The Brassicas

TL;DR. Broccoli, cauliflower, cabbage, Brussels sprouts, kale, and kohlrabi are all the same species, Brassica oleracea, selected in six different directions from one wild Mediterranean cabbage. Their defining chemistry is the glucosinolate system: a stored compound and a separate enzyme that meet only when the tissue is damaged, producing sulforaphane and its relatives. That system has the best mechanistic and epidemiological case of any vegetable-derived compound class, and it is destroyed by boiling. Chop and wait, steam briefly, or add something raw and mustardy.

1. What they are

One of the most striking cases of artificial selection in any crop. All of these are the same species, Brassica oleracea, differing only in which organ was selected for enlargement:

They interbreed freely, which is why home seed savers grow them far apart.

Relatives in other Brassica species: turnip and bok choy (B. rapa), swede/rutabaga (B. napus, a cabbage × turnip hybrid), mustard greens (B. juncea), plus radish, horseradish, wasabi, rocket, watercress, and cress in the wider Brassicaceae family.

2. Where they come from

Wild B. oleracea grows on the limestone sea cliffs of the northern Mediterranean and Atlantic coasts, a tough, bitter, leafy perennial. Selection began with the Greeks and Romans, who had recognisable kales and cabbages. Cauliflower emerged in the Near East and reached Europe in the sixteenth century; Brussels sprouts were selected in the Low Countries around the same period; broccoli was developed in Italy (the name is from broccolo, cabbage sprout) and only reached mass popularity in the English-speaking world in the twentieth century.

Production: cabbages and other brassicas around 71 million tonnes; cauliflower and broccoli around 26 million tonnes. China and India dominate both, followed by the US, Spain, Mexico, and Italy. Spain is the main winter supplier to northern Europe.

3. How they are grown

Cool-season crops. Optimum 15 to 20 °C. Most tolerate frost, and several improve with it: kale, Brussels sprouts, and parsnips genuinely taste sweeter after a frost, because the plant converts starch to sugars as an antifreeze. That is a real, measurable physiological response and not folklore.

Cauliflower is the fussy one: it needs consistent conditions and forms a poor, loose, "riced" or "buttoned" head if stressed by heat, drought, or nutrient shortage.

Soil. They are heavy feeders, wanting rich, firm, well-limed soil at pH 6.5 to 7.5. Lime serves two purposes: brassicas want the calcium and higher pH, and clubroot, the defining brassica disease, is suppressed by alkalinity.

Clubroot (Plasmodiophora brassicae) is a soil-borne organism that causes swollen, distorted roots and stunted plants. Its resting spores survive in soil for up to twenty years, which makes it effectively permanent once established and is the reason brassica rotations are so long (4 to 7 years minimum). It is the single biggest constraint on brassica growing worldwide.

Pests. Cabbage white butterfly caterpillars, cabbage root fly, flea beetle, aphids, and pigeons. Brassicas are among the most heavily protected vegetable crops, usually under fine mesh netting rather than sprays in smaller-scale production.

Blanching cauliflower: traditional varieties needed their outer leaves tied over the head to keep it white, because light turns it cream or purple. Modern "self-blanching" varieties have leaves that curl over naturally.

Harvest and cooling. Broccoli in particular respires extremely fast and yellows within days at room temperature. It is vacuum- or ice-cooled immediately and held near 0 °C.

4. What is inside them

Per 100 g raw:

Notes: Brussels sprouts are the most nutrient-dense of the group and the highest in glucosinolates. Broccoli, kale, and sprouts all deliver essentially a full day's vitamin C per 100 g raw. Calcium from brassicas is unusually well absorbed (around 50 percent for kale and bok choy) because they are low in oxalate (Chapter 38).

Broccoli sprouts are the extreme: they contain 10 to 100 times the glucoraphanin of mature broccoli, which is why almost all sulforaphane research uses them.

5. The glucosinolate system, properly explained

In short: A stored compound and a separate enzyme, kept apart until the tissue is damaged, produce a reactive compound that activates your own defence genes.

