Brussel Sprouts – On bitterness & Glucosinolates

For as long as I can remember Brussel sprouts have been one of my favorite foods to eat. Up to this day that is still the case, I still enjoy eating those mini cabbages, whether it’s boiled, grilled or sauteed. That said, a lot of people, children especially, don’t like them at all. Their bitterness and characteristic flavours set some people off quite intensely.

Those flavours actually are a group of molecules called glucosinolates, so we’ll be taking a closer look at them. Is there a way to bring down that flavour (or increase it of course, if you’re more like me)?

What are glucosinolates?

Glucosinolates are a group of molecules which all have a very similar chemical structure, but vary in our side-group of this molecule (see below, image from Wikipedia). It’s the R-group that can be exchanged for a variety of different groups.

glucosinolate-skeletal, from Wikipedia
The basic structure of glucosinolates. Notice that they contain a glucose group (on the left) which is attached to the rest of the molecule through a sulfide (S) bond and contains nitrogen (N) as well. The presence of both nitrogen and sulfide makes it quite unique.

Sinigrin and other glucosinolates

In Brussel Sprouts one of the most common glucosinolates is sinigrin (see image below, from Wikipedia). You will again recognize the glucose group and the S- and N- atoms. For a lot of these glucosinolates it isn’t very clear whether and how they contribute to flavour development. However, for sinigrin (and progoitrin) it has been shown that they impact the bitterness of Brussel sprouts.

sinigrin

There are a lot more glucosinolates. Broccoli for instance seems to contain mostly glucoraphanin. Progoitrin is another example and can be found in kale and cauliflower. Most Brassica vegetables contain several different glucosinolates at a time though.

The defense system of Brussel sprouts

If you have a close look at brussel sprouts, you will see that they look like tiny cabbages! And that’s correct. Brussel sprouts are actually part of the cabbage family, the same family to which kale, broccoli and cauliflower belong; the Brassica oleracea.

Like any other plant, these Brassicas have developed a defense system to help them fight any damage or attack by the outside world for instance by pests or plant eating animals. In the case of the Brassicas they use glucosinolates as their defense system. The younger the plant, the more active its defense system. This is why the center of a Brussel sprout can be quite a bit more pungent that the outer leaves, this is the actively growing section.

Whenever a brussel sprout (or other Brassica) is damaged, the defense system will set to work. An enzyme called myrosinase will come into contact with glucosinolates and catalyze a hydrolysis reaction. It will split the glucose molecule from the rest of the structure.

Which molecules form exactly will depend on the overall structure of the glucosinolate that reacted. In the case of sinigrin, in our Brussel sprouts, allyl isothiocyanate is formed. This gives vegetables a ‘spicy’ bite. Isothiocyanates are also called mustard oils for their pungent flavor. These components are toxic for a lot of plant eating animals and insects and thus it protects the plant itself.

How best to prepare Brussel sprouts (and other Brassicas)

If you want to get rid of these glucosinolates and the components that form once the enzyme comes in to play, it is best to cook them in plenty of water, or to soak the freshly cut cabbage or sprout in some water. However, since glucosinolates might also have health benefits, that takes away that advantage.

Quickly wokking or sauteing them preserves more glucosinolates. If you cook them quickly at a high heat you might prevent them from transforming into those more pungent isothiocyanates since the enzyme required to do so will be deactivated by the heat.

Overall though, it isn’t really that well known what happens to these molecules during cooking. It’s best to try out and just see what works best for you.

brussel sprouts with stewed meat and french fries
A super simple way to prepare Brussel sprouts, just boil them and eat them with french fries and stewed meat. Jum.

How to cook brussel sprouts

If you do want to a few more specific tips, here are a few to get you going. Start by buying small Brussel sprouts, not those huge ones, the smaller ones just taste a lot better. Then take off any yellow or wilted leaves. Remove the remaining stalk of the sprout as well.

Put a pot of water on the fire and bring it to the boil. Make sure you bring it to the boil first before adding the sprouts. By bringing it to the boil first you will greatly improve the quality of the sprouts. Sprouts contain a lot of enzymes which will cause flavour deviations as described above as well as a change in colour. Enzymes are deactivated by placing them in boiling water. This will limit off-flavours!

Cook the sprouts until they are soft, test this with a knife. I like them when they’re still a little firm, then they also tend to have a slightly stronger flavour. I tend to prefer eating my lightly boiled sprouts with some mustard. After reading this I understand why. The mustard complements all those glucosinolates beautifully!

Sources

van Doorn, H. E., van der Kruk, G. C., van Holst, G. , Raaijmakers‐Ruijs, N. C., Postma, E. , Groeneweg, B. and Jongen, W. H. (1998), The glucosinolates sinigrin and progoitrin are important determinants for taste preference and bitterness of Brussels sprouts. J. Sci. Food Agric., 78: 30-38. doi:10.1002/(SICI)1097-0010(199809)78:1<30::AID-JSFA79>3.0.CO;2-N

Song, Lijiang & Thornalley, Paul. (2007). Effect of storage, processing and cooking on glucosinolate content of Brassica vegetables. Food and chemical toxicology : an international journal published for the British Industrial Biological Research Association. 45. 216-24. 10.1016/j.fct.2006.07.021.

Glucosinolates, Wikipedia, link

ScienceDirect summaries of various articles on glucosinolates, link

Cruciferous vegetables, Oregon State University, link

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