The silk thread produced by a silkworm originally has a bundled structure consisting of two fibroin strands coated with a water-soluble protein called sericin.
The sericin is removed and the fibroin pulled apart to produce the supple, shiny silk we are familiar with.
Once it is stripped, the fibroin is used in clothing, and is subjected to direct friction when washed. The fibrils in the fibroin have a finely twisted structure (imagine a rope), so the friction causes the fibrils to fluff out or become broken. Because of this, diffuse reflection occurs at the surface of the fiber, making it appear different from the original color, and imparting a whitish appearance to the silk. This phenomenon is called “whitening.” The whitening phenomenon also occurs easily in cotton, rayon, flax and other fibers in addition to silk.