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Chemistry


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Broccoli (Brassica oleracea)

Broccoli, main

Broccoli florets in a Schlenk tube, together with a molecule of sulforaphane.

Broccoli is a commonly eaten vegetable in the UK. But how do you eat your broccoli? Do you eat both the leaves and the stalks? If not, are you missing out on key nutrients that these parts could provide? And what about your method of cooking it: Steaming or boiling? Is one method better than the other in terms of nutritional benefit?

1. Broccoli grows on plants with large leaves surrounding the crown that we eat. But should we also be eating the broccoli stalks? The answer to that question is probably yes, as the stalks turn out to be equally nutritious as the florets in terms of vitamins such as vitamin C.

In ancient medicine, broccoli was thought of very highly and was used to treat conditions such as oedema and tetanus around 3rd Century B.C. The Roman Cato even encouraged others to grow broccoli in their orchards to use as an all-purpose medicine.
Often, old wives tales have an element of truth to them and this can be exploited, for example using the chemicals in foxgloves to treat heart conditions. But what techniques were used to determine which chemicals foxgloves and broccoli contain?

2. One such technique is chromatography. Chromatography involves the separation of different components of a mixture according to their different physical properties. Paper chromatography, often done in schools, involves putting a dot of ink on some filter paper and then dipping the paper into some water. The water then travels up the paper and separates the different components of the ink out to create a multi coloured “blob”. The different colours adhere to the paper with different strengths, meaning they travel at different speeds and so become separated.

Figure 1 shows this technique used. 

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Figure 1 Broccoli florets and paper chromatography of ink, resting on a petri dis.

In HPLC (High Performance Liquid Chromatography), the retention time (how long the component takes to travel through a stationary phase in the apparatus) of a component is measured and, using this, the component can be identified. HPLC can be used to check if athletes have been doping, by analysing their urine samples.

In a recent study (Food Additives & Contaminants, 2006, 23, 1088-1098), the quantity of vitamins and minerals in broccoli after cooking were investigated using the HPLC technique. Vitamin C was one of those studied and they measured the differing amounts of Vitamin C in fresh broccoli, steamed broccoli and boiled broccoli. There was a 28.6 mg decrease from fresh to steamed broccoli and a much larger decrease of 156.4 mg from fresh to water-boiled broccoli. Note that the fresh broccoli had 681.2 mg of vitamin C per 100 g. Assuming you don’t fancy raw broccoli, this study indicates that steaming is better in terms of retaining nutrients than boiling.

But why is it better? Vitamin C is water soluble so when boiling broccoli, it will leach out into the surrounding water. In contrast, when you steam broccoli there isn’t direct contact between the water and the vegetable. Table salt (NaCl) is also water soluble. This is due to the polarity of water; water has partially charged atoms (O atoms formally have a partial negative charge due to the difference in electronegativity between O and H) but is still neutral overall. There is attraction between the oxygen atoms in water  and the Na+ ions, and also between the hydrogens and the Cl ions.

However, even boiling (J. Agric. Food Chem. 2008,  56, 139-147) could be better than eating broccoli raw as it makes the compounds more bioavailable, i.e. easier to absorb by the body.

3. Mustard seeds may be used to season a dish and therefore make it tastier. Yet, their addition during the boiling of broccoli has shown another unexpected benefit. The mustard seeds contain an enzyme (myrosinase) which can hydrolyse the sulforaphane precursor to give a greater concentration of sulforaphane, a potential “cancer chemopreventive”(as this study (Synthesis 2013, 45, 1667-1674) was carried out in vitro), than without the seeds. 

Sulphoraphane

Figure 2– The sulforaphane molecule

Sulforaphane is a compound which, as you can see above in Figure 2, contains an isothiocyanate group. Synthesis of the isothiocyanate group involves first the reaction of an amine with a carbonothioyl (this contains a C=S group). Then the product of this forms the isocyanate group as the phenoxide, which is a good leaving group, is generated. Dicholoromethane (DCM) was used as a solvent. Figure 3 shows a possible mechanism for this reaction which was proposed in a study ((Synthesis 2013, 45, 1667-1674).

Mech

Figure 3– A mechanism for making sulforaphane

 

So there you have it: you need to add mustard seeds to your pan whilst steaming your broccoli to add flavour and get most of your nutrients out of this vegetable. Steaming is better than boiling and now you can impress your friends by telling them the reason why! So eat your steamed greens, including the stalks, folks.

Contributors: Lucy Bird (writing, images), Natalie Fey (editing), Chris Arthur (editing).