment the sugar quickly, making plenty of carbon dioxide.
• C (70 °C): the balloon does not inflate. The hot water killed the yeast, so no fermentation takes place.

Conclusion: yeast needs warmth, not heat. The same idea explains why dough is proved in a warm place, and why water that is too hot gives a flat loaf. The temperature is the only thing changed between bottles, so it is a fair test.

Practice