Brussels sprouts are a cultivar group of the same species as broccoli, cabbage, collard greens, kale, and kohlrabi; they are cruciferous (they belong to the family Brassicaceae; old name Cruciferae). Many cultivars are available; some are purple in color, such as 'Ruby Crunch' or 'Red Bull'. The purple varieties are hybrids between purple cabbage and regular green Brussels sprouts developed by a Dutch botanist in the 1940s, yielding a variety with some of the red cabbage's purple colors and greater sweetness. In the 1990s, Dutch scientist Hans van Doorn identified the chemicals that make Brussels sprouts bitter. This enabled Dutch seed companies to cross-breed archived low-bitterness varieties with modern high-yield varieties, over time producing a significant increase in the popularity of the vegetable.