Lignocellulose-rich walnut shell hydrochars for eco-friendly and biocompatible cosmetics: A sustainable strategy for product stability and microbial protection

This study investigates the potential of hydrochars (HC) derived from waste walnut shells (WHC) through hydrothermal carbonization (HTC) for cosmetic applications. WHCs were incorporated into facial cleansing gels and soaps at varying concentrations, and their physicochemical and antimicrobial properties were evaluated. Microbial activity was extensively tested: WHC at a concentration of 0.5 % completely inhibited bacterial and fungal growth over a 30-day period at both 25 °C and 45 °C, achieving a log reduction of >5 against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Microbial challenge tests confirmed no visible colony formation (estimated <10² CFU/mL) at WHC levels ≥0.5 %, compared to >10⁶ CFU/mL in control samples. The incorporation of WHCs also reduced the soap moisture content from 14.4 % in the control to 11.1 % at 1 % WHC, enhancing product stability. Furthermore, the pH of WHC-containing gel formulations remained within the skin-compatible range (5.56–5.85), and foam height increased from 350 mL to 500 mL in gels, and from 12 mL to 17 mL in soaps, indicating improved performance. GC–MS and FT-IR analyses identified 49 bioactive compounds, including phenolics, aldehydes, and organic acids, contributing to antimicrobial action and stability. These findings suggest WHC is a viable natural alternative to synthetic preservatives and additives, supporting the development of greener, longer-lasting cosmetic formulations.