a process for the conversion of lutein or of its esters into zeaxanthin by base - catalyzed isomerization is disclosed . the process is carried out by heating an optionally pre - treated lutein - containing material in a mixture of an aqueous solution of an alkali hydroxide and either dimethyl sulphoxide or an organic solvent based on saturated aliphatic and / or aromatic hydrocarbons at temperatures in the range from about 50 ° c . to about 120 ° c . the process is carried out in the presence of a phase transfer catalyst when an organic solvent based on hydrocarbons is used . being carotenoids , lutein and zeaxanthin are used correspondingly , especially as pigments for egg yolk , the integuments and the subcutaneous fat of poultry , the flesh and the integuments of fish and crustaceans , as well as foodstuffs . zeaxanthin is preferably used in many applications , since in comparable dosages it produces a more intensive golden yellow pigmentation than lutein .