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Page 83 Oca In the Andean highlands, only the potato is a more important root crop than oca (Oxalis tuberosa). But whereas the potato has spread to become the world's fourth largest crop, oca (pronounced oh-kah) is little known outside its ancestral home. This is unfortunate because oca tubers have great consumer appeal: brilliant colors and a pleasant flavor that many people find a welcome change from the potato. Oca tubers look like stubby, wrihkled carrots. They have firm, white flesh and shiny skins in colors from white to red. Most varieties have a slightly acid taste—they have been called “potatoes that don't need sour cream.” Others, however, give no perception of acidity. Indeed, some are so sweet that they are sometimes sold as fruits. An attractive, bushy plant with cloverlike leaves, oca is easy to propagate, grows luxuriantly, requires little care, and is exceptionally tolerant of harsh climates—under which its yield can be twice that of the potato. Moreover, it prospers in poor soils and at altitudes too high for most food plants. From Venezuela to Argentina, oca is still a staple for Indians living at altitudes between about 3,000 and 4,000 m. For them, oca tubers are principally sources of carbohydrate, calcium, and iron. Although scarcely known outside the Andes, oca has found a home in Mexico, where it has probably been grown for more than 200 years. 1 And in the last 20 years it has become popular in New Zealand, where the tubers—sold under the misleading name “New Zealand yam”—are now commercially cultivated. This provides an important glimpse of oca's potential future because the climate, latitude, altitude, and daylength regimes of New Zealand are similar to those of some farming regions of North America, Asia, and Europe. Thus, like the potato before it, oca could become a vegetable for temperate zones. Wherever it is cultivated, this crop is likely to be readily accepted. It lends itself to many culinary traditions because it can be prepared 1 Indeed, Mexico's big, bright-red tubers look better than most specimens found in oca's South American home. In the central highlands, where it is mainly grown, Mexicans call it “papa roja,” red potato.
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Page 84 in numerous ways: boiled, baked, fried, mixed fresh with salads, or pickled in vinegar. New Zealanders now serve it with their national dish, roast lamb. Although at present barely known beyond the Andes, Mexico, and New Zealand, it seems likely, during the coming decades, that oca will become a vegetable familiar to millions of new consumers. First, though, the crop needs improvement. The plants in the Andes are infected with viruses that depress yields and could infect other crops such as potatoes. Fortunately, simple ways to remove viruses are available, and now, before the plant begins to spread, is the time to apply them. Daylength requirements may slow up the crop's acceptance in new areas. Most Andean oca varieties have specific photoperiod responses that limit their culture to equatorial latitudes. If grown elsewhere, they form no tubers. Before oca's potential can be achieved worldwide, varieties that are either daylength neutral or adapted to long days must be located. 2 The plants of New Zealand, the southern end of the Andes, and perhaps Mexico seem likely sources for these. PROSPECTS Andean Region. Despite the fact that oca is an important food and cash crop in upland Andean areas, it suffers unwarranted cultural scorn because it is considered a “poor-person's” plant. Education could rid oca of its “poverty food” stigma, and, given a change in attitude and better marketing, the plant is likely to become a major food, not just for highland Indians but for everyone in the region. For some countries, it also might eventually become a valuable export. Oca already yields well, but research in Britain indicates that elite virus-free stocks give much greater yields. The use of these in the Andes could therefore bring rapid economic benefits to highland farmers, who are among the most destitute in the western hemisphere. Other Developing Areas. Oca seems particularly promising for the highlands of Central America, Asia, and Africa. 3 It is also likely to become a valued crop in other cooler areas of the Third World, 2 The potato probably had similar limitations initially and became a major crop in Europe and North America only after types that would tuberize during long days were selected (see next chapter). 3 Currently, researchers are conducting tests in Nepal; British scientists are testing varieties for introduction to the Ethiopian highlands. Information from S. King and A.A. Brunt.
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Page 85 ~ enlarge ~ The oca plant. (S. King) such as northern India, northern China, southern Africa, and the sprawling region from southern Brazil to Argentina. The plant will probably perform poorly in tropical lowlands because it is susceptible to heat, and also because its soft, fleshy stems are easily infected by bacteria. Industrialized Regions. Given New Zealand's experience, oca seems poised to become a commercial crop in warm-temperate areas of Australia, North America, Japan, and Europe. Products from oca “chips” to oca “fries” seem possible. It will never reach the potato's overall level of consumption, but it has the potential to become a well-known, widely enjoyed, and profitable crop. USES Oca can be used in many ways. In the Andes, a few types are eaten raw. Most, however, are added to stews and soups; steamed, boiled,
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Page 86 ~ enlarge ~ Oca tubers come in many colors and sizes, but they are all cylindrical and have distinctive wrinkles. (S. King) or baked like potatoes; or served as a sweet, either plain or candied. In Mexico, oca is commonly sprinkled with salt, lemon, and hot pepper, and eaten raw. It is also made into bottled preserves (often in vinegar). In the Andes, the tubers are often placed in the sun for a few days, during which they become sweet, and the amount of glucose can nearly double. Bitter varieties are almost always converted into dry products (cavi or caya), during which the bitterness disappears to leave bland-tasting products that can be stored without refrigeration. The freeze-drying chuño process—which involves soaking the tubers in water, exposing them repeatedly to freezing night temperatures, and squeezing the water out by stamping on them—also removes the bitterness. Because of their high dry-matter content (normally about 20 percent, but sometimes as high as 30 percent 4 ), the tubers may have potential for producing starch or alcohol. Theirs is a quality starch with promise for the food industry. Oca plants can also be used as stock feed. Livestock—notably pigs—relish both tubers and foliage. 4 Information from N.W. Galwey.
