What it is
The EU's CosIng database describes Hydrolyzed Keratin as a biological material consisting predominantly of partially hydrolysed keratin, and lists five functions: antistatic, film forming, hair conditioning, humectant and skin conditioning.1 Hydrolysis cuts the protein into shorter chains; one study describes the result as a series of polypeptide molecules, usually derived from wool.2
It is usually animal-derived. A 2015 review notes that keratin hydrolysates are generally prepared from keratin-containing animal parts collected from discarded materials, such as feathers, horns, hoofs, hair and wool. Some manufacturers instead use amino acids from wheat, corn and soy to mimic keratin's composition, and the same review adds that amino acids do not replace or restore damaged keratin structure.3
What it does in hair products
Proteins have many ionic and polar sites that bond to hair, and molecules under about 1,000 daltons can diffuse into it, especially into damaged hair.3 Protein-derived conditioners are popular for damaged hair because protein is drawn to keratin and can hold the split fragments of a damaged end together, but only until the next shampoo.4
A 2025 laboratory study looked more closely. Bleached hair was soaked for four hours in a 2% hydrolysed keratin solution; the protein deposited along the edges of the cuticle scales and partly penetrated the cortex. After six days under a xenon lamp used to mimic sunlight, untreated hair had lost 14.32% of its tensile strength while treated hair had not, which the authors attribute to the film screening UV and to fragments broken down by UV entering the fibre.2 A four-hour soak is far longer than any shampoo or conditioner stays on hair, so the study shows what the ingredient can do rather than what a particular product does.
None of these sources shows it rebuilding hair. The effects they report are a film, a temporary hold on split ends and, in one laboratory study, strength kept after UV exposure.
What official sources say
Hydrolyzed Keratin has no entry in the annexes of the EU cosmetics regulation.5 In 2021 the US Cosmetic Ingredient Review assessed eight keratin-derived ingredients, the family hydrolysed keratin belongs to, which it describes as functioning mainly as skin and hair conditioning agents. The panel concluded that they are safe in the present practices of use and concentration described in its assessment.6
Not the same as a keratin treatment
Salon smoothing services sold as keratin treatments work differently. A 2015 dermatology review explains that their straightening comes from formaldehyde, or from substances such as methylene glycol and glyoxylic acid that release it when heated by the blow-dryer and flat iron, which cross-links the hair's own keratin in its new shape.3 The FDA notes that hair smoothing products often contain formaldehyde, also listed as formalin or methylene glycol, which is released into the air as a gas when the solution is heated.7 Hydrolyzed Keratin on a list does not, on its own, make a product one of these.
How it appears on a label
As Hydrolyzed Keratin, its name in the EU glossary, which lists Keratin, Keratin Amino Acids and longer names such as Hydrolyzed Keratin PG-Propyl Methylsilanediol as separate ingredients.8 The label does not say how far the protein was broken down, which animal it came from, or how much is present. The conditioner ingredients guide shows where proteins sit among a conditioner's other ingredients, and the hair product scanner reads a whole list.