Why is pickled Mucor Douchi sour?

General method: 1, soak soybeans in water to fully absorb water for about 5~8 hours. 2. Steam on a big fire. 3. Let it cool in the shade until the beans are moldy and wiredrawn. It will take three or four days. 4. Wash it with boiling water and air it for another day. 5. Add appropriate amount of liquor, salt, spiced noodles, cooking wine, etc. and gently rub beans. 6. it will be ko after 30~50 days of sealing.