What Silk Is Actually Made Of: Fibroin and Sericin

Silk Is Two Proteins, Not One

A silk thread is made of fibroin, the structural protein that forms the fibre itself, held together by sericin, a sticky protein that coats and glues the strands. A cocoon is fibroin filament bonded with sericin gum.

The distinction is not academic. Almost everything about how silk is processed, and what it can be used for, follows from separating those two proteins.

Fibroin: the Fibre

Fibroin is what people mean by silk. It is the long structural protein the silkworm extrudes from its spinnerets, and it is the part that gives silk its strength and its handle.

The silk gland of a mature larva is one of the most productive protein-manufacturing organs in the animal kingdom — the animal spends its final larval days doing almost nothing else. Silk is not unique to silkworms, incidentally: it is produced by more than two hundred thousand arthropod species including spiders, lacewings and mites. Bombyx mori is simply the one we learned to farm.

Sericin: the Glue Nobody Wanted

Sericin is the coating. In textile production it is boiled off in a process called degumming, because it makes the fibre stiff and dulls the shine. For most of the history of the silk industry, sericin was waste — washed away by the tonne and discarded.

That has changed. Sericin has since been found to have biological properties of its own, interesting enough that the protein textile mills spent centuries throwing out is now studied in its own right for wound dressings and other biomedical uses.

Why Keepers See This Too

The two-protein structure explains something you can watch in a rearing container. A silkworm about to pupate lays down an outer scaffold of loose, sticky strands to anchor itself before spinning the dense inner cocoon — which is why a pupating container needs paper or mesh to grip. That anchoring layer is sericin doing exactly what it does industrially: sticking silk to things.

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