Onions and Other Allium Plants
ONIONS AND OTHER ALLIUM PLANTS
ONIONS AND OTHER ALLIUM PLANTS. Allium crops have been cultivated for millennia by people worldwide for sustenance, flavor, and medicinal purposes. Each of these three properties is closely connected to a suite of unique organosulfur compounds present in Allium crops that make them distinct from other wild and cultivated food plants. These compounds impart the characteristic flavors and odors of edible alliums. A substantial body of scientific literature suggests that these organosulfur compounds likely arose through natural selection for pest resistance. In a fortuitous circumstance, humans find these odors and flavors appealing, thus what confers functional significance to the Allium crop for its survival also confers culinary significance to the Allium consumer for gastronomic pleasure.
Seven major allium crop complexes are recognized, five of which contain a single allium crop (Table 1). These five are bunching onion (fistulosum ), chives (schoenoprasum ), Chinese chives (tuberosum ), garlic (sativum ), and rakkyo (chinense ). The remaining two complexes contain four separate crops (leek, kurrat, great-headed garlic, and pearl onion) in the case of ampeloprasum and two different crops (onion and shallot) in the case of cepa. Each of these crops represents a unique modification of the leaf. In the allium crops where bulbs are prominent, leaf bases are swollen due to the accumulation of carbohydrates from photosynthesis. In those crops where pseudostems are the edible portion, overlapping leaf bases form a hollow column that has the appearance of a stem, such as the base of the leek. For other allium crops the edible portion is the leaf blade, which also serves as the primary photosynthetic organ. These seven crop complexes are grown and consumed worldwide for a multiplicity of uses.
|The seven primary edible allium crop complexes|
|Species Complex||Crop||Variety||Storage Organs|
|cepa||bulb onion shallot||cepa ascolonicum||foliage leaf bases and bladeless leaf sheaths|
|fistulosum||bunching onion||NA||foliage leaf bases, bulbs absent|
|schoenoprasum||chives||NA||foliage leaf bases, bulbs absent|
|tuberosum||Chinese chives||NA||rhizomes, bulbs absent|
|ampeloprasum||leek kurrat great-headed garlic pearl onion||porrum kurrat holmense sectivum||bulbs generally absent, cloves like garlic in great-headed garlic and pearl onion; pseudostem in leek and kurrat|
|sativum||garlic||sativum||swollen, bladeless sheaths (cloves)|
|chinense||rakkyo||NA||swollen, foliage leaf bases, bulbs prominent|
|NA = not applicable|
|SOURCE: Brewster, 1994, as redrawn from Jones and Mann, 1963|
The genus Allium contains more than five hundred species, including many ornamental and edible plants. The genus has been assigned to the family Alliaceae, although for many years it was classified with both the Amarylidaceae and the Liliaceae. Edible alliums are important staples in the diets of many of the world's cultures. Most of the edible alliums are native to the mountains of central Asia, and a number of alliums are still collected from the wild in this region. Distribution of Allium crops ranges widely throughout the Northern Hemisphere and in mountainous regions of the tropics. The area of greatest diversity is the mountains of central Asia, including Afghanistan, Tajikistan, Pakistan, and parts of Siberia and China.
Many edible alliums are classified into two subgenera, Rhizirideum and Allium. In subgenus Rhizirideum the sections Cepa, Schoenoprasum, and Rhizirideum are comprised of the species cepa, fistulosum, schoenoprasum, and tuberosum. In the subgenus Allium the section Allium is comprised of the species ampeloprasum, sativum, and chinense (Hanelt, 1990; Brewster, 1994). Together these seven species contain the primary edible alliums consumed throughout the world. G. R. Fenwick and A. B. Hanley (1985) also describe a number of other minor alliums consumed as vegetables or herbs, including the topset onion, the tree onion, the Wakegi onion, and others. These minor alliums are primarily from the Allium cepa group and are discussed in some detail in Henry A. Jones and Louis K. Mann's Onions and Their Allies (1963).
