Formulation and Physicochemical Quality Evaluation of Purified Used Cooking Oil-Based Solid Soap with Nutmeg Fruit Flesh Extract (Myristica fragrans Houtt.)

Background: Recycling used cooking oil into solid soap offers an approach to oil waste utilization, while incorporating nutmeg fruit flesh extract may influence its antioxidant activity. Objective: This study aimed to develop and evaluate the physicochemical properties and antioxidant activity of solid soap prepared from purified used cooking oil containing different concentrations of nutmeg (Myristica fragrans Houtt.) fruit flesh extract. Methods: Used cooking oil was sequentially purified in a single batch through activated charcoal adsorption and citric acid degumming. Solid soaps containing 1%, 2%, and 3% w/w extract were prepared by sodium hydroxide saponification and cured for four weeks. Free fatty acid content, pH, moisture content, unsaponifiable matter, and antioxidant activity using the DPPH assay were evaluated. Results and Discussion: Free fatty acid content decreases from 5.452 ± 0.325% to 1.128% after adsorption and reaches 1.598 ± 0.163% after degumming. The formulations exhibit pH values of 9.3–9.8, moisture contents of 10.2–10.8%, and unsaponifiable matter of 1.24–1.38%. The extract-free soap (F0) exhibits an IC₅₀ of 988.29 ± 20.00 µg/mL, while F1, F2, and F3 exhibit values of 687.42 ± 18.34, 412.85 ± 14.22, and 263.17 ± 11.56 µg/mL, respectively. Increasing extract concentration corresponds to decreasing IC₅₀ values, although these findings do not establish antioxidant benefits for skin. Conclusion: Purified used cooking oil is a feasible raw material for solid soap, and nutmeg fruit flesh extract improves DPPH radical-scavenging activity. Further quality, stability, and safety evaluations remain necessary.