The Science of Brussels Sprouts

Why do sprouts look so weird? And why do people hate them so much?
22 December 2017

Interview with 

David Hanke - University of Cambridge & Sarah Castor-Perry

brussels sprouts

brussels sprouts


Moving on we get to the most controversial part of Christmas dinner: Brussels sprouts. But what are brussel sprouts and why do they look so weird? Georgia Mills put it to Plant Development expert, David Hanke.  

David - Well, brussel sprouts are the dormant swollen side buds along the stem of a relative of the cabbage. But, because they’re an over-wintering structure, they’re completely dormant, and that’s neat because it means that after we pick them they stay in a kind of pristine state because they’re not metabolising, they’re not going to go downhill. They were achieved as a result of selective breeding. Normally, of course, the wild cabbage over-winters because the terminal shoot comes into this big bud - a single bud, like our cultivated cabbages are larger versions of these, but, if you’re a breeder you can transform the plant. It just so happens that the brussel sprout has this long stem and then that astonishing helical arrangement of little green buttons with the spiralling leaf stalks stuck out there makes the field of brussel sprouts look like a kind of alien landscape. They are remarkable things.

Chris - You say that they were a product of selective breeding so when do you think was the first Christmas at which we saw brussel sprouts on the table then?

Hugh - Oh my goodness! There is a kind of rumour that the Romans had them. They’re first written down in a 16th century document, and then there’s records of them for sale in early medieval times in the low countries around Brussels, in fact, which is why we call them brussel sprouts. They go back a long way.

Georgia - I personally am not a fan of the brussel sprout but I know many people are. Why are they so controversial, why do so many people love them and so many people hate them?

Hugh - The difference has to do with your genes, with your genetic makeup. Sprouts contain an isothiocyanate - these are very nasty chemicals which is called sulphoraphane. And sulphoraphane has got an extra sulphur in it and it’s the reason why, if you overcook sprouts, you get that smell of rotten eggs because the sulphur is released in a reduced form. Some people have the gene that encodes a receptor, a sensor, that interacts with sulphoraphane and gives you the bitter taste. Other people are missing that gene and so they don’t taste it.

Georgia - I see. So whether you like sprouts or not is all in your DNA. Thanks David. 

Chris - Now as most people know, Brussel sprouts can have an unfortunate effect on the human digestive tract. Sarah Castor-Perry...

Sarah - Every Christmas one vegetable divides opinion - brussel sprouts. Some of us love them, some of us hate them but eating them can have some embarrassing consequences. But what actually is flatulence?

Well, some of it is caused by swallowed air. Some of this swallowed air comes back up again as a burp, but any that doesn’t can pass through the digestive tract and emerge again at the other end in the usual tuneful fashion.

But most of the gas that ends up as flatulence is actually formed fresh inside our intestines by the colonies of bacteria that live there as a normal part of their microbial metabolism. They pump out variable volumes of nitrogen, methane, carbon dioxide, and hydrogen. These are, thankfully, all odourless and largely harmless gases, although hydrogen and methane are quite combustible as some party pranksters armed with a lighter and a convenient episode of wind will attest to.

Unfortunately, some of the other gaseous products of bacterial digestion are much less easy on the olfactory system. Hydrogen sulphide reeks of rotten eggs and methyl mercaptan, which stinks of moldy cabbages is the same stuff that’s deployed by skunks as part of their repellent arsenal.

But why are some foods far more fartogenic than others? As a rule, foods that trigger flatulence are those that can’t be completely broken down in the stomach or small intestine. This means that partially digested foodstuffs then make their way into the colon where they can feed a large bowel bacterial banquet with predictable odiferous effects, and this is where the sprouts come in.

Sprouts, along with onions, beans and dairy products are hard to digest in the stomach and small intestine because our bodies can’t produce the enzymes needed to break down some of the chemical components they contain. The main culprit in sprouts is a complex sugar called raffinose, which is also found in cabbage, broccoli, cauliflower, kale and, in fact, all members of the brassica family of vegetables. Raffinose is broken down by an enzyme call alpha-galactosidase, but as we don’t make this enzyme in our guts, the raffinose together with other complex sugars arrive in the large intestine. Some of the bowel bacteria are armed with the necessary chemical knives and forks to break these sugars down but, in the process, they churn out hydrogen, methane and carbon dioxide.

So that’s why sprouts make you produce gas, but why the particularly pungent smell that you often get as an unwelcome addition to the Christmas table?  Well, one thing that all brassicas also have in common is that they’re full of sulphur-containing defence chemicals. They’re there to deter animals from feeding on their leaves and also what cause the strong and sometimes bitter flavours of these vegetables that put some people of eating them altogether. And it’s these sulphur-containing chemicals that the bacteria turn into hydrogen sulphide and methalmacaptan. Added in small amounts to the bulky, sugar fueled fart gas already being produced, these gases are offenders that can clear a room in seconds.

But is there a way of solving the problem? Unfortunately, some people are just more prone to producing their own airborne toxic events owing to the unique community of bacteria with which they’re colonised. Some guts are just more fart friendly, you could say.

And, if this is the case for you, then perhaps Buck Weimer of Pueblo, Colorado can help… He won an Ig Nobel prize in 2001 for his invention of underwear that contains a removable filter to help soak up any nasty niffs. For those who don’t like the sound of charcoal stuffed pants there are some enzyme-containing dietary supplements that can help break down the complex sugars reducing the personal fart risk. But fart experts agree, there is no surefire way to prevent those brussel sprouts sounding a bum note on Boxing Day.

Merry Christmas.


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