Hydrolysed keratin and molecular weight: what does the MW profile reveal about ingredient performance?

Hydrolyzed Keratin on an INCI list tells formulators which category of ingredient they are working with, but does not explain how the material will interact with the hair fibre. Its molecular weight profile is one of the factors that determines the balance between its potential for penetration and the importance of surface interactions. Selecting hydrolysed keratin requires consideration of several interrelated parameters: molecular weight (MW) distribution, peptide composition, degree of hydrolysis and hair condition. This article explains how to interpret these data, what the research actually shows and how to apply these insights when developing hair care formulations.

The same INCI name, different ingredient characteristics

Hydrolyzed Keratin describes an ingredient category, but does not capture its full technical profile. Two materials with the same INCI name may be derived from different protein sources, produced using different hydrolysis methods and contain different proportions of free amino acids and peptides. Their molecular weight distributions may also differ.

These parameters may influence whether, after application, a hydrolysate penetrates into the hair, is deposited primarily on the cuticle or interacts with the fibre at both levels. Lower-molecular-weight fractions may have greater penetration potential, whereas larger fractions are more likely to remain in the outer regions of the fibre or interact with its surface. This is not a rigid distinction, however. Some of the same hydrolysate may penetrate into the fibre, while some is deposited on the cuticle [1,3,4].

When selecting ingredients for hair care, the INCI name alone is not enough. It is also necessary to examine the MW profile, including the distribution of individual fractions, and determine whether penetration, surface interactions or a combination of both mechanisms is more relevant to the project.

The suitability of a selected ingredient can only be confirmed through testing in the intended formulation.Przydatność wybranego surowca potwierdzają dopiero testy w docelowej formulacji.

Would you like to order a sample of the raw material?

We will send you the latest specification and information on the product’s availability. You can also receive a sample for preliminary formulation testing.

Order a sample of the raw material

What does the MW profile tell us about hydrolysate behaviour within the hair fibre?

A study by Malinauskyte et al. illustrates the relationship between molecular weight and hydrolysate localisation. The authors compared three wool-derived keratin hydrolysates: a low-MW hydrolysate with an average molecular weight of 221 Da, a mid-MW hydrolysate of approximately 2,577 Da and a high-MW hydrolysate with an average molecular weight of approximately 75,440 Da [1].

The study used tightly curled textured hair that had previously been chemically straightened with a sodium hydroxide-based relaxer. The researchers examined where the individual fractions were located and how they affected the mechanical and thermal properties of the fibres.

The hydrolysates differed substantially in their location within or on the fibre. For the low-MW hydrolysate, dimensional changes in the fibre suggested substantial penetration. For the mid-MW hydrolysate, microscopy demonstrated penetration into the cortex. The high-MW hydrolysate, by contrast, remained mainly on the surface and reached the inner regions of the hair only to a limited extent [1].

Table 1. Localisation and selected effects of keratin hydrolysates with different average molecular weights [1]

Source: compiled by the authors based on [1].

These MW values relate to the specific hydrolysates and conditions used in the study and do not define universal functional thresholds. Nevertheless, the findings indicate that the MW profile may influence where hydrolysed keratin is located within or on the fibre [1].

Does deeper penetration mean better performance?

Not necessarily. The location of a hydrolysate alone does not determine the resulting effect. If functionality depended solely on penetration depth, the smallest peptides would be expected to deliver the greatest improvement in the parameters assessed. However, the relationship observed in the Malinauskyte study was not that straightforward.

The mid- and high-MW fractions increased Young’s modulus, a measure of fibre stiffness, and reduced hair breakage at 20% and 80% relative humidity. The low-MW hydrolysate, despite the greater penetration suggested by the study, did not produce comparable improvements across all the mechanical parameters assessed. Only the mid-MW fraction affected the thermal properties evaluated [1].

Studies of wool-derived keratin peptides and larger keratin proteins point to a similar conclusion. Both types of ingredient had beneficial effects on moisture content and selected mechanical properties of previously chemically treated hair [2].

A higher molecular weight and localisation closer to the surface therefore do not preclude hydrolysate functionality.

For formulators, the practical conclusion is clear: penetration is not a standalone measure of either ingredient quality or performance. Smaller peptides present in the cortex and larger fragments interacting with the cuticle may contribute different, potentially complementary properties. Selection should be guided by the effect to be achieved and measured.

Are penetration and surface activity mutually exclusive?

No. In practice, the behaviour of hydrolysed keratin cannot readily be reduced to a choice between the interior and the surface of the fibre. Available research shows that both mechanisms can occur simultaneously.

Villa et al. studied a hydrolysate obtained through enzymatic degradation of feather keratin, in which the predominant peptides had molecular weights of approximately 800–1,079 Da [3]. When the hydrolysate was used in hair care formulations, increased fibre hydration was observed, particularly with the application of heat. At the same time, scanning electron microscopy (SEM) revealed deposits of material at the junctions between cuticle scales. The study did not directly localise peptides within the cortex. It does, however, show that a hydrolysate composed predominantly of peptides in the approximately 800–1,079 Da range can leave detectable deposits on the fibre surface.