The topset onion, tree onion, and Egyptian topset onion (A. cepa subvarieties viviparum, bulbiferum, and proliferum respectively) form bulbils in their inflorescences. Bulbils are small, bulblike structures used as vegetative propagules for these alliums.
Onion and shallot. The wild progenitor of onion is not known, although P. Hanelt (1990) and M. J. Havey (1995) have suggested it may be Allium vavilovii. Bulb onion was domesticated from a plant that likely had a long juvenile phase and grew as a perennial. Selection pressure during domestication was for larger bulbs that grew more rapidly, a biennial life cycle that concentrated vegetative growth into one season, and barriers to crossing with other wild species (Brewster, 1994). The cultivated Allium cepa has been placed into two horticultural groups by Hanelt (1990), the Common Onion group and the Aggregatum group. The Common Onion group includes the typical bulb onion, while the Aggregatum group consists of those subspecies or varieties of A. cepa whose lateral buds have been active, thereby forming clusters of smaller bulbs. These types of A. cepa have been further subdivided into multiplier or potato onions and shallots. Multiplier onions may possess as many as twenty small bulbs that are short and wide, whereas shallots form clusters of smaller, narrow separate bulbs (Jones and Mann,1963). Most of the plants in the Aggregatum group are vegetatively propagated for horticultural use. Aggregatum group crops are found at extreme northern latitudes, such as Finland and Russia, and also in the tropics, but for different reasons. The short life cycle of Aggregatum group crops favors the short growing season in northern latitudes, whereas the intense pest pressure of tropical environments favors a short-season crop that can be grown from a vegetative propagule rather than from seed (Brewster, 1994).
Onion plants form bulbs in response to specific day lengths and temperatures. This photoperiodic response includes two primary categories, long day and short day. The long-day onion plant requires a day length of at least fourteen hours (actually a night length of less than ten hours) to initiate bulb formation, while short day onion plants require between twelve and fourteen hours of day length to form bulbs. Long-day onions are grown in the northern latitudes, often from seed or transplants sown in early spring. Bulb formation takes place during the summer months, and bulb harvest is during the later summer and the fall. The short-day onion is grown in warmer climates, where it may be sown in the fall and overwintered in the field. Bulb formation is in the spring, and harvest is in early summer.
Long-day onions were cultivated in Europe for many centuries and adapted to northern latitudes. Like many root vegetables developed for consumption during winter months in cold climates, these long-day onions were harvested in late summer and fall and stored at cold temperatures. For this reason these cultivars or landraces became known as storage onions. Populations of storage onions developed in England and other parts of northern Europe and in the early seventeenth century were taken to the New World, where they were planted in the area around Salem, Massachusetts (Goldman et al., 2000). Selection for proper bulb formation and storability led to the development of popular cultivars, such as the Danvers Yellow Globe, which became a progenitor population for virtually all long-day storage onions in the United States during the nineteenth and twentieth centuries.
Short-day onions were cultivated in the Middle East and in many southern European countries for centuries, and immigrants from Italy and Spain brought these cultivars and landraces to the United States in the late nineteenth century (Goldman et al., 2000). The first southern European onions in the United States were likely the Bermuda types, which originated in Italy and were first grown in the United States in southern Texas. The second type of onion introduced from southern Europe was the Babosa onion from Valencia, Spain, which was likely introduced early in the twentieth century. Fabian Garcia's early breeding work at the New Mexico Agricultural Experiment Station developed the Early Grano onion, an important progenitor of many of the sweet, mild onions grown in the southern United States (Brewster, 1994; Goldman et al., 2000). These onions were further selected into a number of important populations, including the Texas Early Grano and Granex series, which are important short-day onions in the U.S. market.
