Myrosinase is an enzyme found in cruciferous vegetables like broccoli, cabbage, mustard, and kale. Its job is to break down compounds called glucosinolates into smaller, biologically active molecules, including isothiocyanates, the pungent substances responsible for the sharp bite of mustard, horseradish, and raw broccoli. This enzyme is the reason these vegetables have attracted so much attention for their potential health benefits, and understanding how it works can change the way you prepare food.
How Myrosinase Works
Myrosinase is classified as a beta-thioglucosidase, which means it breaks a specific type of sugar bond. Cruciferous plants produce a family of sulfur-containing compounds called glucosinolates. More than 100 different glucosinolates have been identified across various species. On their own, these compounds are relatively inert. They sit quietly in plant cells and don’t do much.
When you bite into a raw radish, slice a cabbage, or chew a broccoli floret, you’re rupturing plant cells and allowing myrosinase to physically contact those stored glucosinolates for the first time. The enzyme strips off a glucose molecule from the glucosinolate, creating an unstable intermediate that quickly rearranges into one of several breakdown products: isothiocyanates, nitriles, or epithionitriles. Which product forms depends on conditions like pH and the presence of certain helper proteins in the plant. At a mildly acidic to neutral pH (around 5 to 7), isothiocyanates are favored. At very low pH (below 3), nitrile formation dominates instead.
The “Mustard Oil Bomb” Defense
This enzyme exists for the plant’s benefit, not ours. Biologists refer to the glucosinolate-myrosinase system as the “mustard oil bomb,” a two-component chemical weapon that deters herbivores and pathogens. In intact plant tissue, myrosinase is stored in specialized cells called myrosin cells, physically separated from the glucosinolates. When an insect chews through a leaf, the compartments rupture, the enzyme meets its substrates, and toxic breakdown products flood the damaged area.
The strategy is effective against many generalist insects. Research has shown that plants with higher myrosinase activity sustain less damage from certain pests. Some specialist insects, however, have evolved ways to disarm the bomb, either by neutralizing the enzyme or by diverting the reaction away from toxic products.
Why Sulforaphane Gets All the Attention
Of the many isothiocyanates myrosinase can produce, sulforaphane is the most studied. It forms when myrosinase acts on glucoraphanin, a glucosinolate concentrated in broccoli and broccoli sprouts. Sulforaphane has been investigated for its role in supporting the body’s detoxification pathways, reducing inflammation, and protecting cells from oxidative damage.
The conversion is not perfectly efficient. In controlled laboratory conditions using myrosinase extracted from Chinese flowering cabbage, researchers measured a molar conversion rate of about 48% from glucoraphanin to sulforaphane after 30 minutes of reaction time. After that point, the reaction reached equilibrium and sulforaphane concentrations plateaued. This means that even under ideal conditions, not all of the raw material becomes sulforaphane.
Heat Destroys It Quickly
Myrosinase is a protein, and like most proteins, it unfolds and loses function when heated. Thermal inactivation begins at temperatures as low as 30°C (86°F) and progresses through 60°C (140°F). Boiling, steaming for extended periods, or microwaving broccoli until soft will destroy most or all of the enzyme. Once that happens, the glucosinolates in cooked vegetables pass through your digestive system largely intact, and far less sulforaphane is produced.
This is the central tension of cooking cruciferous vegetables: the method that makes them palatable often eliminates the enzyme that makes them most nutritionally interesting.
Practical Ways to Preserve the Enzyme
There are two well-supported strategies for getting around the heat problem.
The first is the “chop and wait” method. If you chop, crush, or blend your broccoli, kale, Brussels sprouts, or cauliflower and then let it sit for about 30 minutes before cooking, the myrosinase has time to do its work while still active. The sulforaphane and other isothiocyanates that form during that waiting period are heat-stable, so subsequent cooking won’t destroy them even though the enzyme itself is gone.
The second approach is adding an outside source of myrosinase after cooking. Mustard seeds contain their own active myrosinase. A randomized crossover study with 12 healthy adults found that adding just 1 gram of powdered brown mustard to 200 grams of cooked broccoli increased urinary sulforaphane metabolites by more than four times compared to eating the cooked broccoli alone. Other raw cruciferous foods, like a small amount of raw daikon, arugula, or watercress added to a cooked dish, can serve the same purpose.
Your Gut Bacteria Can Step In
Even without any active myrosinase, eating cooked cruciferous vegetables still produces some isothiocyanates. The credit goes to your gut microbiome. Several species of bacteria living in the human intestine can perform myrosinase-like reactions, breaking down glucosinolates that arrive intact in the colon.
Bacteria from at least four major groups (Firmicutes, Bacteroidetes, Actinomycetes, and Proteobacteria) have demonstrated this ability. Specific strains include Lactobacillus agilis, Bacteroides thetaiotaomicron, and several Bifidobacterium species. The conversion rate from gut bacteria is considerably lower and more variable than what plant myrosinase achieves, which partly explains why studies show wide person-to-person differences in how much benefit people get from cooked cruciferous vegetables. Your gut flora composition matters.
Myrosinase in Supplements
Because sulforaphane is unstable and difficult to put in a pill, many supplements take a different approach: they package glucoraphanin alongside active myrosinase so the conversion happens after you swallow the tablet. One well-studied commercial formulation using this strategy achieved a median bioavailability of about 20%, with much less variation between individuals than glucoraphanin-only supplements that rely entirely on gut bacteria.
Clinical trials have used these combined supplements at doses ranging from roughly 70 to 411 micromoles of glucoraphanin per day, with study durations spanning single doses to year-long daily use. Adding fresh broccoli sprouts as a myrosinase source alongside glucoraphanin-rich preparations has also been shown to synergistically boost absorption. The key variable in all of these approaches is the same: whether active myrosinase is present to drive the conversion.