1, walnut kernel deterioration: walnut kernel in the storage or processing process, subject to moisture, high temperature or long exposure to the air, deterioration will occur. It will lead to black spots or black color on the surface of walnuts, accompanied by odor.

2, mold infection: walnuts in the storage process by mold infection, will lead to walnut surface black. Mold infection is caused by humid environments, inappropriate storage conditions or prolonged preservation.

3, natural discoloration: some walnut varieties will have some natural black pigment inside, resulting in walnuts appearing black after opening. The black color does not indicate that the walnuts have been spoiled or infected, it is a normal pigmentation.