Keratin is one of the most studied proteins in professional hair care, yet it remains widely misunderstood — confused with its hydrolyzed derivatives, reduced to a simple smoothing agent, or overhyped in marketing formulas with no real scientific basis. Hairswiss examines the exact chemistry of this structural protein, its mechanism of action on the hair fiber, and the conditions under which its professional use is genuinely effective.

What Is Keratin? A Biochemical Definition

Keratin is a fibrous protein from the family of scleroproteins, constituting the main structural component of hair (65–95% of its dry mass), nails, and the epidermis. Its fundamental biochemical characteristic is its high content of the amino acid cysteine, which accounts for approximately 14–18% of its total composition. It is cysteine that gives rise to disulfide bonds (S–S) — covalent bonds between two sulfur atoms — which give hair keratin its exceptional mechanical strength and its ability to hold the hair’s shape.

There are two major keratin families:

• α-keratins: present in hair and wool, they adopt an alpha-helix conformation. The polypeptide chains coil into coiled-coil (superhelix) structures, forming protofilaments and then macrofibrils that make up the internal architecture of the hair cortex.
• β-keratins: present in reptile claws and bird feathers, with a pleated beta-sheet structure — less relevant for professional hair cosmetics.

Human hair is therefore essentially composed of α-keratins, organized in a precise hierarchical architecture: amino acids → polypeptide chains → protofilaments → intermediate filaments → macrofibrils → cortex. It is this multi-scale organization that explains both the flexibility and the remarkable resistance of the hair fiber.

Disulfide Bonds: The Chemical Core of Hair Strength

Disulfide bonds (–S–S–) are the most important chemical bonds in hair keratin. They form through the oxidation of two thiol groups (–SH) on adjacent cysteine residues. Their bond energy (approximately 250 kJ/mol) makes them robust covalent bonds, far more stable than the hydrogen or ionic bonds also present in the fiber.

It is precisely these disulfide bonds that are targeted by the most common salon procedures:

• Perming and chemical straightening: reducing agents (ammonium thioglycolate, cysteamine) break the S–S bonds to allow fiber reshaping, then an oxidant (hydrogen peroxide) reforms them in the new configuration.
• Bleaching and coloring: hydrogen peroxide partially oxidizes disulfide bonds and degrades melanin, structurally weakening the cortex with each application.
• Excessive heat: above 230°C, disulfide bonds begin to break irreversibly, causing permanent degradation of the keratin structure.

Native Keratin vs. Hydrolyzed Keratin: A Fundamental Distinction

A common confusion — even among professionals — is equating native keratin with the hydrolyzed keratin used in cosmetics. These are, however, two very different chemical realities.

Native keratin is the intact protein, with a high molecular weight (40,000–70,000 Da). At this molecular weight, it is unable to penetrate the hair fiber: it remains on the surface, unable to cross the cuticle.

Hydrolyzed keratin is obtained through acid, alkaline, or enzymatic hydrolysis of native keratin (derived from sheep wool, feathers, silk, or plant proteins for vegan versions). This process fragments the protein into low molecular weight peptides (300–10,000 Da depending on the degree of hydrolysis). These peptide fragments can adsorb onto the cuticle surface and, for the smallest ones, partially penetrate the cortex to interact with the existing protein matrix.

In other words: no cosmetic product can “reconstitute” broken disulfide bonds or rebuild the native α-helical structure. What hydrolyzed keratin does is temporarily fill the lacunar zones on the hair surface, improve texture, reduce porosity, and mechanically reinforce the fiber — which is already considerable, provided the effect is not overstated.

Mechanism of Action on the Hair Fiber

Hydrolyzed keratin acts through three distinct and complementary mechanisms:

• Adsorption onto the cuticle: peptides bind to the cuticle surface through electrostatic interactions and hydrogen bonds, forming a protective film that smooths lifted scales, reduces porosity, and improves gloss.
• Partial penetration into the cortex: very low molecular weight fragments (< 1,000 Da) can diffuse through damaged areas of the cuticle to reach the cortex, where they interact with the existing protein matrix and contribute to partially restoring mechanical cohesion.
• Hygroscopic effect: free amino acids from hydrolysis (particularly serine, glycine, glutamic acid) are natural humectants that help maintain intrafibrillary hydration — a key factor in hair elasticity.

Measurable Benefits for the Professional

• Increased tensile strength from the first applications, measurable through fiber tenacity tests.
• Reduced porosity: the fiber absorbs less water, shortening drying time and improving color retention.
• Smoother cuticle surface: increased shine, reduced frizz, better comb glide.
• Partial heat protection: the peptide film on the surface acts as a shield against moderate temperatures from heat styling tools.
• Compatibility with technical processes: applied before coloring or bleaching, hydrolyzed keratin prepares the fiber to better withstand chemical aggression.

Plant-Based Keratin: A Vegan Alternative with Similar Properties

Plant-based keratin — often marketed as such — is not chemically keratin, but hydrolyzed proteins of plant origin (wheat, soy, corn, rice) whose amino acid profile shows functional similarities to animal hydrolyzed keratin. Their molecular weight and adsorption capacity are comparable, making them relevant as a vegan alternative. However, their cysteine content is significantly lower, which limits their interaction with the fiber’s disulfide bonds.

Professional Hydrolyzed Keratin Products on cliCHair

Among professional formulations integrating hydrolyzed keratin as a structural active, several references are available on cliCHair.ch, the B2B platform for Swiss hair professionals. Hydrolyzed keratin features prominently in intensive regeneration protocols such as the Regeneration Therapy Mask by Edelstein, where it is combined with Regenine and hyaluronic acid for a full-spectrum reconstructive action, and in the Regeneration Therapy Master Kit, designed for salons wishing to offer this protocol as a high-value technical service.

What Keratin Cannot Do

Scientific rigor requires stating the limits of hydrolyzed keratin in cosmetics clearly. It cannot:

• Permanently reform broken disulfide bonds (only cysteamine or thioglycolate, under controlled conditions, can influence S–S bonds).
• Reverse deep structural damage caused by very aggressive bleaching or excessive heat.
• Replace a haircut on severely damaged hair — fractured ends cannot be “resoldered.”

Understanding these limits is precisely what distinguishes a knowledgeable professional from a mere seller of marketing promises. Hydrolyzed keratin is a powerful and well-documented active — provided it is used at the right concentrations, with the right penetration vehicles, and within a coherent protocol.

Hairswiss follows the evolution of research on structural hair proteins and their applications in professional cosmetics. Hydrolyzed keratin remains, to this day, one of the best-documented and most effective reconstructive actives available in the salon.