A 2025 study by Fan et al. presents a similar picture [4]. In the experimental model examined, hydrolysed keratin formed a layer on the hair surface, while some of the material also penetrated into the fibre. The authors additionally observed the breakdown of the hydrolysate under UV radiation into smaller peptides and amino acids, which promoted penetration in that model. These findings relate to a specific hydrolysate and cannot be directly extrapolated to other ingredients.

These data show that penetration potential can coexist with surface interactions. The MW profile may therefore help determine the nature of a hydrolysate’s interaction with the fibre, but does not, on its own, unequivocally define its function. These findings should not be automatically extrapolated to every ingredient with the INCI name Hydrolyzed Keratin.

What factors other than MW influence hydrolysate behaviour?

Average MW does not describe the entire ingredient. Two hydrolysates with similar average MW values may have different distributions of fractions: narrower or broader, and comprising different proportions of free amino acids, short peptides and larger protein fragments.

When comparing ingredients, the following factors should also be considered:

• Keratin source – influences the amino acid composition of the starting material;

• Method and degree of hydrolysis – influence how the protein is broken down and the resulting peptide profile;

• Peptide structure and charge – may alter electrostatic interactions with the fibre;

• Hair condition – a damaged cuticle and changes in fibre structure may make penetration pathways more accessible;

• Formulation and application conditions – including pH, formulation base, use level, contact time, the presence of other ingredients and the application of heat.

Research into the influence of peptide structure on penetration also highlights the importance of hair condition.

In the experimental model examined, transport depended on electrostatic interactions and was observed in oxidatively damaged hair, but not in undamaged hair [5].

Results obtained with bleached hair cannot be automatically extrapolated to virgin hair.

Average MW should therefore be interpreted alongside the molecular weight distribution, peptide characteristics and a description of the experimental model.

How can the MW profile guide formulation development?

The MW profile should be treated as one criterion for characterising a hydrolysate, rather than as a standalone predictor of performance. Lower-molecular-weight fractions may offer greater potential for interaction in the more accessible regions of damaged fibres, while larger fragments may be more relevant to interactions with the outer regions of the fibre and its surface. However, available research does not establish a single optimal molecular weight or universal functional thresholds [1–5].

TuriKer® – using MW profiles to select a grade

The TuriKer® range, available from MEDICOS, is an example of an ingredient line in which molecular weight profiling is used as one criterion for differentiating hydrolysed keratin grades. This family of turkey-derived β-keratin hydrolysates is produced through controlled hydrolysis. The individual grades differ in their molecular weight profiles; depending on the grade, they contain free amino acids and peptides spanning different MW ranges [6].

The functions listed for Hydrolyzed Keratin in CosIng include hair conditioning, film forming, antistatic and humectant functions.

Table 2. MW profiles and illustrative application areas for TuriKer® grades [6]

Source: compiled by the authors based on technical materials supplied by Atlantis Ingredients [6].

This overview helps narrow down the choice of grades, but does not replace technical documentation or testing. The studies cited [1–5] were not conducted using TuriKer® grades and do not directly substantiate their performance in formulations. They do, however, show that the MW profile may be one of the factors that differentiates how keratin hydrolysates interact with hair, making it relevant from the formulation development stage onwards. The final selection should be based on the manufacturer’s current materials, the formulation objectives and test results [6].

TurEssence® – different MW profiles within one concept

The TurEssence® concept builds on this approach to MW profiling. Where a formulation is intended to address both the more accessible regions of damaged hair and its surface, the complementary use of TuriKer® grades with different molecular weight profiles may be considered [6].

Within the TurEssence® concept, smaller peptides and free amino acids may offer greater potential for interaction in the more accessible regions of damaged fibres. Larger fragments are primarily intended to interact with the surface, where they may adsorb onto the cuticle and support film formation. This approach is consistent with observations reported in the literature indicating that penetration and surface deposition need not be mutually exclusive mechanisms [1,3,4]. Each specific combination of grades, their proportions and use levels should be verified in the intended formulation.

Figure 1. TurEssence® – a two-level hair fibre care concept based on complementary TuriKer® grades with different MW profiles. Image created using AI.

The MW profile narrows the choice; testing determines suitability

The MW profile helps predict how a hydrolysate may interact with hair, but does not determine its performance in the intended formulation. The distribution of fractions, peptide structure, hair condition and the parameters of the formulation as a whole also matter. Selecting a TuriKer® grade should therefore be treated as the starting point for evaluation in the intended base.

MEDICOS provides technical documentation and samples of individual TuriKer® grades, enabling comparison within a specific formulation project. A well-prepared formulation brief helps identify the grades that match the project’s objectives.