Broccoli is a nutritious green vegetable that is a staple in many healthy diets. However, when cooking broccoli, especially by steaming or boiling, it can give off a pungent, almost unpleasant smell. So what causes broccoli to smell when cooked?
The main culprits behind broccoli’s distinct smell are sulfur-containing compounds. When broccoli is cut or chewed, an enzyme called myrosinase is released which acts on glucosinolates in the broccoli to form various breakdown products like sulfur-containing isothiocyanates and thiocyanates. Isothiocyanates like sulforaphane are responsible for many of broccoli’s health benefits but also contribute to its pungent aroma.
Cooking broccoli breaks down sulfur compounds like glucosinolates into various volatile sulfur compounds that are easily vaporized and detected by our sense of smell. The intensity of the smell depends on factors like cooking method, cooking time, and broccoli freshness. Overcooking broccoli can greatly increase smelly sulfur compound production. The stench is exacerbated by broccoli’s shape and structure which allows escape of gaseous byproducts during cooking.
Sulfur Compounds in Broccoli
The main sulfur compounds responsible for broccoli’s smell are:
- Glucosinolates – these sulfur-containing compounds are found in broccoli and other cruciferous vegetables. They impart bitterness and aroma.
- Isothiocyanates – formed when glucosinolates in broccoli are broken down by the enzyme myrosinase. Isothiocyanates like sulforaphane give broccoli its health benefits.
- Dimethyl trisulfide and dimethyl sulphide – smelly volatile sulfur compounds produced when glucosinolates break down during cooking.
- Thiocyanates – pungent byproducts formed along with isothiocyanates when glucosinolates degrade during cutting or cooking of broccoli.
Glucosinolates like glucoraphanin, glucoiberin, and glucobrassicin are the main sulfur containing compounds found in broccoli. The glucoraphanin in broccoli is broken down into the isothiocyanate sulforaphane which gives broccoli its bitter flavor and many health benefits as a cancer-fighting antioxidant. However, glucosinolate degradation also produces smelly volatile sulfur compounds like dimethyl trisulfide, dimethyl sulphide, and thiocyanates which are vaporized during cooking and give broccoli its stench.
Why Cutting/Chewing Broccoli Causes Smell
Cutting or chewing broccoli causes smell by triggering the release of the enzyme myrosinase which acts on glucosinolates:
- In intact broccoli, the myrosinase enzyme is physically separated from glucosinolates like glucoraphanin.
- Cutting/chewing broccoli causes cell damage which allows mixing of myrosinase and glucosinolates.
- Myrosinase reacts with glucosinolates, converting them into various breakdown products like smelly isothiocyanates and volatile sulfur compounds.
- This produces the characteristic pungent smell of cut/chewed broccoli.
So in summary, cutting or chewing broccoli releases myrosinase to act on glucosinolates, forming smelly sulfur compounds detected by our nose. Leaving broccoli intact prevents smell by keeping myrosinase separate from glucosinolates.
Why Cooking Broccoli Causes Smell
Cooking broccoli produces smell for several reasons:
- Heat breaks down glucosinolates into volatile sulfur compounds like dimethyl sulfide and dimethyl trisulfide.
- Cooking denatures odor-reducing enzymes like epithiospecifier protein (ESP) which normally promote formation of less smelly byproducts.
- Longer cooking times increase production of smelly compounds.
- Boiling/steaming releases volatile sulfur gases into the air.
- Overcooking amplifies the smell by liberating more sulfur compounds.
In particular, steaming and boiling broccoli allows direct escape of volatile sulfur gases like dimethyl disulfide into the air we breathe. This makes the smell very apparent. Methods like microwaving and stir-frying minimize smell by cooking broccoli faster and destroying fewer odor-reducing compounds.
Factors Affecting Broccoli Smell
Factors that influence broccoli smell during cooking include:
| Factor | Effect on Smell |
|---|---|
| Cooking time | Longer cooking increases smelly sulfur compound production |
| Cooking method | Boiling/steaming causes more smell by releasing gases. Microwaving reduces smell. |
| Overcooking | Liberates more smelly compounds, increasing odor intensity |
| Broccoli freshness | Older broccoli produces more sulfur smell during cooking |
| Cut vs. intact | Cut broccoli smells more from cell damage-induced enzymatic reactions |
In general, longer cooking times, cooking methods that leach gases, overcooking, and using older broccoli can all increase the intensity of broccoli smell.
Exact Chemical Reactions Behind Broccoli Smell
The specific chemical reactions behind broccoli smell during cooking are:
- Cutting/chewing broccoli ruptures cells, allowing myrosinase enzyme to act on glucosinolates like glucoraphanin.
- Myrosinase converts glucoraphanin into sulforaphane isothiocyanate, glucose, and smelly thiocyanate.
- Cooking denatures epithiospecifier protein (ESP), reducing formation of less smelly epithionitriles.
- Heating degrades glucosinolates into dimethyl sulfide, dimethyl trisulfide and other volatile sulfur compounds.
- Isothiocyanates degrade forming more thiocyanates and nitriles.
- Volatile sulfur gases like dimethyl disulfide escape easily into air during steaming/boiling.
In summary, the myrosinase-mediated breakdown of glucosinolates into isothiocyanates, thiocyanates, and volatile sulfur compounds causes the characteristic smell of cooked broccoli. Cooking conditions promote additional glucosinolate breakdown and leaching of odor compounds.
Health Benefits from Smelly Sulfur Compounds
While being responsible for the unpleasant smell of cooked broccoli, many of the volatile sulfur compounds produced contribute to broccoli’s health benefits:
- Sulforaphane – this isothiocyanate has anticancer and antioxidant effects but also causes broccoli smell.
- Dimethyl sulfide – this smelly compound induces beneficial phase II enzymes like glutathione S-transferase.
- Thiocyanates – these sulfur breakdown products of glucosinolates have antioxidant, anticancer, and anti-inflammatory activities.
So the same sulfur-containing compounds that give broccoli its odor during cooking are also responsible for its health-promoting effects! The smell is an indicator that beneficial sulfur compounds are being produced.
Ways to Reduce Broccoli Smell
There are a few methods that can help decrease or mask the smell of cooked broccoli:
- Steam for short time periods to avoid overcooking.
- Microwave instead of boiling/steaming to minimize leached gases.
- Cook broccoli intact instead of pre-cutting before cooking.
- Add ingredients like lemon, spices, cheese that overcome broccoli smell.
- Increase ventilation over the stove to disperse volatile sulfur gases.
- Consume raw broccoli or lightly steamed to retain more ESP enzyme.
Although the sulfur smell is linked to health benefits, these cooking strategies can help reduce the odor for those who find it too unpleasant.
Conclusion
The distinctive and often unpleasant smell of cooked broccoli is caused mainly by the production of volatile sulfur compounds like dimethyl sulfide, dimethyl trisulfide and thiocyanates. These compounds form from the degradation of sulfur-containing glucosinolates when broccoli is cut or cooked. Heat breaks down glucosinolates while chewing/cutting allows the myrosinase enzyme to act on glucosinolates, forming smelly isothiocyanates and volatile byproducts. Although undesirable, the smell indicates formation of health-promoting sulfur compounds. Using shorter cooking times, different methods like microwaving, or masking ingredients can help reduce broccoli’s odor. So while it may not smell the best, remember that the sulfur stench is a small price to pay for broccoli’s numerous nutritional benefits.