Encyclopedia · protein or peptide
Hydrolyzed keratin
Keratin fragments, usually from wool or feathers, that coat damaged hair and in lab tests partly entered it and raised wet strength; the evidence is small and industry-linked.
What it is
Peptides made by breaking keratin down with alkali, acid or enzymes. Molecular weight varies, and the 2016 tensile test used about 600 Da. It is a cosmetic ingredient and is not the same as K18's engineered peptide (see K18).
How it works
It deposits as a film and partly penetrates the cortex. UV breaks it into smaller peptides that may enter more easily. Fan 2025 Mild film-forming and hydrogen bonding explain smoothness and strength. No new covalent bonds are proposed.
What the studies show
- In a 2016 ex vivo test on bleached, permed and heat-damaged tresses, hydrolysed keratin (600 Da, 0.5%) improved wet tensile strength on all three damage types (Daniels et al.).
- Keratin formed a film and partly penetrated. Treated hair kept its tensile strength after UV, while untreated hair lost 14.32% (Unilever co-authors). Fan 2025
- A feather-keratin hydrolysate improved hydration with heat and deposited at cuticle edges, with no strength outcome. Villa 2013
How it's used in studies
Typical study conditions are 0.5-2% in a conditioner emulsion, applied to tresses and rinsed.
Side effects and safety
Protein hydrolysates in hair conditioners have caused contact urticaria, mainly in people with atopic dermatitis. Niinimaki 1998 It is animal-derived (wool or feather), so it is not vegan.
Bottom line
Useful for bleached or heat-damaged hair as a smoothing and mildly strengthening ingredient. We are only moderately sure, because independent replication and results on real heads are missing. See proteins and hair.
Studies 3
Fan J et al. Performance and mechanism of hydrolyzed keratin for hair photoaging prevention. Molecules. 2025
Tested: hydrolysed keratin treatment before UV
Result: Hydrolysed keratin deposited on the cuticle and partly penetrated; treated hair kept its tensile strength after UV while untreated hair lost 14.32%.
Read the study ↗Villa AL et al. Feather keratin hydrolysates obtained from microbial keratinases: effect on hair fiber. BMC Biotechnol. 2013
Tested: feather keratin hydrolysates
Result: Hydrolysates improved hydration with heat, deposited at cuticle edges and gave a smoother look under the microscope; no tensile or on-head outcome was measured.
Read the study ↗Niinimaki A et al. Contact urticaria from protein hydrolysates in hair conditioners. Allergy. 1998
Result: Twelve patients, all women with atopic dermatitis, reacted to a hydrolysed collagen derivative in hair conditioners with contact urticaria.
Read the study ↗Related
- Hydrolyzed wheat proteinA common conditioner protein that raised dry strength of bleached and heat-damaged tresses in one lab test, with a small but real allergy risk.
- Hydrolyzed silkSilk-derived peptides added for gloss and softness; the cosmetic safety panel judged most silk proteins safe, but there is no hair efficacy study.
- K18 peptide (sh-Oligopeptide-78)A short cysteine-rich peptide modelled on hair keratin; inventor lab data look promising, but the one independent test found it acted mostly as a surface coating.
- Amino acidsFree amino acids in conditioners are small enough to enter hair, but there is little evidence that they strengthen it.
- Proteins and hairHydrolysed proteins can modestly strengthen damaged hair in the lab, but the popular protein-moisture balance rule has never been tested.