Broccoli Microgreens: Glucoraphanin, Myrosinase, and Why Form Matters

Sulforaphane appears frequently in articles about broccoli and cruciferous vegetables. But there's a question that is almost never answered well: why do sprouts concentrate so much more than the adult plant, and what needs to happen for sulforaphane to form and be absorbed by the body?

The following explains the complete mechanism: glucoraphanin, myrosinase, conversion, cooking, and bioavailability. With data from studies.

What is glucoraphanin and why it exists in the plant

Glucoraphanin is a glucosinolate: a sulfur compound that broccoli accumulates in its cell vacuoles. By itself, it has no biological activity. Its function is defensive: when an insect pierces the plant tissue, glucoraphanin comes into contact with myrosinase—an enzyme stored in different cells—and the reaction produces isothiocyanates, including sulforaphane. These are irritating compounds that deter herbivores.

Something similar happens in the kitchen. When cutting or chewing, the tissue breaks and the reaction begins. Without that physical damage, there is no sulforaphane.

Glucoraphanin is present in all cruciferous vegetables—broccoli, kale, collard greens, radish, watercress—but in very different concentrations depending on the species, variety, and stage of development.

Why sprouts concentrate much more glucoraphanin than mature broccoli

In 1997, a team from the Johns Hopkins University Brassica chemoprotection laboratory measured the glucoraphanin content in 3-day-old broccoli sprouts compared to mature broccoli. Sprouts contained between 10 and 100 times more glucoraphanin than the adult plant of the same variety (Fahey et al., PNAS 1997, DOI: 10.1073/pnas.94.19.10367).

A nuance that almost no article mentions: this range comes from cultivars selected in the laboratory, not from sprouts bought in the supermarket. The actual concentration depends on the variety, germination time, growing conditions, and post-harvest storage. That said, broccoli sprouts are still one of the most concentrated sources of glucoraphanin in regular diet.

The biological explanation is simple: in the first days of germination, glucosinolates have not yet been redistributed to leaves and flowers. They are concentrated in the cotyledons, which are what are consumed.

→ Glucoraphanin variability in adult broccoli: Glucoraphanin in broccoli: variety, cultivation, and actual concentration

How sulforaphane is formed: glucoraphanin + myrosinase

Glucoraphanin and myrosinase coexist in the same cell but in separate compartments. When plant tissue is damaged—by cutting, chewing, or crushing—the compartments break, the two compounds come into contact, and myrosinase hydrolyzes glucoraphanin to produce active sulforaphane. The reaction occurs within minutes.

Myrosinase is thermolabile: it is inactivated above approximately 70°C. Boiling broccoli for several minutes practically destroys all enzymatic activity. What remains on the plate is intact glucoraphanin, with no possibility of plant conversion.

When myrosinase is not available, partial conversion can occur in the colon, through the gut microbiota. The problem is that efficiency varies greatly among people: some individuals convert more than 40% of ingested glucoraphanin; others, less than 10%.

Cooking and bioavailability: the data

Vermeulen et al. (2008) measured the bioavailability of sulforaphane in 8 men who consumed 200 g of raw or cooked broccoli in a randomized crossover trial. With raw broccoli, 37% of glucoraphanin was recovered as sulforaphane metabolites in blood and urine. With cooked broccoli, only 3.4% (DOI: 10.1021/jf801989e). The plasma peak was reached at 6 hours with cooked broccoli versus 1.6 hours with raw.

Cooking does not destroy already formed sulforaphane—in fact, sulforaphane is more thermostable than myrosinase. What happens is that heat inactivates the enzyme before it can act, leaving glucoraphanin unconverted.

An experimentally supported strategy: cut the broccoli and let it rest for 40 minutes before cooking. Myrosinase acts during this period and converts part of the glucoraphanin before heat inactivates it. Adding ground mustard seeds to cooked broccoli provides exogenous myrosinase that can help convert the glucoraphanin in the already cooked broccoli, although the amount of enzyme varies depending on the type of mustard used.

Glucoraphanin supplements: the detail that changes everything

Many "sulforaphane" supplements sell glucoraphanin without myrosinase. The relevant question is how much sulforaphane actually circulates in our body.

Clarke et al. (2011) compared bioavailability in 12 people who consumed 40 g of fresh sprouts versus the same people taking 6 capsules of a broccoli supplement without myrosinase activity. Fresh sprouts produced significantly higher plasma levels; the peak and urinary excretion were also delayed with the supplement (DOI: 10.1016/j.phrs.2011.07.005).

Fahey et al. (2015) confirmed that when endogenous myrosinase is active—as in sprouts or seeds consumed directly—sulforaphane is 3 to 4 times more bioavailable than when glucoraphanin is administered without the enzyme (DOI: 10.1371/journal.pone.0140963).

What this implies: if a supplement does not explicitly state that it contains active myrosinase, most of the conversion depends on the colonic microbiota—variable, slow, and inefficient compared to the plant enzymatic pathway.

→ How food form changes absorption: Nutrient bioavailability: why you absorb 5% of some supplements and almost 100% of others

How to consume broccoli microgreens or sprouts

The way with the highest documented bioavailability is to consume them raw and well-chewed. Chewing is the physical damage that activates the reaction.

• Raw in salads or on any cold dish. The taste is spicy and slightly bitter—similar to radish—but it integrates well when mixed with other ingredients.

• Blended in a cold smoothie. The blender performs the necessary physical damage. Without adding heat.

• Alongside cooked broccoli. A small amount of raw sprouts on the dish provides active myrosinase that can help convert the glucoraphanin in already cooked broccoli.

• Broccoli microgreens. More tender and easier to integrate than 3-day-old sprouts, with a glucoraphanin concentration similar to that of sprouts.

For those looking for a concentrated and stable source without relying on daily preparation, low-temperature freeze-dried microgreens retain both glucoraphanin and myrosinase in the original plant matrix. Freeze-drying removes water without destructive heat, so the enzymatic conversion capacity remains intact. This is what differentiates SYNERGIC from an industrialized extract: the precursor and the enzyme coexist in their biological context, available to be activated upon contact with saliva and the digestive environment.

Conclusion

Microgreens concentrate more glucoraphanin than adult broccoli because in the early stages of germination, glucosinolates have not yet been redistributed throughout the plant. But the amount of glucoraphanin is only one part of the equation. Without active myrosinase—which heat destroys and microbiota only partially and unpredictably replaces—glucoraphanin is not efficiently converted to sulforaphane.

How the source is prepared or consumed largely determines how much sulforaphane actually circulates. The bioavailability data are clear enough to make it worth considering before choosing how to obtain glucoraphanin: 37% with a source with active myrosinase versus 3.4% boiled, or 3–4 times less with a supplement without the enzyme.

→ Why cruciferous vegetables have their own biochemistry: Why cruciferous vegetables are different from other vegetables

→ What exactly is glucoraphanin: What is glucoraphanin? The precursor to sulforaphane explained
→ Glucoraphanin vs sulforaphane: the differences that matter: Glucoraphanin vs sulforaphane: why they are not the same