Responsible for the lactonization of acidic sophorolipids (SLs). Catalyzes the intramolecular esterification (ring closure) between the carboxyl group of the hydroxy fatty acid and the hydroxyl group of sophorose C6'' of acidic SLs. Lactonizes acetylated acidic SLs, producing di-acetylated C18:2, C18:1 and C18:0 lactonic SL, and mono-acetylated C18:1 lactonic SL. Does not catalyze the reverse reaction, the hydrolysis of lactonic SL into acidic SL, nor hydrolysis of triglycerides or paranitrophenyl esters. Optimum pH is 3.5-6. Stable from pH 3.5 to pH 8. Optimum temperature is 20-50 degrees Celsius. Glycosylated. Results in the production of exclusively acidic sophorolipids and a complete absence of any lactonic sophorolipids. Sophorolipids (SLs) are a family of fungal biosurfactants that have been commercialized as biodegradable detergents in ecological cleaning solutions. They naturally exist in two forms, a closed lactone and an open acidic form. Each form has distinct properties, acidic SLs have better foaming properties, while lactonic SLs are better in surface tension reduction and have antimicrobial activity. Lactone esterase, catalyzing the final conversion of the open acidic to the closed lactonic form, provides a biotechnological opportunity to engineer and regulate the production of SLs in S.bombicola, which naturally produces a mixture of acidic and lactonic SLs. Belongs to the AB hydrolase superfamily. Lipase family.