Do you go wild over your garlic and smile through your tears when chopping onions? Members of the allium family, such as onion, garlic and chives, have long been used around the world to add flavour and vitamins to food, despite the next day’s “garlic breath”, but additional medicinal properties have been much harder to prove.
1. The picture above shows onion and garlic bulbs together with some chives on a petri dish; you can also see the bottom of a measuring cylinder with some allium flower stalks and a volumetric flask in the background. While the petri dish is more often used in biological and biochemical labs, both measuring cylinders and volumetric flasks are key to chemical experiments involving liquids and solutions. On the left hand side is the structure of a molecule common to all members of the allium family, called alliin, which can be converted to the molecule on the right, allicin, by the action of an enzyme when the plant needs to protect itself, for example because the cell wall has been broken/chopped open.
Archaeological finds suggest that onions have been used as food for more than 7000 years and the early Egyptian pharaos more than 4500 years ago seem to have liked garlic, so members of the allium family have been popular for a long time and feature in the dishes of many countries. Onions and garlic bulbs and their relatives, such as leeks and chives, are reasonably easy to grow and the bulbs store well if kept at room temperature and quite dry. As such, they can provide vitamins, fibre and sugar over the winter when other fruit and vegetables are not available. In addition, the defense mechanism of this family of plants produces a range of chemicals which add strong pungent and spicy flavours to food and which might have other benefits as well.
When the cell walls of alliums are damaged, an enzyme in the plant (allinase) performs a chemical reaction on a molecule called alliin (shown in the image above), which converts it into a range of compounds that are chemically and biologically active as irritants, but quite unstable. In garlic, this includes allicin, also shown above, which causes pain to many mammals. Onions and leeks don’t just rely on allicin, but also produce an additional compound that irritates the eye so tears are formed (you can read more about this by following this link, although more recent work, summarised here, suggests that another enzyme is involved). Unfortunately, these defensive strategies haven’t been successful against humans, although they can cause considerable irritation when preparing a meal. They may also lie at the root of suggestions about companion planting of onions with carrots, and have been used to produce an insecticide derived from garlic extract (this article is by the producers, so should perhaps be taken with a pinch of salt).
Alongside their culinary uses, onions and garlic especially have been used in traditional/folk medicine, and alliin, allicin and related compounds have also been tested as potential anti-inflammatory, anti-cholesterol, anticancer and antioxidant compounds, with recent press reports suggesting that they may help against the common cold as well. Unfortunately, several reviews of such studies (e.g. “Garlic for the common cold” and “Garlic in clinical practice“) have found the evidence for such beneficial effects small and often unconvincing. Many of the compounds are short-lived and may not survive digestion, so dosing yourself up with onion soup and garlic bread when you feel a bout of the sniffles coming on may be comforting (and the hot liquid and vitamin content might help), but there is not enough medical evidence to lead to a prescription from your GP any time soon.
2. Sulfur compounds are often associated with bad smells and can be quite reactive, helping to explain why plants might have needed to evolve a mechanism where they store the chemical precursors and carry out the necessary chemical reactions to produce the active defensive chemicals as and when needed. In alliums, the compound stored is alliin, (2R)-2-amino-3-[(S)-prop-2-enylsulfinyl]propanoic acid, which is a derivative of the naturally occuring amino acid cysteine (Fig. 1). This molecule has two chiral centres (recall from the rose post that the different forms of chiral molecules have the same connectivity, but different three-dimensional arrangements of the atoms in space, giving rise to non-superimposable mirror images at each chiral centre) and it is a particularly interesting chiral compound, because it has both carbon and sulfur stereochemistry, as shown in Fig. 1.
In addition, there is scope for hydrogen-bonding, both within the molecule, as shown by the lines in the calculated molecular structure (Fig. 2), and also between molecules and with some solvents. Hydrogen bonding arises when the hydrogen connected to an electronegative atom (most commonly oxygen or a halide) interacts with another atom that has a lone pair of electrons (e.g. O, S, N). The interaction can be quite strong and can alter the properties of molecules interacting in this way, most notably for water where the strong network of hydrogen bonds in ice means the density of ice is lower than that of liquid water.
Alliin is decomposed to allicin, 2-propene-1-sulfinothioic acid S-2-propenyl ester, which is not particularly stable and decomposes further to 2-propenesulfenic acid (shown in the picture on the left) and other sulfur compounds. In the body, these sulfur compounds are slowly (over several hours) converted to allyl methyl sulfide (shown in the picture below), which is then exhaled and excreted by the body, giving rise to “garlic breath” and “garlic sweat”, the less pleasant side effects of a tasty meal.
3. While decomposition pathways for these sulfur compounds had long been postulated, the technology to observe the relevant intermediates has only recently been developed and applied to crushed garlic. Direct analysis in real time mass spectrometry (DART-MS) has been used to analyse the decomposition products of crushed garlic and to demonstrate the short life-times of various intermediates (J. Agric. Food Chem. 2010, 58, 4617–4625) – 2-propenesulfenic acid, which might be a useful peroxyl radical scavenger (Angew. Chem. 2009, 121, 163 –166), is only around for less than a second after crushing a clove of garlic, limiting its usefulness if ingested.
While some of the compounds contained in members of the allium family may find useful applications in medicine one day, the human metabolism is quite good at dismantling these sulfur-containing compounds, perhaps helping to explain why it has been so difficult to prove conclusively that eating onions and garlic can help to fight various illnesses. However, both raw and cooked members of this successful family of plants are nutritionally useful and their flavours can be brought out and enhanced by cooking or frying them. Indeed, frying caramelises the sugars in foods, and the chemical reactions taking place during this process are complex and not fully understood, perhaps something we should revisit in a series about the chemistry taking place during cooking… In the meantime, here are some articles about the chemistry taking place: chemistry of caramel, caramelisation, new science, new possibilities, food chemistry from the Chemistry World blog.
Contributors: Jenny Slaughter (photography), Natalie Fey (images and writing)