Catalyzes the optional acetylation of acidic sophorolipids (SLs) at the 6' and/or 6'' carbon atom of the sophorose head group to produce acetylated acidic SLs in sophorolipid biosynthesis (PubMed:21702032, PubMed:23516968, PubMed:26298016). Has no activity toward lactonic sophorolipids, confirmimg that acetylation must precede lactonization (PubMed:26298016). Induced in early stationary phase (at protein level). Produces purely unacetylated sophorolipids. Yields mainly lactonic SLs, in addition to minor amounts of acidic SLs. Belongs to the transferase hexapeptide repeat family.