Garlic. Garlic almost certainly originated in the mountains of central Asia. Although its exact progenitor is unknown, it may be A. longicuspis. For thousands of years garlic has been propagated by asexual means because fertile flowers were extremely rare. At the end of the twentieth century several reports of fertile garlic clones, both in the wild and in cultivation, initiated intense interest in seed propagation of garlic, which appears to be a realistic proposition (Pooler and Simon, 1994). Seed production is important for several reasons, including the possibility of reducing the spread of viral diseases by going through a reproductive phase and the potential for breeding garlic for improved characteristics. Despite the fact that little to no sexual reproduction of garlic has taken place under cultivation, much phenotypic variation exists among cultivars, likely due to the selection of interesting and favorable mutants.
Leek, kurrat, great-headed garlic, and pearl onion.
The ampeloprasum group includes four primary crops, all of which can be freely crossed and interbred. Wild A. ampeloprasum is found in a broad geographic range that includes western Iran through the Mediterranean to Portugal (Brewster, 1994).
The leek group is characterized by the development of a pseudostem, which is actually the edible part of the leek plant. The pseudostem is so called because the concentric overlapping leaf bases fold over each other to create a hollow stemlike structure, although botanically the edible portion of the leek is not a stem. Leeks are well adapted to cool climates and are grown throughout northern Europe. Unlike bulb onions, leeks do not have specific photoperiodic requirements for pseudostem formation and can thus be grown in a wide range of latitudes.
The kurrat group includes those A. ampeloprasum selected for edible leaves and short pseudostems. The crop is popular in Egypt, where the leaves are repeatedly cut and harvested every three to four weeks over an eighteen-month period (Brewster, 1994).
The great-headed garlic group includes plants that form large cloves similar to garlic cloves, however the inflorescence is large and leeklike. Called elephant garlic by many commercial growers, it typically is much larger than a garlic plant and can produce up to six large cloves at the base of the flower stalk. When the plant does not flower, only a single large clove, known as a "round," is produced.
Pearl onion is a minor Allium crop grown in certain parts of Europe. The plant forms a cluster of small, spherical, white-skinned bulbs. In the United States, products marketed as pearl onion may in fact be small bulbs of A. cepa rather than the true pearl onion. Among U.S. horticulturists, the true pearl onion is often called Portuguese leek.
Bunching onion. The bunching onion has been the primary Allium crop in many parts of Asia for millennia and is still a major component in the diet in China and Japan. Bunching onions appear similar to bulb onions during early growth stages, but bunching onions do not form bulbs and instead are harvested for their green foliage.
Allium fistulosum may derive from the wild Allium altaicum, which grows in the mountains of southern Siberia and Mongolia and is interfertile with A. fistulosum.
In the U.S. market the consumer is usually presented with green onions or scallions that are likely the bulb onion A. cepa harvested for its green foliage. It is also possible that these green onions were produced from an interspecific hybrid between A. cepa and A. fistulosum known as "Beltsville Bunching" that has been a mainstay of bunching onion production in the United States for more than fifty years. Thus the green onions or bunching onions in the U.S. market may be of several different genetic backgrounds.
Bunching onion cultivars have been developed for a variety of market classes. These classes are primarily separated by the geographic areas in which the plants can be grown and by the quality of the foliage. Certain consumers prefer blanched pseudostems instead of green pseudostems. Cultivation of the former is accomplished by mounding soil on the developing plants, thereby reducing the amount of chlorophyll and producing a more tender, lighter-colored pseudostem.
Rakkyo. This allium crop is grown mainly in Japan and China, where it produces small bulbs that are mostly consumed pickled. The plants resemble chives but develop elongated bulbs in the summer.
Chives. Chives, which grows wild in Eurasia and in America, is the most widely distributed allium species. It is extremely cold hardy and is winter dormant, and it can grow in latitudes as high as 70 o (Brewster, 1994). Chives form a cluster of low-growing, narrow, hollow leaves. After every two or three leaves have formed, axillary buds form side shoots that then allow for the development of a cluster of shoots (Brewster, 1994). The shoots are attached to each other on a rhizome.
