Garlic and Other Alliums: The Lore and The Science
Why am I, a chemist specializing in the organic chemistry of the element sulfur, writing this book on garlic and its relatives the alliums? Forty years ago I began laboratory work on the unusual sulfur-containing compounds found in garlic and onion. With the aid of talented students and coworkers, secrets of the often smelly and sometimes tear-provoking compounds slowly revealed themselves. The surprising complexity found in the chemistry of compounds extracted from the plants and synthesized in the laboratory served as the basis for my group's scientific publications and my students' doctoral theses. I took advantage of opportunities to talk about our work to audiences of specialists, as well as to high school students, audiences at garlic festivals and, sometimes informally, to participants at garlic-themed dinners.
To enliven my talks, I interspersed information on the history of these plants and their medicinal and culinary use with the science. On occasion I was invited to speak at professional meetings in the fields of horticulture, botany, medicine, and other areas where the science of garlic and its relatives were of interest. To prepare for these meetings, I educated myself on historical and cultural aspects of these plants, often journeying into areas far afield from chemistry. I found myself a student again, learning as much as I could from colleagues I met and those whose work I read, as well as from my international travels, which sometimes brought me to exotic produce markets that invariably included alliums. In the course of my self-education I amassed a wealth of information on garlic and its botanical allies. The more I researched the subject, the more it became apparent just how fascinating was the interplay between the science of these plants and their incorporation into different cultures, since these plants were first noticed by our ancient ancestors.
I was hardly the first to take a fancy to the chemistry of genus Allium plants. Indeed, Allium chemistry attracted such luminaries as August von Hofmann, first Director of the Royal College of Chemistry in London, now Imperial College, and founder of the German Chemical Society; Arthur Stoll, President of the Swiss pharmaceutical company Sandoz Ltd., and a pioneering researcher on chlorophyll with Nobel Laureate Richard Willstatter; and Finnish chemist Artturi Virtanen, winner of the 1945 Nobel Prize in Chemistry, among a host of other international scientists. I found it humbling to follow in the footsteps of such scientific giants.
In my reading I learned that garlic, onion, leek, chives and other members of the genus Allium occupy a unique position as edible plants and herbal medicines, appreciated since the dawn of civilization. Alliums have been featured through the ages in literature, where they are both praised and reviled, as well as in architecture and the decorative arts. Planting ornamental alliums adds beauty to gardens while simultaneously protecting nearby plants against pests. Garlic pills are among the top-selling herbal supplements, while garlic-based products show considerable promise as environmentally friendly pesticides. Careful observations by early scholars, as well as recent research, has established that the remarkable and varied properties of the alliums can be well understood based on the occurrence of a number of relatively simple chemical compounds ingeniously packaged by Nature in these plants to protect them against attack by predators.
This book represents an effort to examine and document the fascinating past and present uses of these plants, sorting out fact from fiction based upon the detailed scrutiny of historic documents, as well as numerous studies conducted in laboratories around the world. Every effort has been made to explain in the clearest terms the sometimes complicated scientific concepts and results in a nonspecialist language. At the same time, I have sought to provide sufficient additional detail and full referencing to the older original sources, as well as the most recent literature, to satisfy readers who want to know more, as well as researchers in disciplines as diverse as archeology, culinary arts, medicine, ecology, pharmacology, food and plant sciences, agriculture, and organic and analytical chemistry. It is my hope that those just beginning their careers in chemistry, or related areas, will find this book useful in illustrating how scientific research is conducted, and how an understanding of organic chemistry proves very useful in many different areas of science. I am very grateful that my publisher encouraged me to include a variety of photographs and drawings illustrating the subject matter in ways not possible with words alone.
This book is also a story of a personal journey, both figurative and real. My training at Harvard University with Nobel Laureate E. J. Corey prepared me for a career in the areas of natural products and organic sulfur chemistry, the latter being the subject of my 1967 doctoral dissertation and of a 1978 monograph. My interest in Allium chemistry was purely serendipitous. In the late 1960s, as a young faculty member at the University of Missouri-St. Louis, looking for a subject for laboratory research, I recalled from my graduate studies the notable chemistry of dimethyl sulfoxide (DMSO; CH3S(0)CH3) as a powerful solvent, a reagent for chemical processes, and a controversial treatment for arthritis. I thought it would be of interest to explore the chemistry of a relatively little studied analog of DMSO, namely dimethyl thiosulfinate (CH3S(0)SCH3), differing from DMSO by one additional sulfur atom. The properties of this easily prepared compound proved to be quite fascinating chemically. What soon caught my attention was the fact that dimethyl thiosulfinate was formed naturally from a variety of common vegetables, including members of the Allium, as well as Cruciferea genera. Furthermore, I discovered that the knowledge I gained exploring the chemistry of dimethyl thiosulfinate was directly relevant to the chemistry of the more complicated compound allicin. Allicin (CH2=CHCH2S(0)SCH2CH=CH2), the active compound formed when garlic is cut or crushed, has six carbon atoms instead of the two found in dimethyl thiosulfinate.
