Why the Muga Silk Is Golden

Silk is not a fabric you weave from plant fibres like cotton. It is a protein — a long chain of tiny building blocks called amino acids — produced inside the body of a caterpillar. Specifically, the caterpillar makes a protein called fibroin in two long glands that run the length of its body, filled with a thick, gooey solution of dissolved protein.

When the caterpillar is ready to build its cocoon, it squeezes this protein solution through a tiny nozzle on its head called a spinneret. Think of it like squeezing toothpaste through a tube — except as the fibroin comes out, something remarkable happens. The narrow opening forces the tangled protein chains to line up in parallel, like combing messy hair into neat rows. Once aligned, the chains lock together into flat, stacked sheets called β-sheet crystals, held by millions of tiny hydrogen bonds.

A second protein called sericin coats the fibroin as it emerges, acting like glue to hold the two strands together and stick the cocoon into shape. One caterpillar produces a single continuous thread 300 to 900 metres long — nearly a kilometre of silk from an animal smaller than your little finger.

Check yourself: A silkworm’s spinneret works at room temperature, using water as a solvent. Nylon factories need temperatures above 250°C and toxic chemicals. Which process is more energy-efficient? What does that tell you about what nature has figured out that we haven’t?

Most silk is white. Mulberry silk from Bombyx mori caterpillars is white. Eri silk is cream. Tasar silk is brown. But muga silk is golden — the only naturally golden silk on Earth. Why?

The answer is chemistry. The Antheraea assamensis caterpillar feeds on the leaves of som and sualu trees. These leaves contain an amino acid called tryptophan. Inside the caterpillar’s body, tryptophan gets converted into a golden pigment called xanthurenic acid. Here is the crucial part: this pigment doesn’t sit on the surface of the thread like a dye on a cotton shirt. It is chemically bonded into the fibroin protein itself — woven into the molecular structure.

Imagine the difference between writing on a wall with chalk (surface dye) and mixing yellow paint into wet plaster before it dries (structural colour). The chalk washes off in the rain. The coloured plaster is yellow all the way through — you cannot remove the colour without destroying the wall. Muga silk’s gold works the same way. You cannot wash it out. Sunlight cannot break it down, because the bond between xanthurenic acid and fibroin is UV-stable.

Even more remarkably, muga silk gets more lustrous with every wash. This is because the sericin coating (which is slightly dull) gradually wears away, exposing more of the golden fibroin underneath. Most fabrics fade with use; muga silk gets more beautiful.

A prediction you can test: If you could somehow raise an Antheraea assamensis caterpillar on mulberry leaves instead of som leaves, its silk would likely be pale or white — because without the specific tryptophan chemistry from som leaves, the caterpillar cannot produce enough xanthurenic acid. The gold comes from the diet, not from the caterpillar alone.