Volatile oil is the product

You are not selling ground plant matter. You are selling the oil held inside it, and every decision from purchase to jar is about how much of that oil survives to the customer.

What is actually being graded

Volatile oil content is the quality measure the trade actually uses. ASTA publishes a minimum acceptable level per spice, and black pepper, cinnamon and oregano are the ones most often quoted against it. It is a percentage by volume of the ground material, determined by steam distillation.

This is the number a serious buyer asks for and a certificate of analysis carries. If your supplier cannot give it to you, that tells you what kind of supplier they are.

Why it is a minimum and not a target

A minimum defines the floor of salable, not the shape of good. Two lots can both pass and be very different in the jar, because volatile oil is a quantity and flavor is a composition. Origin, harvest year, drying method and storage all move the composition without necessarily moving the number.

So use the figure to reject, not to select. It will tell you a lot is weak. It will not tell you a lot is interesting.

How you lose it

Three ways, in order of how much damage they do. Grinding heat takes roughly 30 percent in one pass at ambient temperature. Time takes the rest, steadily, faster in ground than in whole. Air and light accelerate both and also oxidize what is left, which changes the flavor rather than merely reducing it.

Nothing puts it back. There is no recovery step anywhere in this craft, which is why the whole process is defensive.

What you can measure

Steam distillation is a laboratory method and it is not a thing you do on a bench in the back. What you can do is buy against a certificate, record what you received, and smell your own stock on a schedule against a reference sample kept sealed and cold.

That last practice is unglamorous and it is the one that catches a declining supplier before a customer does.

References