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28 July 2010

978 0 85404 190 9
Royal Society of Chemistry
GBP 29.95

Garlic and Other Alliums: the Lore and the Science

Eric Block

There is a long history of references to onions and other alliums in literature and art. In his 2007 biography, Peeling the Onion, Günter Grass eloquently uses the metaphor of the onion to expose past memories for inspection: “so we can read what is laid bare”. In 1889, Van Gogh painted Still Life with Onions and Annuaire de la santé, which, in addition to the onions, contained a letter from his brother, his pipe and a copy of Annuaire de la santé by the French chemist and physiologist François-Vincent Raspail. The painting was significant, as it was made when Van Gogh was trying to recover from the breakdown that had led to his hospitalization. As he makes clear to his brother, he wanted to prove his illness had not robbed him of his identity or power as an artist. (“As far as I can judge I’m not mad, strictly speaking. You’ll see that the canvases I’ve done in the intervals are calm and not inferior to others” letter 7511 to Theo Van Gogh). Onions have also been included in other works by Van Gogh as well as by Gauguin, Cezanne, Renoir and Velásquez to name but a few. Examples from the latter two artists and Van Gogh’s painting are included in Garlic and Other Alliums: the Lore and the Science by Eric Block, professor of chemistry at the University of Albany.

Members of the genus Allium include garlic, onions, leeks and chives, and, as Eric Block describes, their use as food and medicinal herbs is as old as human civilization. Block starts with an introduction to the different species and their cultivation from ancient times to the present. The second chapter deals with cultural references and contains an eclectic range of sources from literature, art, philately and architecture. The latter includes the expected onion domes of orthodox churches and the Royal Pavilion in Brighton and the more quixotic garlic bulb found on the tower of Gaudi’s Casa Batlló. Interestingly, the architectural use of the onion shape was to make structures appear taller.

These earlier chapters serve as the appetizers for the main course which is the chemistry of secondary metabolites found in onions and garlic species. The chemistry makes up about a third of the book and constitutes the two central chapters. Block sets the scene well describing the early studies in the late 19th and early 20th Centuries and the researchers involved in isolating and characterizing the volatile compounds from garlic and onions. For example, there is the Finnish Nobel laureate who had a “very simple assay” for the tear inducing compound from chopped onions: “holding the vial … to the eye”. In addition to the lachrymating effect when cut, other common properties associated with alliums, including smell and sharp taste are the products of organosulfur-containing secondary metabolites. The onion lacrymating factor (LF) (CH3CH2CH=S=O) is synthesized from isoalliin, which is related to alliin, the non-protein amino acid S-allyl-L-cysteine sulfoxide. Interestingly, silencing the enzyme LF synthase results in a ‘tearless’ onion. In garlic, alliin is converted into allicin (CH2=CH2-S-S(O)CH2CH=CH2), which gives garlic its characteristic small and taste. Block writes well and passionately and does not shy away from a detailed discussion of the biosynthesis, metabolism and chemistry of these compounds, which he explains in an illuminating manner. This is to be expected as he has spent a 35 year research career in this area.

In the penultimate chapter, Block gives a very balanced assessment of the claims and evidence for the health benefits of eating or taking allium supplements, primarily garlic. The evidence from in vitro studies for antibacterial and antifungal activity of garlic extract and even inhibition of cancer cell proliferation is impressive. However, the translation to human studies is at best inconclusive. The last chapter intriguingly discusses the possible use of alliums as natural herbicides and pesticides. The co cultivation of allium species with other crops, based on anecdotal evidence from ancient agricultural practices together with more recent controlled field studies reveals some promising results. But, as Block is careful to state, more work is required in this area.

The book is well written and illustrated: a particular bonus is the inclusion of 27 coloured botanical prints from a volume of Flora Germanica. It will probably be of most interest to students and researchers familiar with plant biochemistry, but there is also something for those curious about this group of plants that play a prominent role in cooking, culture and chemistry.


1. www.vangoghletters.org/vg/

Iain J. McEwan (University of Aberdeen)



 
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