What Is Silk Protein, What Does It Do? Hydrolyzed Silk and the Evidence

A silk fibre is actually made of two proteins. The strong central thread is fibroin, and the sticky sheath that surrounds it is sericin. Although proportions vary with the cocoon and method, fibroin is reported at roughly 60-80% and sericin at 15-35%. In cosmetics these proteins are broken into pieces small enough to dissolve in water, which is where the word "hydrolyzed" comes from. So the ingredient on the label is not a silk thread, but protein fragments derived from it.

How do sericin and fibroin differ?

Hydrolyzed Silk, which you often see on labels, does not state which part is present in what proportion. It also does not state the molecular size. In a safety assessment of commercial samples, the average molecular weight could go down to a few hundred daltons, but you cannot assume this for every product.

How will you see it on labels?

The cosmetic safety assessment covers ten silk-derived ingredient names. The ones you will most often encounter in hair products are the water-soluble hydrolyzed forms: Hydrolyzed Silk, Hydrolyzed Sericin and Hydrolyzed Fibroin. Silk Powder, however, is powdered silk; it is not expected to behave like a hydrolyzed protein in a liquid hair product.

A product name containing "silk", or a claim that it makes hair silky, does not mean these ingredients are in it. Check the list, not the name. The proportion is not stated on the label, and it is hard to find a generally accepted source for a typical usage rate in cosmetics.

What does it do in hair, and how strong is the evidence?

Hydrolyzed proteins are expected to attach to hair because they carry many ionic and polar regions. Studies with keratin and wheat protein show that small pieces can enter the strand while larger pieces leave a thin film on the surface. It is reasonable to assume silk behaves similarly.

The problem: independent, peer-reviewed hair studies specific to silk protein are hard to find. The shine and softness promised on the label rest more on general protein knowledge and on the formula as a whole. An honest reading should be:

• Reasonable expectation: Within the whole formula, hair feels slipperier and split ends stay neat for a while.
• Not to expect: Broken strands rejoining, hair becoming stronger and resistant to breakage, shedding decreasing.
• Duration: The effect of protein-containing care fades with the next shampoo; it needs to be repeated to last.

For a discussion of what loading hair with too much protein does, see the hair protein care page.

What is known about skin?

The most direct finding is for sericin. In a small study with healthy volunteers, skin treated with a sericin-containing gel showed increased moisture, reduced water loss and a smoother appearance. The authors attributed this to sericin forming a sealing layer on the surface. This single formula and small group cannot be directly generalised to any skin cream. The antioxidant and UV findings for sericin are mostly at laboratory level.

Is it safe, and who should be careful?

A scientific panel evaluating the safety of cosmetic ingredients found eight silk proteins, including Hydrolyzed Silk, Hydrolyzed Sericin and Hydrolyzed Fibroin, safe under current conditions of use. For two substances (MEA-Hydrolyzed Silk and Silkworm Cocoon Extract) it stated that the data were insufficient.

Silk allergy is mostly reported through inhalation and in occupational settings; reactions on skin contact are less common. Even so, protein hydrolysates in hair products can rarely cause hives. In a study on this, everyone who tested positive was a person with atopic dermatitis. If you have atopic dermatitis or have reacted to a protein product before, try the new product first on the inner arm or elbow crease, twice a day, for 7-10 days. If a symptom such as facial swelling or breathlessness appears, seek emergency help without delay.

And remember its source: silk protein is an animal-derived ingredient, so if you are looking for a vegan product, check the label for it.