Yeast produces carbon dioxide by fermenting sugar substances in flour with microbial yeast, which makes dough fluffy and delicious. Usually, people like to make dough with "old noodles". Old noodles are also dry dough containing yeast. In addition to yeast, they also contain mixed bacteria such as lactic acid bacteria and acetic acid bacteria. Therefore, when using old noodles to make dough, these miscellaneous bacteria will produce some acid, which makes the dough taste very sour, so it is necessary to add alkali to neutralize the acid.

The chemical name of baking soda is: sodium bicarbonate, which is an edible alkali. The function of adding baking soda is to neutralize the acid and make the dough not sour. However, due to the poor dosage control of baking soda, it often destroys the nutrition of flour and changes the color of flour, which leads to the yellowing of dough and a bit bitter taste.