prepare a noodle basin, put about a catty of flour into it, and then add the prepared warm water, the temperature of which is about 4 degrees. When adding warm water to the flour, stir the flour with chopsticks until the flour appears cotton wool-like, so don't add any water. Then knead the flour by hand, and knead the flour into dough. After kneading the dough, you need to prepare a clean towel to cover it tightly, and then put it at a temperature of about 3 degrees to ferment the dough. The fermentation time is preferably more than 1 hours. When the dough has obvious expansion and a large number of honeycombs appear, it is proved that the dough has been fermented, so the fermented dough is the old yeast surface.

method 2: cooked steamed bun fermentation

the cooked steamed bun fermentation method is adopted, which requires preparing three steamed steamed buns with old yeast in advance, and then breaking these steamed buns into small pieces for later use. At the same time, prepare a washbasin, add the required flour into it, and add warm water to stir it into cotton wool. Then, add the prepared steamed bread pieces into these flour, knead them, and knead the flour into dough. Finally, cover it with a clean towel (it is best to cover it with plastic wrap in winter). Put the dough in a place where the temperature is above 3 degrees, and when it is fermented to twice its original size, the old yeast is ready.