Keratin hydrolysate characterized in that it comprises at least 94%, preferably at least 96%, and more preferably 100% by weight of free amino acids relative to the total weight of the amino acids of the hydrolysate, said hydrolysate comprising at least the following amino acids: at least 93%, preferably at least 95%, of valine in free form by weight relative to the total weight of valine in the hydrolysate, at least 90%, preferably at least 95%, of isoleucine in free form by weight relative to the total weight of isoleucine in the hydrolysate, and at least 95%, preferably 100%, of leucine in free form by weight relative to the total weight of leucine in the hydrolysate, said hydrolysate being obtainable by a preparation method wherein the keratin material is a poultry keratin material, comprising at least the following steps, in this order: - subjecting the keratin material to at least one chemical hydrolysis by means of an acid under conditions suitable for obtaining a hydrolysate comprising at least 88% by weight of free amino acids relative to the total weight of the amino acids of the hydrolysate, the remainder of the amino acids of the hydrolysate being in the form of peptides having a molecular weight of less than or equal to 800 Daltons, the chemical hydrolysis being carried out in two steps: - a first chemical hydrolysis carried out at a temperature ranging from 60 to 80°C for a period ranging from 4 to 5 hours, then - a second chemical hydrolysis carried out at a temperature ranging from 100 to 115°C for a period ranging from 5 to 8 hours, it being possible for the two hydrolyses to be carried out without an intermediate pause step or by performing an intermediate pause step of between 1 hour and 7 days - extracting the tyrosine and the cystine from said hydrolysate, preferably by means of a base; - desalifying said hydrolysate; - carrying out a nanofiltration by means of a filtering membrane with a maximum calibre of 300 Da, at a pressure of between 15 and 40 bar and at a flow rate ranging from 300 to 900 I/h; - optionally drying. at least 93%, preferably at least 95%, of valine in free form by weight relative to the total weight of valine in the hydrolysate, at least 90%, preferably at least 95%, of isoleucine in free form by weight relative to the total weight of isoleucine in the hydrolysate, and at least 95%, preferably 100%, of leucine in free form by weight relative to the total weight of leucine in the hydrolysate, said hydrolysate being obtainable by a preparation method wherein the keratin material is a poultry keratin material, comprising at least the following steps, in this order: - subjecting the keratin material to at least one chemical hydrolysis by means of an acid under conditions suitable for obtaining a hydrolysate comprising at least 88% by weight of free amino acids relative to the total weight of the amino acids of the hydrolysate, the remainder of the amino acids of the hydrolysate being in the form of peptides having a molecular weight of less than or equal to 800 Daltons, the chemical hydrolysis being carried out in two steps: - a first chemical hydrolysis carried out at a temperature ranging from 60 to 80°C for a period ranging from 4 to 5 hours, then - a second chemical hydrolysis carried out at a temperature ranging from 100 to 115°C for a period ranging from 5 to 8 hours, it being possible for the two hydrolyses to be carried out without an intermediate pause step or by performing an intermediate pause step of between 1 hour and 7 days - extracting the tyrosine and the cystine from said hydrolysate, preferably by means of a base; - desalifying said hydrolysate; - carrying out a nanofiltration by means of a filtering membrane with a maximum calibre of 300 Da, at a pressure of between 15 and 40 bar and at a flow rate ranging from 300 to 900 I/h; - optionally drying. - subjecting the keratin material to at least one chemical hydrolysis by means of an acid under conditions suitable for obtaining a hydrolysate comprising at least 88% by weight of free amino acids relative to the total weight of the amino acids of the hydrolysate, the remainder of the amino acids of the hydrolysate being in the form of peptides having a molecular weight of less than or equal to 800 Daltons, the chemical hydrolysis being carried out in two steps: - a first chemical hydrolysis carried out at a temperature ranging from 60 to 80°C for a period ranging from 4 to 5 hours, then - a second chemical hydrolysis carried out at a temperature ranging from 100 to 115°C for a period ranging from 5 to 8 hours, it being possible for the two hydrolyses to be carried out without an intermediate pause step or by performing an intermediate pause step of between 1 hour and 7 days - a first chemical hydrolysis carried out at a temperature ranging from 60 to 80°C for a period ranging from 4 to 5 hours, then - a second chemical hydrolysis carried out at a temperature ranging from 100 to 115°C for a period ranging from 5 to 8 hours, - extracting the tyrosine and the cystine from said hydrolysate, preferably by means of a base; - desalifying said hydrolysate; - carrying out a nanofiltration by means of a filtering membrane with a maximum calibre of 300 Da, at a pressure of between 15 and 40 bar and at a flow rate ranging from 300 to 900 I/h; - optionally drying.