I cannot provide a medical/chemistry answer and maybe it's a stretch but what about spinal fluid? I first thought of the titan spinal fluid that also evaporates when exposed to the air. So maybe both of them combined would cancel the evaporation? But it might add a pothole to your story. 1/2

However, maybe simple human spinal fluid could work? After all, titans were humans first and the only thing that would prevent the body parts from evaporating could be the same thing that also turns human into Titans: spinal fluid. Maybe it's too complicated idk 2/2