Fibroin and sericin: what really remains in your silk

In short: a raw silk thread is made of two proteins, fibroin and sericin. Degumming removes most of the latter, and that is precisely what gives silk its feel and its light. Yet most of the “benefits” attributed to sericin have been observed on laboratory extracts, not on a fabric. Here is what your pillowcase really contains, and how to read the promises without being misled.

I am often asked, in the shop as in writing, whether silk “nourishes” the skin. It is a fair question: half the market’s product pages talk about sericin, amino acids and proteins that supposedly moisturise overnight. I will tell you what I have learned from spending my days with my hands in the material, and what the scientific literature actually says.

One silk thread, two proteins

The mulberry silkworm, Bombyx mori, does not spin one thread: it spins two, side by side, which it coats with a gum as it goes. What is harvested from the cocoon is therefore an assembly.

Fibroin, the core

Fibroin is the two filaments. It is the structural protein: long, highly ordered, remarkably strong for its fineness. It is what gives the thread its strength, its suppleness and its ability to drape. Everything you love to hold in your hand comes from it.

Sericin, the glue

Sericin is the gum that binds the two filaments together and stiffens the cocoon. It represents roughly a quarter to a third of the weight of the raw thread. To the touch, silk that is still gummed is matt, a little stiff, almost cardboard-like, nothing like what you imagine when you say “silk”.

	Fibroin	Sericin
Role in the cocoon	Structural filament	Coating gum
Share of the raw thread	About 70 to 75%	About 25 to 30%
Appearance	Smooth, luminous	Matt, stiff
After degumming	Remains: it is the fabric	Largely removed
Use in research	Textiles, biomaterials	Extracts, hydrogels, formulations

Degumming: the step that makes all the difference

Degumming, which is also called boiling-off, consists of dissolving the sericin in a hot bath, most often soapy or mildly alkaline. The thread then loses a quarter of its weight, and gains everything else: the fluid hand, the drape, that light which moves as you move the fabric.

It is a step that calls for skill, not a formality. Degummed too little, silk stays dry and grey; degummed too harshly or for too long, the fibroin is damaged and the fabric loses strength. When a piece feels rough at purchase and “softens in the wash”, it is generally incomplete degumming finishing its work in your machine, taking a little of the material with it.

Above all, remember this: finished silk, the kind in your pillowcase or your duvet cover, contains hardly any sericin any more. That is intentional. It is the condition of the feel.

What science studies, and on what

Sericin genuinely interests researchers. A review published in 2025 in Macromolecular Bioscience (Stradczuk-Mazurek et al., Silk Sericin in Dermatological Diseases: From Preclinical Studies to Future Clinical Applications, DOI: 10.1002/mabi.202500058) takes stock of this work. Its title already says the essentials about where things stand: we are at the stage of preclinical studies, and clinical applications are yet to come.

Above all, this work concerns extracted sericin that has been processed: films, hydrogels, dressings, cosmetic formulations. In other words, an isolated, dosed, applied ingredient. That is not the same thing as a fabric from which this protein has precisely been removed. Carrying results from one over to the other is a shortcut, and unfortunately it is the most widespread shortcut in the sector.

My view, stated cautiously: if sericin one day proves itself in dermatology, it will probably be as a treatment, not as a sheet.

The clinical trial nobody cites

There is, however, a serious trial on silk textiles worn on the body. The CLOTHES trial (Thomas KS et al., PLoS Medicine, 2017, DOI: 10.1371/journal.pmed.1002280) followed 300 children aged 1 to 15 with atopic eczema for six months, randomly assigned to “silk clothing plus usual care” or “usual care alone”.

The result: no clinically significant difference in eczema severity (ratio of geometric means of the EASI score: 0.95; 95% confidence interval: 0.85 to 1.07). The authors conclude that adding these garments brings no measurable benefit.

I find this trial valuable, and I would rather cite it myself than leave it to the competition. It draws a clear line: silk is a material of comfort, not a treatment. No textile cures a skin condition, and any page that suggests otherwise is telling you a story. If your skin worries you, it is a dermatologist you need to see, not a pillowcase.

So what remains true?

Plenty, only it is physical rather than biochemical. Well-degummed, well-woven silk is:

• A very smooth surface, and therefore little friction against the cheek and hair as you move during the night. It is mechanical, measurable, and it is the strongest argument for the material.
• A moisture regain of about 11%, higher than that of cotton, which contributes to the bed’s microclimate without giving that damp-fabric sensation.
• Fineness at equal warmth: you get the same thermal comfort with far less thickness, which changes how it feels under the duvet in winter.
• A lint-free surface, which does not hold on to short fibres the way a cotton jersey does.

These are qualities of the material. They can be checked by hand, and they last as long as the piece is cared for properly.

Five points to check before buying

1. Momme first. It is the unit of silk weight, and the first indicator of durability. I have devoted an entire guide to it: understanding momme and choosing your weight.
2. “100% mulberry silk”, stated in black and white. “Satin” on its own only describes a weave, which can be made of polyester. The distinction is explained here: silk or satin, what are the differences.
3. The feel on arrival. Properly degummed silk is supple and luminous from the very first touch, not after three washes.
4. Be wary of medical vocabulary. “Repairs”, “heals”, “treats”: on a textile, these words have no foundation.
5. Ignore sericin promises. Now you know why: it is no longer there, and that is all the better for the feel.

To go further on what silk changes, and does not change, for reactive skin, Mei has written a companion article: sensitive skin and silk, what the science says.

Frequently asked questions

Does my silk pillowcase still contain sericin?

Very little. Degumming removes most of this gum, and that is precisely what gives the fabric its suppleness and sheen. Silk that still contained a lot of it would be stiff and matt.

Is sericin good for the skin?

Preclinical studies are exploring its properties as an extract, in formulations or in dressings. Clinical evidence in humans remains limited, and this research does not concern worn fabric. It would be overreaching to draw a promise for a pillowcase from it.

Should I prefer “non-degummed” silk?

No. Raw, undegummed silk is stiff, matt and unpleasant against the skin. Degumming is not a loss; it is the step that makes silk what we know it to be.

How can I tell if silk has been poorly degummed?

It looks dry, a little grey, and slightly “squeaks” under the fingers. It softens unusually in the first washes, a sign that the gum was finishing its departure.

Can fibroin cause an allergy?

Reactions to silk are rare and have mostly been associated, historically, with sericin residues. Well-degummed silk contains very little. If you are unsure about a skin reaction, seek the advice of a healthcare professional.

In closing

Silk does not need powers invented for it. Two proteins, a carefully managed degumming step, an honest weight: it is already a rare material, and that is enough. I would rather offer you a fine, well-woven fibroin than an imaginary sericin.

If you would like to judge for yourself, start with a piece your skin touches eight hours a night: our mulberry silk pillowcases, or the whole Mulberry silk collection if you would like to compare weights.

— Colette, for the StudioSoie Journal