Reviving Traditional Dyes: The Role of Lawsonia inermis L. in Modern Hair Coloration

Abstract

This work aimed to improve the convenience and performance of plant-based henna hair coloration. In the EU, henna is mostly sold as powder to be mixed into a paste, limiting formulation innovation and user convenience. To simplify application, the release of lawsone (2-hydroxy-1,4-naphthoquinone) was investigated. Lawsonerich extracts and pastes stored for 8 weeks showed marked degradation and loss of dyeing efficacy due to light-induced degradation and oxidation to quinonoid species, underscoring that lawsone content alone is an unreliable performance and safety metric. This highlighted the need to shift focus to naturally occurring and stable precursors – the monoglycosidic hennosides. Coloration systems reliant on hennosides necessitated precursor extraction from raw material followed by enzymatic hydrolysis shortly before application. Extract analysis was enabled by isolating hennoside A (1,2,4-trihydroxynaphthalene-1-O-glucoside), the only monoglycosidic lawsone precursor present in the Moroccan henna used, via ethanolic Soxhlet extraction and reverse-phase column chromatography. The isolate was characterized (NMR, HPLC, ESI-TOF) and used as a quantitative standard for the developed and validated HPLC method. To contextualize hennoside-based coloration, a benchmark system using a 4% w/v henna suspension (pH 4.3–4.8, 15 min extraction at 23±2 ◦C, 30 min coloration) was established. Soxhlet extraction yielded the highest total hennoside amount regarding input mass (5.2±0.7%), while optimized ethanolic maceration (40 ◦C, 3 h) gave comparable yields. A pilot-scale ethanol:water (8:2, v/v) maceration delivered a hennoside-rich extract (27% w/w) suitable for product formulation.Hydrogel formulations containing the pilot-plant extract were activated immediately before application with a fermentation product containing fungal β-glucosidase (5–10%), triggering in situ conversion of hennosides to lawsone. Over 8 weeks, the gel matched fresh, traditional paste in color intensity and wash fastness while improving texture and ease of use. Overall, the work establishes hennoside as a stable, quantifiable precursor and provides a pathway to henna-based formulations that increase convenience without compromising performance.