People buy and grow broccoli microgreens for sulforaphane. The plant does not actually contain ready-made sulforaphane. It holds glucoraphanin and an enzyme called myrosinase, and the two meet when the greens are chewed or chopped. How much sulforaphane you end up with depends on three things: the day the greens were cut, how much light they saw, and what happened to them after harvest.
The younger the plant, the more glucoraphanin
In 1997, researchers at Johns Hopkins measured 10 to 100 times more glucoraphanin in 3-day-old sprouts of some broccoli varieties than in the mature plant. That finding started the sprout craze, and later the one for broccoli microgreens. We told the story in our piece on broccoli sprouts and Paul Talalay.
In 2002, a Portuguese team working with the Hopkins lab tracked the compound day by day. Glucosinolates, the family glucoraphanin belongs to, peak in the dry seed. As the plant grows, their concentration falls, and younger sprouts are consistently richer than older ones. Inside the sprout, the cotyledons hold the most and the roots the least.
Day 7, day 9, or day 12
For microgreens, the question is how fast that decline runs. In 2015, a team in Portugal cut broccoli at 7, 9, and 12 days after sowing, grown at 77°F (25°C) under 16 hours of light a day. For most of the brassicas in the trial, day 7 was the richest. Broccoli grown in light was the exception: on day 9 it held more aliphatic glucosinolates, the group glucoraphanin belongs to, than on day 7. By day 12, aliphatic glucosinolates had dropped noticeably.
A Spanish group at CEBAS-CSIC compared nine brassicas in 2012 and named day 8 the best day to eat them. In a 2023 US study, broccoli microgreens were cut on day 10 and held about 13 µmol of glucoraphanin per gram, in line with the 16.6 the authors cite for sprouts.
In practice, that puts the broccoli window at days 8 to 10. It matches the harvest signs in our broccoli microgreens profile: cotyledons fully open, first true leaf not yet showing. Holding the tray to day 12 for extra weight means selling greens with a lower concentration.
One caveat. All of these numbers measure glucoraphanin per gram of greens. None of these studies measured how much a whole tray yields on different days while its weight is still climbing.
Darkness costs a quarter of the glucoraphanin
In the same Portuguese trial, some of the broccoli was grown in total darkness right up to harvest. It held 23% less glucoraphanin than greens grown in light. This is not about the blackout period in the first days after sowing: in the trial, the plants stayed dark the whole cycle. The practical point is simpler. Once broccoli microgreens are up, they belong under light, not finishing in a cupboard or under a lid.
The kind of light matters too. In a 2023 Turkish study, blue LED light raised glucoraphanin in broccoli microgreens. Our light spectrum guide covers what blue and red each do for the plant.
Another 2002 finding concerns temperature. Broccoli grown at 86°F (30°C) by day and 59°F (15°C) by night built up more glucosinolates than at 72/59°F (22/15°C) or 64/54°F (18/12°C). Heat or cold stress raised glucoraphanin as well. For a farm this is more of a curiosity than advice: a hot regime has its own cost in growth and quality.
After harvest: the fridge does no harm
Good news for anyone who sells them. In the 2023 study, broccoli microgreens spent six days at 39°F (4°C), and neither glucoraphanin nor sulforaphane changed in any meaningful way. A 16 g (0.56 oz) serving of those greens gave participants 100 µmol of sulforaphane, the same amount broccoli sprouts supplied in earlier trials. How much microgreens to eat in a day overall is worked out in our guide to a daily serving.
What happens in the kitchen
There is a catch here. When broccoli is chopped at room temperature, the main product is sulforaphane nitrile rather than sulforaphane. The nitrile has no biological activity. A protein called ESP, which works alongside myrosinase, is responsible.
ESP is sensitive to heat. In 2004, researchers at the University of Illinois heated broccoli sprouts to 140°F (60°C) before chopping them, and got more sulforaphane and less nitrile. In 2022, a team in Chile worked out the exact settings for 11-day-old sprouts, which is microgreen age. The best result came from 4.8 minutes in water at 142°F (61°C): 3.3 times more sulforaphane than in fresh greens.
More heat does not mean more benefit. The Chilean authors cite data showing that blanching fresh broccoli at 185°F (85°C) for 3 minutes cut isothiocyanates by 43%. In the 2004 study, florets heated to 158°F (70°C) and above also yielded less, while sprouts did not, so the upper limit for microgreens is less clear. And the Chilean greens were heated in vacuum bags in a water bath with precise temperature control. Without a thermometer, you will not reproduce that result at home.
If you eat the greens raw, chew them well. Our piece on sulforaphane and the broccoli “magic pill” explains why chewing beats powdered capsules.
When to harvest broccoli microgreens: the takeaways
- Glucoraphanin per gram peaks in the seed and falls as the plant grows.
- For broccoli grown in light, day 9 beat day 7 in the 2015 trial, and levels were lower by day 12. The practical window is days 8 to 10.
- Total darkness up to harvest cost 23% of the glucoraphanin. Once up, the greens belong under light.
- Six days in the fridge at 39°F (4°C) did not change the content.
- Heating to 142°F (61°C) for 4.8 minutes gave 3.3 times more sulforaphane, but that is hard to hit at home without a thermometer.
