Reinventing henna: Enzyme-catalysed colour release from stabilized Lawsonia inermis L. extracts

Improvement of application, performance, colour predictability and stability of plant-based hair colourants derived from Lawsonia inermis L. by investigating the release mechanism of lawsone during extraction and optimizing processing conditions to prevent its premature formation. A two-component system was developed, consisting of naturally occurring lawsone precursors (hennosides) in one formulation and a catalytic agent (β-glucosidase) in another.

Henna raw material was extracted using maceration, Soxhlet, and industrial-scale methods with various solvents. Extracts were analysed for yield and content of lawsone, hennosides, polyphenols, metals and reducing sugars. HPLC quantified lawsone and hennosides; reducing sugars and polyphenols were measured using dinitrosalicylic acid and Folin-Ciocalteu assays, respectively. Yak hair was dyed with different formulations and evaluated for colour outcome and wash fastness using the CIELab colour system.

Formulations containing pre-released lawsone were unstable and ineffective after 4 weeks. In contrast, gel formulations containing hennosides without β-glucosidase remained stable for over 8 weeks and produced consistent colour when combined with the enzyme before application. Ethanolic extraction effectively yielded hennoside-rich extracts without lawsone formation. Aqueous extraction at boiling point also produced hennoside-rich extracts. Soxhlet extraction resulted in β-glucosidase activity remaining in the starting material. The two-component gel formulation demonstrated comparable wash fastness and colour intensity to the conventional paste, with improved ease and speed of application and rinsing.

Optimizing henna extract processing and formulation design enabled the development of a more user-friendly, effective and stable plant-based hair dye, supporting broader consumer adoption.