From Culinary Dud To Stud: How Dutch Plant Breeders Built Our Brussels Sprouts Boom : The Salt : NPR

There are hundreds of these old varieties. The companies grew them in test plots, and they did, in fact, find some that weren't as bitter. They cross-pollinated these old varieties with modern, high-yielding ones, trying to combine the best traits of old and new spruitjes. It took many years. But it worked. "From then on, the taste was much better. It really improved," 

https://www.npr.org/sections/thesalt/2019/10/30/773457637/from-culinary-dud-to-stud-how-dutch-plant-breeders-built-our-brussels-sprouts-bo

_- Steve