Valorization of Waste Frying Oil for Cost‐Effective and Sustainable Sophorolipid Production

Sophorolipids (SLs), the most widely produced class of microbial biosurfactants, have considerable industrial potential; however, their large‐scale application is constrained by high production costs. In this study, waste frying oil was employed as a low‐cost and sustainable carbon source for the efficient production of SLs by Starmerella bombicola . The effects of carbon and nitrogen sources on SLs production were systematically investigated using orthogonal design, followed by Box–Behnken response surface methodology for further optimization. The optimal fermentation conditions were determined as 110 g/L glucose, 78.77 mL/L waste frying oil, and 3.29 g/L yeast extract, achieving an SLs yield of 52.13 ± 1.66 g/L, representing a 54.22% improvement compared with the initial medium. HPLC analysis revealed the presence of acidic and lactonic SLs components mainly containing C18 unsaturated fatty acids. Surface activity evaluation demonstrated that the produced SLs possessed favorable surfactant properties. Lactonic and acidic SLs exhibited lower γCMC values (31.40 and 31.74 mN/m, respectively) than SDBS (34.37 mN/m). Notably, lactonic SLs showed a remarkably lower CMC value (0.042 g/L), indicating superior surface activity efficiency. Furthermore, both SL types exhibited rapid foam dissipation and lower foam stability than SDBS, while their emulsions showed rapid macroscopic phase separation under the tested conditions. This study highlights the potential of waste frying oil as a cost‐effective feedstock for SLs production and supports its application in sustainable industrial biotechnology.