As I continued this research at the University at Albany, State University of New York, in Albany, New York, I was amazed at how, from a stable, odorless white solid, an onion enzyme could instantly release a highly reactive three-carbon compound, which, through action of another enzyme, could rapidly reorganize its atoms forming a second, profoundly lachrymatory (tear-inducing) compound of unique molecular structure. In the absence of this second enzyme, a cascade of chemical events occur, resulting in a doubling in size of the first-formed three-carbon compound to one with six carbon atoms. This chemical event is immediately followed by a remarkable sequence of molecular reorganizations, ultimately giving stable substances with aesthetically pleasing structures -- "zwiebelanes" -- possessing a delicious, sweet onion flavor. Other parallel processes result in a tripling in size through combination of these same three-carbon molecules yielding new molecules with nine carbon atoms, "cepaenes," if from onion, and "ajoene" if from garlic. Both nine-carbon compounds show significant bio-logical ability. Ajoene is currently under clinical study for the treatment of leukemia and fungal infections, while formulations containing cepaenes are being used for the healing of scars.
The novelty of the chemical transformations that occur when garlic or onion are crushed seemed to me to be a story worth communicating to a broader audience than just my fellow chemists. In 1985, while on a fellowship from the John Simon Guggenheim Foundation, I published "The Chemistry of Garlic and Onions" in Scientific American. In the process of preparing that article, as well as more recent Encyclopaedia Britannica articles, I learned how best to explain complicated chemical phenomena to a general audience. I even had the unusual opportunity to participate in three short documentary films dealing with garlic and onion. With the above experiences, I feel comfortable telling the full story of genus Allium plants here.
The journey associated with my Allium research has quite literally taken me around the world, allowing me to pursue my passions for travel, botany and photography. The first two chapters document with photographs, mostly mine, the many exquisite decorative alliums found in botanical gardens, the cultivation and marketing of edible alliums, and the incorporation of the shapes of onion, and even garlic, in architecture. The central two chapters chronicle the chemical detective work involved in understanding, at a molecular level, what happens when these plants are cut or crushed. These two chapters also take the form of a journey both through time and space, because chemists and biochemists around the world have been studying Allium chemistry since the earliest days of the science of chemistry. The final two chapters complete this journey, focusing on alliums in folk and complementary medicine through the ages, and on alliums in the environment, including promising recent work on the use of Allium extracts as environmentally benign pesticides.
I am extremely grateful to the National Science Foundation for more than 30 years of continuous, generous support for my own research program in the area of Allium chemistry, as well as for support from the American Chemical Society's Petroleum Research Fund and Herman Frasch Foundation, the North
Atlantic Treaty Organization, United States Department of Agriculture, the National Institutes of Health, the Jack H. Berryman Institute and the American Heart Association. I am also very much indebted to the John Simon Guggenheim Foundation, to corporate sponsors of my research, particularly McCormick & Company, Inc. (United States), Societe Nationale Elf Aquitaine (France), and ECOspray Ltd. (United Kingdom), and to my hosts at Harvard University, the University of Bologna, the Weizmann Institute and Cambridge University where I conducted Allium-related research during sabbatical leaves. Current University at Albany funding in conjunction with my Carla Rizzo Delray Professorship is also deeply appreciated. I also thank: Wolfson College of Cambridge University for hosting me as a Visiting Fellow in 2006 and 2007, where I formally began work on the book; Dr. Tim Upson, Superintendent of the Cambridge University Botanic Gardens, for providing full access to their botanical library and encouraging me to include copies of historical botanical prints in this book; Dr. Dmitry Geltman of the St. Petersburg (Russia) Botanical Gardens for his hospitality and assistance researching the early botanical studies in St. Petersburg by Eduard Regel; and Dr. Dorothy Crawford Thompson and Dr. John Emsley, both of Cambridge University, for helpful discussions.
It was been particularly inspiring for me to get to know in person Dr. Chester J. Cavallito, whose 1945 discovery of allicin from garlic set the stage for the chemistry described in this book. It was a happy coincidence that he made his seminal discoveries in a laboratory in Rensselaer, New York, located a short distance from my own university. I warmly thank Chester Cavallito for sharing with me details of his discovery of allicin and giving me his col-lection of early references pertaining to garlic. Because research on Cultivation of leek, onion and garlic is as old as the history of the human race, and as extensive as civilization itself. References to these plants in the Bible and the Koran reflect their importance to ancient civilizations both as flavorful foods and as healing herbs. We individually have vivid personal memories of the smell of wild onions in the meadows each spring, of a steaming pot of leek and potato soup, of the aroma of sauteing onions, of a garlic-scented roast, or other olfactory recollections of Allium plants from our own life experiences.
Species sharing similarities are grouped together as a genus. So it is that leek, onion and garlic are all members of the genus Allium, a word said to come from the Greek AAECO, to avoid, because of its offensive smell (Boswell, 1883). One of the largest plant genera, the genus Allium includes 600 to 750 species. Whilst a few alliums are of concern as invasive weeds, most are edible and have been consumed by indigenous populations for thousands of years, and some Allium species are cultivated as important food crops (Kiple, 2000; Peffley, 2006). Many members of this genus are popular with gardeners as hardy perennial ornamentals, though the smell of some can be off-putting. The smell is due to the presence of sulfur-containing compounds, which is a characteristic of this genus, and a focal point of this book.