Brassicas store glucosinolates (glucoraphanin in broccoli, sinigrin in mustard and sprouts, glucobrassicin in cabbage) in the cell vacuole. In a separate compartment they store the enzyme myrosinase. Both are inert alone.

When an insect chews the leaf, the compartments rupture, myrosinase meets glucosinolate, and the product is an isothiocyanate: sulforaphane from glucoraphanin, allyl isothiocyanate (the heat in mustard, horseradish, and wasabi) from sinigrin. These are reactive, pungent, and toxic to insects. It is a two-component chemical weapon armed by damage.

In your body, sulforaphane is one of the most potent known dietary activators of Nrf2, a transcription factor that switches on a battery of your own antioxidant and detoxification enzymes: glutathione S-transferases, NAD(P)H quinone oxidoreductase 1, heme oxygenase-1. The result is a strengthened endogenous defence system that persists long after the compound is cleared (Chapter 17). You are not getting antioxidants from broccoli so much as being provoked into making your own.

The human evidence is better than for most food compounds:

• A randomised trial in Qidong, China (Egner and colleagues), gave broccoli sprout beverage to people in a highly polluted area and measured accelerated urinary excretion of benzene and acrolein metabolites, a direct demonstration of enhanced detoxification in humans.
• Consistent observational associations between cruciferous vegetable intake and lower risk of several cancers, particularly colorectal, lung, and prostate, though the associations are modest and confounded like all such data.
• Small trials on markers in autism, schizophrenia, and asthma, all preliminary.

Why cooking method decides the outcome

Myrosinase is a protein and is destroyed above about 60 °C. Boiled broccoli therefore delivers its glucosinolates unconverted to the colon, where gut bacteria perform the conversion at perhaps 10 to 20 percent efficiency. Boiling also leaches 20 to 60 percent of the glucosinolates into the water.

Measured retention of the sulforaphane-producing capacity, roughly:

The three practical workarounds:

1. Chop and wait. Cut the vegetable and leave it 40 minutes before cooking. Myrosinase does its work first, and sulforaphane itself is considerably more heat-stable than the enzyme.
2. Steam briefly, 3 to 4 minutes, which is enough to soften and not enough to destroy the enzyme.
3. Add a raw myrosinase source to cooked brassicas: a pinch of mustard powder, grated horseradish or wasabi, rocket, watercress, or raw radish. This has been demonstrated to restore much of the conversion in cooked material.

Frozen broccoli is blanched before freezing, which destroys myrosinase, so it produces very little sulforaphane on its own. Adding mustard powder to frozen broccoli is the direct fix and has been shown to work.

6. The smell, the bitterness, and the genetics

The smell of overcooked brassicas is hydrogen sulphide and other sulphur compounds released as glucosinolates and S-methyl cysteine sulphoxide break down. The longer they cook, the more is released, roughly doubling between five and seven minutes of boiling. Overcooking is the entire cause, and it is why a generation of British children learned to hate cabbage.

The bitterness is partly genetic. The TAS2R38 gene encodes a bitter taste receptor, and common variants make people supertasters, tasters, or non-tasters of the compounds PROP and PTC, which are chemically related to brassica glucosinolates. Roughly a quarter of people are supertasters, who genuinely experience brassicas as far more bitter than others do. Telling such a person they are being fussy is inaccurate.

Brussels sprouts genuinely used to be worse. In the 1990s, Dutch researchers identified the specific glucosinolates responsible for the harshest bitterness (sinigrin and progoitrin) and seed companies bred varieties with lower levels. Modern sprouts are measurably less bitter than those of the 1980s. Roasting rather than boiling did the rest. The rehabilitation of the Brussels sprout is a genuine plant-breeding success story.

Suppressing bitterness works through three levers: fat (butter, oil, cheese, bacon), salt (which suppresses bitterness perception directly), and acid or sweetness (lemon, vinegar, balsamic, honey). Roasting adds a fourth: caramelisation and Maillard browning generate sweet and savoury compounds that mask bitterness. This is why roasted sprouts with bacon and balsamic converted so many people.

7. Who should eat more, who should be careful

Good for: almost everyone. This is the vegetable family with the strongest mechanistic case.

