Because water is produced.

reason: water is generated by the respiration of yeast, which can decompose starch into maltose, water and carbon dioxide.

leavening: the process in which the dough is allowed to fully propagate and generate gas under certain temperature and humidity conditions, and the dough is expanded.

when yeast converts starch into sugar and consumes it in an aerobic environment inside the dough, it will release carbon dioxide gas. At this time, the volume of the dough will expand and it will rise.

Extended information:

Flour making method:

1. While adding water, stir with chopsticks until the flour is flaky.

2. Knead the dough with your hands until the "three lights", namely, face light, basin light and hand light, will do.

3. Rub some water on the dough surface to keep it moist.

4. Cover and let it ferment in a warm place.

5. The fermentation time depends on the temperature, shorter in summer and longer in winter. When the dough volume increases to two or three times the original, it will be fine.

6. To test whether the fermentation is successful, you can dip some flour with your fingers and poke holes in the dough. If the holes rebound immediately, it means that the fermentation is not enough and you still need to ferment. If the surface of the dough collapses, it means that it is over-fermented, and the dough will have a sour taste, which can be neutralized with alkali, such as baking soda.

7. If you look inside the dough, there will be honeycomb holes.

Baidu encyclopedia-famian