This table shows the solubilities of some common flavor ingredients in various solvents.
PG (Propylene Glycol) is a preferred solvent because it is non-flammable and low cost. However larger molecules are not soluble in it.
Triacetin is a very effective solvent and is non-flammable, but has a bitter taste greater then 0.1%, and higher cost. Some essential oils and large molecules are soluble in Triacetin, but not in PG.
A mixture of PG and Triacetin provides increased solubility of PG, and provides solubility of ingredients that would not be soluble in PG alone. Mixtures of PG/Triacetin can be optimized to achieve solubility and cost. 60/40 and 40/60 mixtures are soluble.
MCT (Medium Chain Triglycerides) is not a good solvent for most solid flavor ingredients. Triacetin is soluble in MCT at 20 %. Adding ingredient solutions in Triacetin to an MCT based flavor, is an efficient way to add existing Triacetin solutions to a flavor.