Resource Recovery from Waste Cooking Oil Through Cold-Process Saponification for Sustainable Solid Soap Production

Waste cooking oils represent an important household and institutional waste stream with significant environmental impacts when disposed of improperly. In the context of the circular economy, this study investigates their valorization through conversion into solid soaps by cold-process saponification. The oils were pretreated by mechanical filtration, chemical purification with 10% NaCl solution and natural purification using eggshells, and their quality was assessed through total polar compounds, acid, peroxide and iodine values. Raman spectroscopy showed similar molecular profiles of the soaps obtained from the two purification treatments. The purified oils were converted into solid soaps using sodium hydroxide with a 5% superfat, and essential oils of Pimpinella anisum L., Ocimum basilicum L. and Coriandrum sativum L., characterized by GC-FID, were incorporated to provide an additional functional characteristic. The soaps were evaluated for physicochemical characteristics and antimicrobial activity against Staphylococcus aureus and Escherichia coli. All soaps showed antibacterial activity, with inhibition zones up to 10.87 mm against S. aureus and 7.77 mm against E. coli for basil essential oil soap. Two-way ANOVA confirmed significant effects of sample type and concentration (p < 0.001), the essential-oil soaps exceeding the base soaps by 0.83 and 1.88 mm, while no statistically significant difference was detected between soaps prepared from the two purification treatments. Waste cooking oils thus constitute a potential secondary raw material for functional soap production. The proposed approach demonstrate that, following appropriate pretreatment, waste cooking oils constitute a viable secondary raw material for the production of solid soaps with satisfactory quality and preliminary in vitro antimicrobial activity, supporting waste valorization and contributing to the development of circular economy approaches.