Be careful if:

• You have hypothyroidism or low iodine intake. Brassica breakdown products include thiocyanates, which compete with iodine for thyroid uptake. In people with adequate iodine this is not a problem at normal intakes, and cooking reduces goitrogenic activity substantially. The documented cases of brassica-induced hypothyroidism involve extreme intakes: there is a case report of a woman who developed myxoedema coma after eating 1 to 1.5 kg of raw bok choy daily for months. Normal consumption is fine (Chapter 18).
• You take warfarin. Brassicas, especially Brussels sprouts and kale, are very high in vitamin K. Consistency, not avoidance.
• You have IBS. Brassicas are high-FODMAP: broccoli stems, cauliflower, cabbage, and Brussels sprouts all contain fructans and, in cauliflower's case, mannitol. Cauliflower is one of the most reliable IBS triggers among vegetables. Broccoli florets are lower-FODMAP than the stems, which is a useful and little-known distinction.
• You are prone to bloating. Raffinose and other oligosaccharides in brassicas are fermented in the colon and produce gas. This is normal, it is the microbiome doing its job, and it reduces as the gut adapts to a higher-fibre diet over weeks.
• You are on a low-potassium diet for kidney disease.

Ages. Excellent from 6 months, cooked soft. Introduce early and repeatedly: children's aversion to bitterness is developmentally normal and repeated exposure (research suggests 8 to 15 exposures) is the evidence-based approach, not one attempt and surrender. Cook them well, add fat, and do not boil them into sulphur.

8. When and how to eat them

• Roast them. High heat, oil, salt, and space on the tray so they brown rather than steam. This is the single most effective way to make brassicas popular.
• Steam briefly rather than boil.
• Chop 40 minutes ahead when you can.
• Add mustard, horseradish, wasabi, or rocket to cooked brassicas.
• Eat the stems and leaves. Broccoli stems peeled and sliced are excellent and are routinely binned. Cauliflower leaves roast beautifully. This is a large source of avoidable food waste.
• Eat some raw: coleslaw, shaved sprouts, raw cauliflower, kohlrabi batons. Raw gives full enzyme activity.
• Ferment them. Sauerkraut and kimchi are cabbage plus salt plus time (Chapter 56). Fermentation converts glucosinolates to isothiocyanates and adds lactic acid bacteria.
• Red cabbage is an acid-base indicator. Its anthocyanins turn blue-purple in alkaline conditions and bright pink in acid, which is why red cabbage cooked without vinegar turns an unappetising blue. A splash of vinegar or apple keeps it red. This is chemistry, not cooking superstition.
• Broccoli sprouts are the concentrated option: growable at home in a jar in 4 to 5 days, and the richest practical source of glucoraphanin. Note the sprout food safety caveat in Chapter 48.

9. Choosing and storing

Broccoli: tight, dark green or purple-tinged heads with no yellowing. Yellowing means the flower buds are opening and it is past its best. Firm, not rubbery, stems. 3 to 5 days in the fridge, unwashed, loosely wrapped.

Cauliflower: creamy white and tight, with fresh green leaves attached (the leaves are the best freshness indicator). Brown spots are oxidation and can be trimmed. A week in the fridge.

Cabbage: heavy for its size, tight, glossy outer leaves. The best-keeping vegetable in this group by far: whole cabbage keeps for weeks to months in the fridge, and traditionally kept all winter in a cellar. Cut cabbage oxidises at the surface; slice off a thin layer.

Brussels sprouts: tight, firm, bright green, small (large sprouts are more bitter and coarser). On the stalk keeps considerably longer than loose. A week or two in the fridge.

Kale and greens: as in Chapter 38.

All brassicas are ethylene-sensitive: keep them away from apples and bananas, which accelerate yellowing.

10. The varieties worth knowing

Broccoli: calabrese (the standard head), purple sprouting broccoli (overwintered, harvested in early spring, more tender and arguably better flavoured), broccolini/tenderstem (a broccoli × gai lan hybrid, developed in Japan in the 1990s), gai lan (Chinese broccoli), romanesco (a fractal-patterned type between broccoli and cauliflower).

