Valorisation of <i>Citrullus lanatus</i> rind for eco-friendly lipstick: physicochemical characterisation and safety evaluation

Abstract The utilisation of agro-industrial by-products for cosmetic purposes corresponds with the increasing demand for multifunctional formulations. This study investigated watermelon ( Citrullus lanatus ) rind as a source of bioactive compounds with antioxidant properties in lipstick. Sequential extraction using solvents demonstrated that ethanol most efficiently extracted polar bioactive compounds, such as flavonoids, alkaloids, and cardiac glycosides. A D-optimal mixture design methodology was utilised to optimise the proportions of oils and waxes, with a focus on melting point stability as a key formulation parameter. The optimised lipstick formulation exhibited a melting point of 52 °C, along with strong thermal resilience and physicochemical stability under varying storage circumstances. The lipstick maintained its shape, consistency, color, and fragrance after being stored under different temperatures (5 °C, 27 °C, and 40 °C) for three months, confirming its stability. Microbiological assays verified the absence of pathogenic pollutants, with a bacterial count of 50 cfu/g and yeast and mold counts below 100 cfu/g, which is well below the ≤1000 cfu/g limit set by ISO 17516:2014, ASEAN, and Malaysian cosmetic regulations. Heavy metal concentrations were within regulatory limits, with arsenic (3.68 ppm), lead (&lt;0.04 ppm), cadmium (&lt;0.02 ppm), and mercury (&lt;0.02 ppm), confirming product safety. The use of phytochemicals possessing established antibacterial and antioxidant capabilities may synergistically enhance the formulation’s stability. The findings substantiate the practical use of watermelon rind extract in creating safe, thermally stable, and environmentally friendly lip cosmetics, presenting an innovative approach for bio-waste valorisation in the personal care industry.