Why Silk Cocoons Are Different Colours

The Colour Comes From the Food

A silkworm does not manufacture its cocoon pigment. It eats it. The colours are drawn from mulberry leaf, transported through the animal and laid down in the silk — which is why cocoon colour is ultimately a question about diet as well as genetics.

Three Categories

• Yellow to red — coloured by carotenoids, principally lutein and β-carotene, both derived from mulberry leaves.
• Green — coloured by flavonoids, also from mulberry.
• White — trace pigment or none at all.

The green series is chemically the more interesting. Yellowish-green cocoons owe their colour to unusual compounds called prolinylflavonols, while lighter greens contain simpler flavonol glucosides — and the mulberry flavonoids appear to need chemical modification inside the silkworm, such as glycosylation, before they end up in silk.

So the animal is not a pipe. It is a processing step.

The Genetics Are Textbook

Yellow and cream-yellow cocoons — lutein and β-carotene respectively — are governed by the Y, C and F genes, following classical Mendelian inheritance. This is a trait you can predict from a cross with a Punnett square, which is exactly why cocoon colour became a teaching example in silkworm genetics.

Why White Won

Commercial silk is overwhelmingly white, and the reason is dyeing. A white cocoon takes any colour; a yellow one fights whatever you put on it. Centuries of selection for a blank canvas is why the default image of silk is undyed white, even though the animal is perfectly capable of producing gold and green.

The Wider Point

This is one more entry in a pattern: mulberry is not just fuel. The leaf supplies pigment, and pigment ends up in the product. What the animal eats becomes what the animal makes, quite literally.

Related: how silk is reeled · why silkworms only eat mulberry