Cauliflower: white, plus orange (higher beta-carotene, a natural mutation), purple (anthocyanins), and green. Riced cauliflower is a low-carbohydrate substitute that is genuinely useful and nutritionally not equivalent to rice: far fewer calories, far less starch, and far less satiety.

Cabbage: green/white (dense, for slaw and sauerkraut), red (anthocyanins), savoy (crinkled, tender), napa/Chinese cabbage (B. rapa, milder, the kimchi cabbage), pointed/hispi.

Kale: curly, cavolo nero/lacinato/Tuscan (flatter, darker, less bitter), red Russian.

Kohlrabi: green and purple; eaten raw like an apple or cooked, and one of the more underrated vegetables in Europe.

11. Myths and confusions

Don't be confused: broccoli, cauliflower, cabbage, kale, kohlrabi, and Brussels sprouts are one species. They are as closely related as different breeds of dog. Their nutritional differences come from which organ was enlarged, not from different plants.

• "Brassicas cause thyroid problems." Only at extreme intakes or with iodine deficiency. Normal cooked consumption is fine.
• "Frozen broccoli is as good as fresh." For vitamins, broadly yes. For sulforaphane, no, because blanching kills the myrosinase. Add mustard powder.
• "Cauliflower rice is a healthy version of rice." It is a low-calorie vegetable dish. It is not a carbohydrate replacement with equivalent energy or satiety, and treating it as one is how people end up hungry.
• "You should not eat broccoli stems." They are the best part by fibre content.
• "Brussels sprouts taste bad." Both the vegetable and the cooking have changed. Bred lower in bitter glucosinolates in the 1990s, and roasted rather than boiled.
• "Boiling is the traditional way and therefore fine." Boiling for ten minutes destroys the compound class the whole family is interesting for, and releases the sulphur smell people associate with them.

12. The bottom line

• Six of the most familiar vegetables in Europe are a single species selected in six directions.
• Their defining chemistry is a two-part system: glucosinolate plus myrosinase, armed by damage, producing sulforaphane and relatives that activate your own detoxification and antioxidant genes.
• Myrosinase dies above 60 °C. Steam briefly, chop 40 minutes ahead, or add mustard, horseradish, or rocket to cooked brassicas. Frozen broccoli has no active enzyme.
• Brussels sprouts are the most nutrient-dense of the group; broccoli, kale, and sprouts each deliver a day's vitamin C per 100 g; brassica calcium is unusually well absorbed.
• The bitterness is partly your genes, the sulphur smell is entirely overcooking, and roasting with fat and salt solves both.
• Cauliflower and cabbage are among the most reliable IBS triggers; broccoli florets are lower in FODMAPs than the stems.

Sources and notes

Composition figures are from USDA FoodData Central; production figures are approximate FAOSTAT values for the early 2020s. Brassica oleracea domestication and the divergence into six crops follow Maggioni's and Mabry et al.'s genomic work. Clubroot persistence, lime response, and rotation requirements follow standard plant pathology and extension guidance. Frost-induced sweetening follows plant cold-acclimation physiology. Glucosinolate and myrosinase compartmentation, sulforaphane formation, and Nrf2 activation follow Fahey and Talalay's work at Johns Hopkins; broccoli sprout glucoraphanin concentration is Fahey, Zhang, and Talalay, PNAS, 1997. The Qidong broccoli sprout beverage trial is Egner et al., Cancer Prevention Research, 2014. Cooking effects on myrosinase and the chop-and-wait and added-mustard strategies follow Ghawi, Methven, and Niranjan, Food Chemistry, 2013, and Dosz and Jeffery, Food and Function, 2013. TAS2R38 bitter taste variation follows Kim et al., Science, 2003. The reduction of bitter glucosinolates in Brussels sprout breeding follows Dutch seed company and Wageningen accounts from the 1990s. FODMAP content follows the Monash University database.

Open questions. Whether the observational cruciferous-vegetable cancer associations reflect sulforaphane or something else about people who eat brassicas is not resolved, and sulforaphane supplement trials are small and short.

👉 Next: the alliums, where cutting the vegetable is what creates the flavour.