Onion and garlic (Allium spp.)
ALLIACEAE, Onion Family
Everyone can think bad things about onions and garlic--the onion breath or garlic breath so hated on a date, or tears brought on when raw onions are sliced. But try now to remember only the good--think instead of the romantic times you enjoyed French onion soup or laughed while watching a comedy and eating fried onion rings or potato chips coated with California onion dip. Of course, the Gibson is an alcoholic drink like a martini but which requires a small pickled onion (Allium chinense).
Onions are cooking ingredients around the world and have been used for thousands of years. Huge quantities of onions were consumed by ancient Egyptians, and several tombs of pharaohs included carvings of onion and garlic, intended for meals in the After Life. Today strong-tasting onions are an essential part of the typical Egyptian diet, which has basically no other pungent species.
The genus Allium has more than 400 species, occurring in temperate climates, mainly of the Northern Hemisphere. All species are perennial herbs having bulbs with strong smells and pungent flavors, so it is no surprise that native tribes learned to use these as foods or as seasonings. Examples of uses by native peoples are recorded from northwestern North America, where numerous Indian tribes (e.g., the Salish of British Columbia) collected and consumed several species of native onions. Onions were eaten raw, but more often steam-cooked in underground pits overnight or roasted. Some of the strong flavor and smell was lost once properly cooked, and cooked onions were eaten immediately, used as an ingredient to flavor meat and salmon meals, or dried in strings for use at a later time. In California wild onions were eaten raw after soaking in salt water, roasted, or boiled, and raw onions were rubbed on skin as an insect repellent.
Species that were successfully cultivated arose in Asia and the western Mediterranean region. The most important was Allium cepa, the source of large onions, yellow (brown), white, and red. This is the "typical" bulb, consisting mostly of fleshy leaves with watery parenchyma that contains the pungent chemicals, e.g., methyl and n-propyl disulfides and trisulfides, which volatilize when cells are cut open or crushed. The brittle covering on the outside of a yellow onion is the epidermis of the outermost leaf, which forms lignified cell walls; in other words, it is sclerenchyma. The thin "onion skins" found within the bulb are likewise the epidermis of the more tender leaves. The stem is a small, hard plate located at the base of the bulb, where the adventitious roots arise.
Another species with similar bulbs is called the shallot, A. ascalonicum, which is often used for pickling. Leek, A. ampeloprasum, has cylindrical bulbs and broad, flat leaves, and these are widely used in soups; the Welsh onion, A. fistulosum, is very similar. Chives, A. schoenoprasum, are harvested for the green leaves and are commonly grown in herb gardens. The most unusual species is garlic, A. sativum, which has a cluster of bulbs, called "cloves." Each clove is actually a 1-leaved bulb, and the leaf is more solid than in most other species.
Most gardeners know that onions and garlic can be cloned by removing small bulbs, called setts, and planting these in the spring to obtain a late-summer cluster of large bulbs. Commercial production instead relies on planting the characteristic black seeds, which are produced in an inflorescence that looks somewhat like an umbrella. Onions and garlic are usually harvested by machines, which scrape up the shallow bulbs with soil and then separate them. In the Central Valley of California, where much domestic garlic and onion growing occurs, the produce is shipped to market in long, open trucks, which occasionally drop bulbs and skins along the roadsides, and leave a trail of pungent smell-keep an eye and nostril open on Interstate 5!
There are many different sulfur-containing compounds in each species, and it has been difficult to study them because the molecules change when the cells are opened or as they volatilize. In Allium cepa, a chemical called 1-propenesulfenic acid is transformed into the "lacrimatory factor," propanethial S-oxide. When the lacrimatory factor becomes dissolved in eye fluid, it is hydrolyzed into propionaldehyde, hydrogen sulfide, and sulfuric acid, which cause the burning sensation. This produces tearing from the lacrimal glands and lacrimal sacs, whose function is to clean, lubricate, and moisten the eyeball by producing a watery solution containing salts. Here the secretion helps to dilute the acid and thereby wash the irritating substance from the eye. The effect of the lacrimatory factor can be sharply diminished by freezing the onion or submerging the onion in water (diluting the chemical, which is soluble in water) before cutting. Cutting onions near a flame also helps by reducing the amount of chemical vapor reaching the (weeping) chef.
Garlic is a plant with a long history of use not only as a food, but also as a folk medicine. The list of past medicinal uses of garlic is very long, but most interesting are the claims that eating garlic reduces heart problems. Medically speaking, the most important compound in garlic is allicin, which only represents about 0.24% of the weight. Allicin is changed via several pathways into ajoene (named after the Spanish name for garlic, ajo) and thioacrolein. In tests performed with human subjects eating whole garlic and tests with the above by-products of allicin, scientists have substantiated that especially ajoene is a strong antithrombotic factor, i.e., it increases the length of time it takes for blood to clot. This is important in preventing heart disease, in which blood clots become lodged in blood vessels, causing stroke. Oils extracted from garlic and onion definitely interfere with agglutination of blood platelets. In garlic, the allicin also acts as an antifungal agent, thereby stopping fungi from attacking the fleshy bulb. All this said, the consumer is not guaranteed that recent garlic pills and dietary supplements will necessarily duplicate results in scientific trials--buyer beware!