First, the food eaten by swallows is not fully digested, and it is spit out with saliva and becomes a part of bird's nest.

Second, mildew. Some consumers think that mildew is black and gray, but it is not. It can be black, gray, white, red, blue and green, depending on which mold it is.

If the bought bird's nest smells musty, it needs to be preserved. It should be placed in a place where sunlight cannot reach and humidity can be avoided. Bird's nest with high water content should be dried and preserved, otherwise it will be moldy. If there is moisture in bird's nest, it can be placed in air conditioner for one night, or dried by fan and dehumidifier, and the moisture will disappear. Then put it in a cool place, out of direct sunlight. After drying, it can be stored in the refrigerator for a long time.