Oil-soluble flavour
Most spice flavour and colour molecules dissolve in fat, which is why oil is the best carrier.
Also: fat-soluble, fat soluble flavour, lipophilic aroma
Many compounds responsible for spice flavour and colour, such as capsaicin, curcumin, piperine, carotenoid pigments and numerous terpenes, dissolve more readily in fat than in water. That is the chemical basis of tadka, blooming and spiced oils, and the reason a curry tastes better with some oil or ghee. Fat extracts the compounds from the spice and distributes them through the dish, and prolongs the flavour on the palate. Alcohol also dissolves many, which is why spices are used in liqueurs and tinctures. Water-soluble elements, such as sweet and sour notes, tannins and some pigments, emerge in broths and teas. Good cooking uses both.