Glucosinolates
The sulfurous compounds in Brassica plants!
The structure of a generic glucosinolate, the R can be a variety of attachments
Glucosinolates are organosulfur compounds found in the Brassica family of plants, which includes broccoli, Brussel sprouts, and kale. These compounds impart a bitter taste, and are considered to be a defense mechanism against insects.
When cruciferous cells (leafy tissue) are ruptured, the enzyme myrosinase (also called beta-thioglucosidase glucohydrolase) hydrolyzes (breaks apart by adding water) glucosinolates, forming isothiocyanates and indoles.
High heat decomposes these compounds, while the relatively low heat of boiling water does not. This is why roasted Brussel sprouts are delicious and boiled Brussel sprouts are swampy and gross.