Oxalis tuberosa Molina.
Apio blanco, Cuba, Cuva (Venez.); Huisisai, Ibias (S. Am.); Macachn, Miquichi (Arg.); Papa extranjera (Mex.); Quiba (Venez.)
A small compact annual tuberous herb, usually 20-30 cm high, with cylindrical succulent stems which can vary in colour from various shades of green to a purplish-red, and normally arise from the base of the plant in the form of a cone or hemisphere. The leaves are spirally arranged, phyllotaxis 2/5, and may be green or purple, with or without hairs and also showing a great variation in form. The flowers are trimorphic with long, short or mid-length styles. The fruits are 5-celled capsules with 1-3 tiny seeds in each, but are rarely formed. The tubers are rhizomes, developing as terminal thickenings of the stolons, generally ovoid, 5-7.5 cm long, but highly variable in shape and size, and characterised by long transverse shallow depressions in which the eyes are situated. Oca is a very ancient food plant of the Andes and in Peru alone over 140 clones have been recognised.
Origin and distribution
Oca grows in the high Andes and is limited to southern Colombia,
Ecuador, Peru and northern Chile, between 4°N and 17°S and an altitude of 2 800 - 4 500 m.
Moderately cool day conditions and cold (possibly frosty) nights, frequently misty or cloudy weather, favour the growth of the plant. The optimum day-length for tuber formation in oca is 9 hours, while 13 hours is suitable for its vegetative development. Under short days the stolons penetrate into the soil and form tubers, under long days they grow into above-ground stems.
Material - usually propagated by means of cut pieces of tuber, each piece bearing one to three eyes.
Method - usually planted at the beginning of the rainy season and generally interplanted or rotated with other tuber crops (eg ulluco or potato), cereals or legumes. The Indians of the high Andean plateau cultivate it by hand in a manner similar to potatoes, and keep it free from weeds and earth it up three or four times during its growth.
Field spacing - planted in rows 50-90 cm apart with 20-40 cm between the plants.
Pests and diseases
In Peru the most serious pests reported are nematodes and the tuber borer Crisomelidas. Many fungi are reported to infect the crop, including Colletotrichum spp., Phyllosticta spp., Puccinia oxalidis, Urocystis oxalidis, Phoma oxalidicola, Septoria sp., and Cercospora oxalidiphila. A strain of arracacha virus B has been found to infect the plant, though no vector has been identified, and mycoplasma-like bodies have been found in seriously diseased material.
Oca tubers mature in approximately 8 months.
Harvesting and handling
The tubers are dug by hand and generally, especially those of the bitter varieties, are left to cure for several days in the sun to eliminate most of the bitterness due to calcium oxalate. Storage life is markedly affected by temperature: at 4°C storage life is stated to be 20 weeks, but at 21°C sprouting occurs and the flavour deteriorates, limiting storage life to about 12 weeks. Converting the fresh tubers to 'chua' (see Processing) gives a product with long storage properties.
Tubers - the starchy tubers are similar to potatoes in size, usually 5-7.5 cm in length and 2.5-3.75 cm in diameter, normally cylindrical or somewhat turbinate in shape. Fleshy overlapping leaf-scales give the tubers a cone-like appearance and conceal the buds in deep set eyes. There is great variation in the colour and flavour of the tubers, some of which are very bitter, others sweet, and the colour can be white, yellow or red.
Yields are reported to average 4-5 t/ha in Peru, but could be raised to 20 t/ha with improved cultivation methods and disease-free planting material.
Oca tubers are one of the principal carbohydrate foodstuffs amongst the Indians of the high Andes and are boiled, roasted or candied.
The semi-perishable tubers are often dehydrated to produce 'chua'.
The bitter forms of oca contain appreciable amounts of calcium oxalate, and it has been reported that most forms are an important source of calcium and iron in the diet of the Andean Indians, though the few available figures do not show exceptionally high proportions of these elements. Composition of the edible portion of oca tubers is: energy 264 kJ/100 g; water 83.8 per cent; protein I per cent; fat 0.6 per cent; carbohydrate 13.8 per cent; fibre 0.8 per cent; ash 0.8 per cent; calcium 4 mg/100 g; iron 0.8 mg/100 g; phosphorus 34 mg/100 g; vitamin A trace; thiamine 0.05 mg/100 g; riboflavin 0.07 mg/100 g; niacin 0.4 mg/100 g; ascorbic acid 37 mg/100 g.
Chua, a form of dried tuber, is commonly made, especially from the bitter varieties. For details of its manufacture see the section on Processing under Potato (Solanum tuberosum). For use, the chua is soaked in water and then eaten in soups or stews. An approximate analysis of chua is: water 13.6 per cent; protein 4.2 per cent; carbohydrate 77 per cent; fibre 1.8 per cent; ash 3.6 per cent.
Sweet types of oca are sometimes dried to form a product known as 'cavi', which is cooked with honey or cane sugar syrup.
Production and trade
Oca is consumed mainly by the rural population of the Sierra regions of the Andes and there is little information relating to production. It has been estimated that in Peru production is about 32 600 t/a, of which approxi mately 22 000 t are used for human consumption, 4 800 t are used for seed and losses due to spoilage amount to about 5 800 t.
In the high Andes oca is of considerable importance and often rivals the potato. However, unlike the latter crop, oca has not spread to other parts of the world, mainly because of the perishability of the tubers when exposed to lowland tropical temperatures. However, germplasm is being collected and is held under the supervision of IICA, Turrialba, Costa Rica.
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