Sericin
The sticky protein gum that holds the two silk strands of a cocoon together, and which is washed out to make silk soft.
Also: silk gum
A silkworm spins two fine strands of fibroin, the silk protein, and coats them with sericin, a gum, so that they stick together as one thread. Raw silk still has its sericin, so the thread is stiff, slightly rough and dull. Removing the gum by boiling is called degumming. Some silk cloths are woven with the gum left in the thread and washed afterwards, which is how crisp sheer silks such as kora organza are made. Sericin is also used in some skin products, but in the saree trade it matters mainly as a measure of how soft or crisp the silk will be.