A Rice Water-Enriched Liquid Soap: A Natural and Sustainable Approach to Skin Care through Valorization of Kitchen Waste

Abstract

The discarding of rice washing water (Oryza sativa L.) represents a substantial and globally underutilized resource, with an estimated 510 billion liters discarded annually. This nutrient-rich liquid contains amino acids, inositol, vitamins B and E, and antioxidants with well-documented dermatological benefits. Simultaneously, used cooking oil constitutes a prevalent domestic waste stream that contributes to environmental degradation when improperly disposed of. This study presents the development and characterization of a rice water-enriched liquid soap formulated through cold-process saponification of used cooking oil with potassium hydroxide (KOH), with rice water substituting distilled water as the aqueous phase. The objectives were to: (1) evaluate the dermatological benefits of rice water as an active ingredient in liquid soap; (2) determine the physicochemical feasibility of the formulation; and (3) assess the economic viability and market potential of the product. The formulation comprised 100 mL used cooking oil, 500 mL rice water, 20 g KOH, activated charcoal, and lavender essential oil (1.5 mL). Dermatological literature synthesis indicated that rice water application significantly improves skin texture (reported by 75% of users), enhances skin luminosity (70%), and reduces wrinkles by up to 12.4% through inositol activity. The product's anti-irritant properties were further attributed to rice starch's capacity to mitigate sodium lauryl sulfate-induced skin damage. Economic analysis yielded a production cost of approximately Rp. 27,441 per 250 mL unit, with a proposed retail price of Rp. 55,000, generating a profit margin of approximately 100%. A consumer survey (n = 89) revealed that 92.1% of respondents expressed purchase intent. These findings demonstrate that rice water-enriched liquid soap constitutes a scientifically grounded, economically viable, and environmentally sustainable cosmetic product aligned with circular economy principles.