What makes Brussels sprouts bitter?

I originally looked this up in the hopes of giving people advice on how to make their Thanksgiving Brussels sprouts more palatable.  I hadn't really expected what I found.

Brussels sprouts (Creative Commons,
courtesy of Wikipedia/Eric Hunt)

The simple answer is that it's from sulfur compounds common to the broccoli family (Brassicaceae).  The highest-profile of these is sulforaphane, one of several similar chemicals that brassicas use to repel chewing insects and animals, because they taste bad.  (Go figure).  But why are sprouts sometimes bitter and sometimes not, and how can you cook them so they aren't bitter?

Plants respond to stress in various ways.  When they can't get enough water, or the weather is too hot, or they're getting chewed on by bugs or diseases, they produce chemicals to make themselves hardier and less tasty.  Brussels sprouts like lots of water and cool weather, to the point that a few light frosts make them sweeter.  (They build up sugars in the leaves to act as antifreeze when the temperature dips below freezing.) So stressed sprouts are more likely to be bitter, as are ones harvested when they're too mature.  Letting them dry out or sit too long on the produce counter doesn't help either.  If you want the best sprouts, get ones from a major producer or a local farm which are small, rock-hard, and bright green.  The sprouts-on-a-stalk aren't bad either, as they tend to be younger and the stalk keeps them from drying out quickly.  Keep them cold until you're ready to use them.

As for how to cook them, that's more complicated.  Sulforaphone is a potent anti-cancer agent, which goes beyond the usual free-radical scavenging job of antioxidants and actually has the potential to turn on tumor-suppressor genes in cells which are turning cancerous.¹ So really, that bitter taste is good for you, in a much more direct way than just "eating your vegetables".  What's more, it's made by breaking apart a bigger molecule with an enzyme called myrosinase, and that doesn't happen until the plant is injured (such as by chewing).  Myrosinase, like many enzymes, is a pretty fragile protein that is damaged by heating, so the more you cook the sprouts, the less there is.  If you cook them too much, it doesn't matter how much of sulforaphone's parent chemical is in the sprouts, it will never be activated.  (Though you can introduce more myrosinase to cooked Brussels sprouts with the addition of horseradish, mustard, or something like broccoli sprouts.²)

So if you want that anticancer activity, cook them lightly, either by steaming, microwaving on low power, or frying them gently in a pan with a little oil; boiling pulls a bunch of the good compounds out into the water, and heats the enzymes too much. In addition, chewing thoroughly activates myrosinase to make sulforaphone. The catch is that you have to put up with some bitterness, which some people can taste more than others (though we haven't found a clear genetic link yet³). On the other hand, if you have really fresh, young sprouts, they have more than enough sweetness to take off that bitter edge. Kids probably still won't like them, but if you can handle a little hops in your beer or a nice dark chocolate, brussels sprouts shouldn't be any more of a stranger to your diet.

¹ Sulforaphane, a Naturally Occurring Isothiocyanate, Induces Cell Cycle Arrest and Apoptosis in HT29 Human Colon Cancer Cells
² Enhancing sulforaphane absorption and excretion in healthy men through the combined consumption of fresh broccoli sprouts and a glucoraphanin-rich powder
³ Genetic influences on the selection of brassica vegetables by elderly individuals