Glucoraphanin

Broccoli does not contain sulforaphane. It contains glucoraphanin, and the two are routinely confused, including on product labels.

What glucoraphanin is

Glucoraphanin belongs to a family of compounds called glucosinolates, which cruciferous plants produce and store in quantity. Structurally it carries a sugar group joined through a sulfur atom to a chain ending in a methylsulfinyl group, with a sulfated oxime in the middle. The sugar and the sulfate are both leaving groups, which is the whole point of the design.

In the intact plant it is stable and does nothing. It sits in the cell vacuole, chemically inert, waiting.

Why the plant keeps it separate

The enzyme that acts on glucoraphanin, myrosinase, is stored in different cells entirely. As long as the tissue is intact, the two never meet.

This is a defense system with a trigger. An insect chewing a leaf ruptures both compartments at once, myrosinase reaches glucoraphanin, and the plant produces a sharp, reactive isothiocyanate precisely where it is being eaten. Sulforaphane is a chemical weapon released on contact, and the fact that people find it interesting for other reasons is incidental to why it exists.

What this means for a label

Most products marketed for sulforaphane contain glucoraphanin from broccoli seed or sprout extract, because glucoraphanin survives manufacturing and storage while sulforaphane does not.

That leaves the conversion step to be solved somewhere else. Some products add myrosinase from a separate plant source. Others rely on the enzyme activity of gut bacteria, which varies enormously between individuals and is not something a label can promise. Two capsules stating the same glucoraphanin content can deliver very different amounts of sulforaphane to two different people, or to the same person on different days.

A label quoting a "sulforaphane equivalent" or "sulforaphane potential" is stating an upper bound assuming complete conversion. Complete conversion does not happen. The page on how much sulforaphane per day sets out the doses human trials have used and how much of a glucoraphanin dose people actually excrete as sulforaphane metabolites.