Chinese chives. Allium tuberosum grows wild in East Asia and is cultivated for its garlic-flavored leaves and immature flowers. It forms rhizomes similar to those of chives, and the leaves arise as dense clumps from these rhizomes (Brewster, 1994). Unlike true chives, which has a hollow stem, the leaves of Chinese chives are flat.
Chemical Constituents and Culinary Significance
The word "allium" derived from the Greek phrase "to leap out," thereby suggesting a strong interaction between the crop and its consumer. Worldwide the edible alliums are prized for their unique flavors. These flavors are derived from a suite of unique organosulfur compounds that likely have their origin as a defense strategy, protecting allium plants from pests. Although they possess some toxicity for insect and microbial pests, their levels of toxicity for humans and larger animals is far less and in most cases nonexistent. Several exceptions exist, however, and these are discussed under the pharmacological properties of allium crops below.
Like all plants, alliums uptake the necessary element sulfur as sulfate from the soil. Sulfate is then used to form the amino acids cysteine and glutathione, which in turn form the gamma glutamyl peptides. These peptides serve as building blocks for the allium flavor precursors, known collectively as the alk(en)yl-L-cysteine sulfoxides or ACSOs. The ACSOs are present in the mesophyll storage cells, inside the cell's cytoplasm. Each allium crop is characterized by a different number and ratio of these ACSOs, which ultimately determine its flavor. For example, onion contains three ACSOs, while garlic contains four. The balance of these ACSOs in onion and garlic are different, resulting in flavor differences between the vegetables. Knowing the ACSO profile can help predict the flavor of an allium, because they are directly responsible for the flavor components as described below.
The ACSOs are considered flavor precursors because they do not impart flavors directly. Rather, it is only upon tissue disruption that allium crops yield their unique flavors and odors. This is accomplished through a chemical reaction that begins when the enzyme alliinase, stored in the bundle sheath cells and protected from the ACSOs by a membrane, comes into contact with the ACSOs after tissues are cut. Thus the scent of an unchopped onion bulb is completely different from the scent after tissues have been chopped, because the enzymatic lysis of the ACSOs initiates the development of the thiosulfinates, which are actually responsible for allium flavors.
As soon as allium tissues are disrupted, such as by a kitchen knife, the transient sulfenic acids are formed as well as the by-product pyruvic acid. Pyruvic acid has been used extensively as an indirect measure of onion pungency because the amount of enzymatically formed pyruvate is positively correlated significantly with a taste perception of pungency (Wall and Corgan, 1992). In all alliums except onions the sulfenic acids are rapidly converted into thiosulfinates, which are the compounds responsible for the flavors of allium vegetables. This conversion also takes place in onions, however, immediately prior to thiosulfinate formation, the compound propanethiol sulfoxide is formed. Propanethiol sulfoxide, also known as the lachrymatory factor, is responsible for the formation of tears in the eyes of those close enough to the chopped onion to intercept the airborne compound. Presumably the formation of tears is caused by the interaction of this sulfur compound with the eye's nerve cell membrane, causing the formation of sulfuric acid.
Thiosulfinate accumulation in crushed or cut allium tissues is perceived by the nose, the eyes, the tongue, and the skin. Because allium crops contain different ACSOs, they also contain different kinds and amounts of thiosulfinates, which in turn leads to different flavors. Thiosulfinate formation at room temperature is complete after thirty minutes (Thomas and Parkin, 1994). Chopped allium tissues therefore have differential thiosulfinate profiles depending upon the length of time they have been allowed to sit and the temperature at which they have been prepared.
Typically most consumers process cut alliums by boiling or by frying or sautéing in oil. These two processes yield different by-products. Boiling chopped garlic results in the formation of various sulfides, including diallyl disulfide and diallyl trisulfide. Sautéing in oil produces the vinyl dithiins and ajoene (Lawson,1998). The transformation of sulfur compounds into thiosulfinates and downstream reactions that transform the thiosulfinates into sulfides and dithiins is an area of great interest and investigation in the allium research community.
