Identity
Hydrolyzed Keratin is keratin protein, most often sourced from wool, broken down by acid, alkaline or enzymatic hydrolysis into peptide fragments. It is not a single substance but a distribution of chain lengths, and commercial grades span an enormous range: materials characterised in the published fibre work ran from 221 Da through roughly 2577 Da to approximately 75 440 Da. All of them are declared identically on an ingredient list.
Its appeal is intuitive. Hair is keratin, so keratin is offered back to it. The evidence supports a narrower and more interesting story than that symmetry suggests.
Development & history
Protein hydrolysates have been used in hair care since the middle of the twentieth century, long predating the bond-building category and most of the actives that now command higher prices. Wheat and rice hydrolysates arrived alongside keratin, and each has been studied for penetration behaviour.
The molecular weight question was investigated systematically at TRI Princeton and published in 2021, testing defined fractions against both an untreated control and a free amino acid, on chemically relaxed textured hair. That design is what makes the data useful: it separates the effect of the peptide from the effect of simply wetting and treating the fibre, and it isolates chain length as a variable.
Mechanism (as proposed)
Two mechanisms are proposed and they operate at different depths.
Lower-molecular-weight peptides are small enough to diffuse into the cortex, where they are proposed to occupy and reinforce regions left porous by chemical processing, interacting with the keratin matrix through hydrogen bonding and electrostatic association rather than covalent attachment.
Higher-molecular-weight fractions largely cannot enter. They adsorb to the fibre surface and form a film, which is proposed to reduce friction, limit further damage and add measurable stiffness to the fibre as a whole.
Neither mechanism is reconstruction. Deposited peptide reinforces the fibre from inside or outside; it does not restore the cross-linked architecture that processing destroyed, and it is removed by washing. The observed mechanical gains are consistent with reinforcement, and the observation that the smallest fraction penetrated best while strengthening least is difficult to reconcile with any repair account.
Related reading
Our comparison of hydrolysed proteins in hair care sets keratin against wheat and rice hydrolysates. For the premium end of the same promise, see our examination of the bond-building category and what independent testing found there.