ppm

ppm (parts per million) is a unit of concentration, used in particular to express the amount of a solute contained in a solution. When 1 liter of solution contains 1 milligram (mg) of solute by mass, the concentration is expressed as 1 ppm. This unit is well suited to indicating the concentration of extremely small quantities of components, and in Japanese sake it is used in analyzing trace components that affect flavor and quality. For example, certain aroma compounds and off-flavor compounds can be present at ppm levels, and brewers take this into account when making sake.