Biology of Vegetable Alliums
Most of the edible alliums possess some variation of the leaf that creates their vegetable form. Those that form bulbs do so because the base of the leaf begins to swell with carbohydrates from photosynthesis. The concentric swollen leaf bases make up the structure of the onion bulb. Similarly the swollen leaf base and its protective leaf sheath make up the clove that is a part of the garlic bulb. Thus the bulb is nothing more than fleshy leaves or leaf bases on top of a short, flattened stem. Those alliums that form a pseudostem, such as leek, do so because the overlapping leaf bases form a hollow stemlike structure. These leaf bases do not swell but make up the edible portion of the crop just the same. And of course those vegetable alliums that are consumed for their leaf blades typically do not form swollen structures such as bulbs. In all cases a short, flattened stem is found under these leaf bases. It is likely that this stem has been shortened during the evolution of these allium crops, resulting in larger and more-prominent leaf bases and storage tissues.
These many variations on leaf morphology illustrate an important principle of crop domestication. Many of our most important crop plants occur in complexes where multiple morphological forms of a single species have been selected by humans to serve a variety of different needs. For example, the crop species in the Brassica oleracea complex, including cabbage, broccoli, cauliflower, kale, collards, and Brussels sprouts, all possess variations that partition their photosynthate into different storage organs. In the case of cabbage the photosynthate is stored in leaf tissue packed into a compact rosette. In the case of Brussels sprouts the axillary buds are activated, and smaller headlike structures become the item of commerce. In the case of broccoli photosynthates are partitioned into the thickened stem and immature inflorescence. Similar to allium crops, the variations in morphology serve the purpose of producing different crops for different uses. Among cultivated plants the alliums are unique in that their multiplicity of forms is based on how and where the photosynthate is partitioned in the leaf.
Another unique biological feature of the alliums is the presence of bulbils in the inflorescence. Bulbils are simply small bulblike structures that form in leaf axils, particularly in the inflorescence, of many allium crops. Bulbils are also comprised of leaf tissue but are not formed from swollen leaf bases as onion and garlic bulbs are. Rather, they represent a unique form of propagule from which alliums can be vegetatively propagated in a clonal fashion. The presence of bulbils in the inflorescence can also be a diagnostic character for differentiating alliums. They are common in garlic, are not present at all in Chinese chives and rakkyo, and occasionally are present in leek and kurrat.
And if the boy have not a woman's gift to rain
a shower of commanded tears,
An onion will do for such a shift,
which in a napkin being close conveyed,
Shall in despite enforce a watery eye.
(William Shakespeare, The Taming of the Shrew, cited in Block, 1992)
Alliums may have been cultivated originally for their medicinal properties and only later developed into flavorants, although it is possible these two discoveries occurred simultaneously or in close proximity. Many of the most common vegetable crops, such as lettuce, tomato, and others, were originally used as medicinal plants prior to their widespread use as food (Rubatzky and Yamaguchi, 1997).
The Egyptians made extensive use of alliums to treat a variety of ailments, many of which are recorded in the Codex Ebers, a document at least 3,500 years old (Block,1985). Documentation of their use in ayurvedic medicine in India, Western-style medicine in Greece, and Eastern-style medicine in China abounds (Lawson, 1998). Evidence also indicates that early Olympic athletes were fed alliums to promote blood circulation and to attain peak performance and that Europeans have used alliums to treat blood clots in horses and other domestic animals for many centuries (Block, 1985, 1992). Alliums were important in warding off plague and other microbial infections over the centuries (Lawson, 1998) and in treating dysentery, smallpox, and many other maladies during the twentieth century. Indeed edible alliums have been more widely used than many synthetic medicines for an incredibly diverse array of conditions.
The rising importance of synthetic, mono-molecular drugs during the twentieth century, particularly in the United States, resulted in a dearth of interest in naturally derived whole foods as medicines during that period. However, in the last decade of the century many people in the West became increasingly interested in the potential health functionality of food. In fact many consumers began to purchase food for these very properties (Sloan, 2000). The folkloric documentation of these crops as curatives is highly informative with respect to the potential health functionality of alliums in the diet. Many properties mentioned in this way have been partially confirmed by modern medical investigation.