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Page 87 NUTRITION Oca tubers show high variability in nutrition levels. However, by and large they have a nutritional value as good as or better than that of potatoes. On average, they contain 70–80 percent moisture, 11–22 percent carbohydrate, and about 1 percent each of fat, fiber, and ash. Protein levels vary greatly among different types; certain high-protein tubers contain more than 9 percent on a dry-weight basis. This is excellent for a root crop, and the protein is of high quality, with a good balance of essential amino acids (valine and tryptophan are the limiting ones). The carbohydrate is usually rich in sugar and is easy to digest. The sour or “bitter” tubers contain amounts of oxalic acid varying up to 500 ppm. Some sweet types, on the other hand, have only an ~ enlarge ~ Produce auction, Auckland. In New Zealand, oca has become popular, and its production and marketing are well advanced. (G. Samuels)
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Page 88 insignificant trace (79 ppm)—merely one-twentieth that found in standard potatoes. 5 AGRONOMY Oca is usually propagated by planting whole tubers; however, aerial stems are sometimes employed. Apparently, it is never propagated by seed. Like potatoes, the edible tubers form on subterranean outgrowths of the stem, called stolons. Farmers mound dirt over the base of the plants to encourage stolon formation, which starts usually about 4 months after planting and peaks at about 6 months. As more stolons appear higher up the stem, more dirt is heaped over them. 6 The tubers normally take another 2–3 months to mature, after which the aboveground part of the plant usually dies back. Mexican types apparently mature more rapidly—6 months from time of planting to time of harvest. Photoperiod and temperature both influence the rate of plant growth and tuber formation. Although many types collected from Peru and Ecuador are daylength sensitive, the ones in New Zealand (most likely originating from southern Chile in the 1860s 7 ) are apparently unrestricted by daylength. They are grown commercially, for example, at Feilding (latitude 40.13°S) and Invercargill (latitude 46.24°S). 8 Oca tubers are harvested like potatoes, but they tend to be more fragile, and they must be dug and handled carefully. Yields average about 5 tons per hectare under traditional Andean husbandry. Commercial yields average 7–10 tons per hectare in New Zealand and Peru. These figures probably do not indicate the plant's true potential, however. Reports from Cuzco indicate that, under experimental conditions in small plots, some clones yielded 40 tons per hectare. 9 Oca seems less affected by pests and diseases than potatoes. However, this is probably because of the small scale of its current cultivation. Even so, problems do occur. In the Andes, the principal insect pest is a tuber-boring beetle related to the potato beetle. 5 Information from S. King. Spinach, by comparison, can contain 5,000 ppm of oxalic acid. 6 In the Andes, farmers usually do it at about 5 weeks after planting and then again at about 9–10 weeks as new stolons appear higher up the stem. 7 Information from P. Halford. 8 There is a possible relationship between photoperiod and temperature. For example, in some instances low temperatures may reduce a plant's sensitivity to short daylengths. Indeed, a few researchers think that low temperatures might sometimes be more important than daylength for stimulating tuberization. Information from A.J. Martínez. 9 Information from H. Cortes Bravo.
