Chemistry of Brussels Sprouts

There’s one vegetable at the Christmas dinner table that’s always bound to elicit strong and contrary opinions: brussels sprouts. Much like marmite, they seem to conjure up a ‘love it or hate it’ sentiment; however, if you fall into the latter camp, there may actually be a chemical and genetic reason why you can’t stand the taste. Sulforaphane is the featured molecule today in the Chemistry Advent Calendar, but here we take a closer look at the some of the other chemicals found in brussels sprouts.

Before discussing brussels sprouts specifically, we actually need to look at a chemical that isn’t even found in them, phenylthiocarbamide (PTC). This synthetic compound is an oddity in that it tastes bitter – but only to around 70% of people. To the other 30%, it’s completely tasteless. This property of PTC was discovered by accident in 1931, when Arthur Fox, a chemist working for the chemical company DuPont, accidentally spilt some of the compound whilst he was working with it. His colleague, working nearby, complained of the bitter taste, but Fox was unable to detect it.

Fox went on to carry a series of tasting tests with his friends and family, further confirming that it tasted bitter to some, but not all. Additionally, These studies, along with further work, confirmed the ability to taste PTC as a dominant genetic trait, and one that is heritable. In fact, this genetic link to being able to taste PTC was so strong, it was commonly used in paternity cases until DNA testing became more readily available. Another compound, propylthiouracil (PROP), is similar in that it also tastes bitter to some and not others, and is now the more commonly used compound in taste research.

At this point, you’re probably wondering what this has to do with brussels sprouts. Whilst the compounds PTC and PROP aren’t found in the vegetables, they both contain within their molecular structure a thiocyanate group (nitrogen, carbon and sulfur bonded in series) which is thought to be related to their bitter taste. This same group is also present in compounds called glucosinolates, which occur naturally in brussels sprouts, as well as broccoli, cabbage and kale (collectively known as cruciferous vegetables). The glucosinolates are thought to form part of the plant’s defence systems , for example against diseases and creatures that feed on the plant. Additionally, the breakdown products of glucosinolates, the isothiocyanates, also contain the thiocyanate group within their molecules.

So why is this structural similarity significant? Well, there seems to be a strong association between the ability to detect the bitter taste of PTC and PROP and a sensitivity to the bitterness of these vegetables. Of course, it’s not the case that sensitivity to PTC and PROP automatically implies a dislike of cruciferous vegetables; other factors may also have an effect, such as a person’s particular flavour preferences. However, if you’re a brussels sprout hater, you can now suggest a potential chemical reason for your dislike if you’re ever berated for your perceived vegetable prejudice.

On a final note, brussels sprouts also contain compounds that have been investigated for potential health benefits. Chief amongst these is sulforaphane, an isothiocyanate breakdown product of the glucosinolates. Research has focused on sulforaphane’s potential as a protector against neurodegenerative diseases, and whilst more research is still required, neuroprotective effects have been observed both in lab-cultured cells and within animals.

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References & Further Reading