Historically allium crops have been used to treat a wide range of ailments, but the most prominent has been cardiovascular disease. Within the rubric of this disease, allium intake has been associated with significant reductions in blood pressure, cholesterol, and platelet aggregation (Block, 1992). Reductions in each of these parameters have been measured in clinically relevant feeding trials (Lawson, 1998), suggesting the potential for dietary intervention with edible alliums for cardiovascular disease. The antiplatelet, or antiothrombotic, potential of onions appears to be quite potent in vivo following onion administration in canines (Briggs et al., 2000).
Significant applications also have been noted for the antimicrobial property of allium extracts, which extends to bacteria, fungi, and viruses; reductions in carcinogenesis; reductions in blood sugar and increases in insulin; and general antioxidant activity (Lawson, 1998). For many of these properties the thiosulfinates have been implicated as the suspected causal agents, although much work is required to determine the involvement of other compounds with significant potential for biological activity. One such example is the flavonoids, which are present in high concentrations in colored onion tissues and have significant potential as antioxidants, show reductions in tumorigenesis, and remain at relatively high levels following thermal processing. Among the most prominent flavonoid in onions is the flavonol known as quercetin, which has shown great promise as an in vivo antioxidant and platelet inhibitor but less promising results when studied in vivo (Janssen et al., 1998).
It is important that the conversion of the ACSOs into thiosulfinates is enzymatically controlled. Therefore this reaction may be significantly hindered by cooking. Furthermore the thiosulfinates are volatile compounds that are likely altered and possibly reduced in concentration with thermal processing. For these reasons evaluation of the above-mentioned medicinal properties in feeding studies with cooked alliums are important in determining the extent of the health implications of dietary alliums.
Nutritional Components and Utilization
The dry matter content of many allium vegetables is in the range of 7 to 15 percent, with the exception of the 35 to 50 percent dry matter found in garlic (Brewster, 1994). Approximately 1 to 2 percent is protein, 0.2 percent is fat, and 5 to 12 percent is carbohydrates. Garlic bulbs may contain up to 6 percent protein. The caloric value is approximately 35 calories per 100 grams but is much higher for garlic. Onion contains a variety of secondary compounds, such as flavonols, anthocyanin pigments, sterols, and saponins (Brewster, 1994).
A large number of processed products are made from vegetable alliums, and these find their way into a wide array of processed foods. Concentrated oils are produced from steam distillation of fresh onion and garlic, and these are used to deliver onion or garlic flavor to processed foods. Dehydrated products make up a sizable portion of the onion and garlic processing industry. Dehydration requires fairly low temperatures due to the potential for carmelization under high heat. The fresh product is ultimately dried to 4 percent moisture. The resultant dried flakes can be further ground into powder and mixed with salt and calcium stearate to produce onion or garlic salt. In a number of countries, particularly in Asia, pickled allium bulbs are consumed. These are produced in a fermentation process and then bottled in vinegar and salt to make a sour pickle or vinegar and sugar to make a sweet pickle.
During the late twentieth century the popularity of mild, sweet onion bulbs rose dramatically in many markets around the world, indicating a desire for a less-pungent onion. Reduced pungency can be obtained through a combination of genetic backgrounds and favorable environments, in particular those soils where sulfur supply is low or is deliberately reduced. As discussed above, since the flavor pathway in alliums begins with the uptake of sulfate, high levels of soil sulfur can result in the production of more pungent onion bulbs (Randle et al., 1995). Only certain short-day onion cultivars grown in the southern United States have made significant inroads into the consumer market as a sweet or mild product, and these are not available year round in many markets. Since the accumulation and partitioning of sulfur and organosulfur compounds in alliums is not fully understood, it is difficult to predict the potential for mild onion cultivars in other market classes.
See also Vegetables .
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Irwin L. Goldman
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