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Page 89 Nematodes also affect the crop. As noted previously, viruses cause chronic yield reduction. Also, mycoplasma-like organisms have been identified in severely diseased Bolivian samples. Although various fungi occur on oca plants, they seem unimportant in the field. After harvest, however, molds can cause major losses. Although more perishable than potatoes, oca tubers, if properly handled, can be stored at room temperature with little deterioration for several months. Dry tubers easily store over winter, and they will sprout precociously when temperatures rise in the spring. LIMITATIONS The major limitations have already been mentioned. They are viruses, daylength restrictions, and the presence of oxalates. Viruses. Virus infections are a major constraint and must be removed before the plant can be used outside its current locations. Daylength. For cultivation in many new locations, types that are daylength neutral or that come from an equivalent latitude must be used. In this regard, ocas from Mexico and New Zealand are important sources of germplasm for nontropical latitudes. Also, the plant is found in Chile at least as far south as the island of Chiloe, and this, too, is a promising source of daylength-insensitive clones. Oxalic acid. Types for use in new areas must be carefully chosen. Whereas some have been shown to have much lower levels of oxalic acid even than potatoes, many traditional varieties in the Andes accumulate oxalate. All levels, however, are far below those of spinach and some other widely eaten green vegetables. RESEARCH NEEDS Oca offers superb research opportunities for root crop specialists, graduate students, the Centro Internacional de la Papa (CIP), and other agricultural research facilities throughout the Andean region and the world. Now is the time to seize the opportunity to develop a thorough understanding of the crop and to explore its promise. To help oca achieve its potential, researchers should assess types gathered from throughout the Andes, New Zealand, Mexico, and
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Page 90 Europe. 10 Existing collections need to be maintained, and the diversity of oca from distinct geographical areas systematically evaluated. In this, as in many Andean crops, there is great potential for international cooperation. To aid in the wider testing, procedures for “virus cleaning” should be publicized, 11 and selections of virus-free clones made available for direct use in the Andes, Mexico, and New Zealand, as well as for experimental trials elsewhere. Institutes, corporations, and nurserymen, both inside and outside the oca-growing nations, can foster this. Attention should be paid to advances already made by Andean researchers. Individual plants should be closely examined for uniformity, growth patterns, and desirable tuber qualities such as size, shape, shallow eyes, and color. There is much genetic diversity in this crop. However, flowering and seed set are uncommon, which limits the opportunities for genetic improvement. Techniques for seed production are needed. Without them, breeding will be slow and difficult. Trial shipments of New Zealand oca have been turned away by U.S. agricultural inspectors on the grounds that the tubers “look like potatoes.” If export trade is to be developed, the possibility of introducing diseases to the potato industry will have to be resolved. Oxalidaceae and Solanaceae are not closely related families, and oca viruses probably will not affect potatoes. Research is needed to settle the question, one way or the other. SPECIES INFORMATION Botanical Name Oxalis tuberosa Molina Family Oxalidaceae (oxalis, or wood sorrel family) Synonym The name Oxalis crenata is used in some older literature, but is now assigned to another species. 12 Common Names Quechua: O'qa, okka Aymara: apiña, apilla, kawi Spanish: oca, ibia (Colombia); quiba, ciuba, ciuva (Venezuela); huisisai, ibias (South America); papa roja (Mexico) English: oca, sorrel; kao, yam (New Zealand) French: truffette acide German: Knollen-Sauerklee 10 For over 100 years, oca has been grown in Britain and continental Europe as a homegarden ornamental. It was also once grown in the south of France as pig feed. Until a decade or so ago, it sometimes appeared in Paris produce markets. Although available through nurserymen, few people realize that the tubers are edible. 11 Researchers in Britain have successfully propagated virus-free plants using meristem (tissue) culture. Initial observations indicate that these outgrow and outyield normal (infected) stocks. Information from A. A. Brunt. 12 O. crenata is a diploid with 2n = 14; oca is an octoploid with 2n = 64. Information from A.J. Martínez.
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Page 91 Origin. Oca may be one of the oldest Andean crops. Tubers have been found in early tombs on the coast, hundreds of kilometers from its native highland habitat. 13 Although wild relatives exist throughout much of South America, the ancestral plant is unknown. Description. Oca is a compact, perennial, tuberous herb, usually 20–30 cm high, with cylindrical, succulent stems that vary in color from yellow and green to a purplish red. The stems normally rise from the base of the plant. Oca has an efficient plant “architecture” for photosynthesis because of its extremely high leaf area (due to its growth form and leaf angle, shape, and thickness). Under long days, the stolons grow as above-ground stems; under short days, they penetrate the soil and form tubers. As days shorten, the stolons swell into rhizomic tubers that generally range in length from 3–20 cm and are produced in abundance. As in the potato, tiny scale leaves border the deep-set eyes. Horticultural Varieties. The Andean Indians recognize about a dozen cultivars and more than 50 distinguishable types. The Colección de Ocas—over 400 accessions—is housed at Cuzco, Peru. There are also major collections at Puno and Huancayo, Peru, and Quito, Ecuador. Environmental Requirements Daylength. The common Andean types generally require days shorter than 12 hours to initiate tuber formation; in most cases longer days promote only foliage development. Rainfall. In the Andes, the crop is grown where annual rainfall is 570–2,150 mm, distributed evenly throughout the growing season. Altitude. Oca grows near sea level in New Zealand, but in the Andes of Peru, Bolivia, and Ecuador, it is found at 2,800–4,000 m elevation. Low Temperature. Although oca is resistant to low temperatures and thrives in moderately cool climates, freezing kills back its foliage. However, the plant's tubers have exceptional regenerative capacity. High Temperature. Temperatures above about 28°C cause the plant to wilt and its leaves to die; resprouting can occur, but tuber production is consequently delayed. Soil Type. Oca seems indifferent as to soil and is reported to tolerate acidities between about pH 5.3 and 7.8. 14 Not unexpectedly, a light, rich soil is best for tuber production. 13 There are even some tantalizing suggestions that oca may have reached Polynesia before the arrival of European ships. 14 Information from J. Duke.
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