Broccoli does not store much ready-made sulforaphane. Instead, it keeps two ingredients apart: glucoraphanin, a relatively stable precursor, and myrosinase, an enzyme that converts that precursor into sulforaphane when plant cells are damaged by chopping or chewing.

That arrangement works well in raw broccoli. It works less well after boiling, microwaving, or other sustained heating, because the enzyme is much easier to damage than the precursor. The useful twist is that the missing enzyme does not have to come from the broccoli itself. Mustard seed carries its own myrosinase, so a little dry mustard powder added after cooking can restart the reaction.