By Harold McGee
The flavorful material in an herb or spice is traditionally called its essential oil. The term reflects an important practical fact: aroma chemicals are more similar to oils and fats than they are to water, and are therefore more soluble in oil than they are in water. This is why cooks make prepared flavor extracts by infusing herbs and spices in oil, not water. They do infuse herbs in watery vinegar and in alcohols, but both alcohol and vinegar’s acetic acid are small cousins of fat molecules, and help dissolve more aromatics than plain water could.