CHINESE WATER CHESTNUT, Matai, Waternut.
Eleocharis dulcis (Burm. f.) Trin. Ex Hensch. var. tuberosa (Schult.) Koyama syn. E. tuberosa Schult.
Aplid, Buslig (Philipp.); Cabezas de negrito (Sp.); Chtaigne d'eau (Fr.); Chikai, Dekang (Indon.); Kalangub (Philipp.); Kohekohe (Haw.); Mati (China); Nilaga (Philipp.); O-kuroguwai (Japan); O-yu, Peci (China); Pipi-wai (Haw.); Pi't'si (China); Potok (Philipp.); Po-tsai (China); Sibosibolasan (Philipp.); Tek, Teki-tik (Indon.); Wu-yu (China).
A variable, annual, stout, tufted, aquatic, sedge plant, characterised by its lack of leaves, their photosynthetic activity having been transferred to the numerous upright tubular septate stems, 50-100 per plant, which normally reach a height of 0.9-1.5 m. Inflorescences containing about 50 insignificant flowers are produced at the top of these stems. The female (pistillate) flowers appear when the stem tips reach 15 cm above the water and are followed later by the male (staminate) flowers. Tiny 'seeds', in fact achenes, are produced but are of no economic importance. Two types of subterranean rhizomes are produced. Rhizomes spread from the base of the plant: the first appear 6-8 weeks after planting, grow horizontally under the surface of the soil, and then turn upwards to form suckers and ultimately daughter plants; others, starting somewhat later, bend down and produce corms at the tip (one per rhizome), about 12 cm below the soil surface. The young corms are white, becoming scaly and brown when mature, subglobose, somewhat flattened, 1-4 cm across.
Two very distinct forms of E. dulcis are recognised. One is a wild form, which generally grows in stagnant water, produces very small, very dark skinned, almost black secondary corms and is sometimes referred to in the literature as E. plantaginea or E. plantaginoides. The second form occurs only under cultivation and produces larger, sweeter, secondary corms and was originally described as a separate species. E. tuberosa.
Origin and distribution
Eleocharis dulcis grows wild in many parts of India, South-East Asia and Polynesia. It was first cultivated in South-East China in humid monsoon areas and is now also grown commercially in Japan, Hong Kong, the Philippines, Hawaii and other Pacific islands, India and the southern USA.
Temperature - a long warm growing season is required, with at least 220 frost-free days, and a soil temperature of 14-15.5°C is necessary for germination of the corms.
Rainfall - the plant is aquatic and thrives in areas where there are well-controlled irrigation facilities giving a continuous supply of water throughout the year.
Soils - for optimum yields a rich clay or peaty soil with a pH of 6.9-7.3 is required; slightly more acid soils may be successfully neutralised with limestone. It has been shown that this crop has a high uptake of certain nutrients; in experiments in which corm production was approximately 4 700 kg/ha the uptake in kg/ha was nitrogen 108, calcium 6.9, magnesium 37.5, though requirements for phosphorus and potassium are relatively low. In the USA the application of a high grade complete fertiliser (including magnesium) at a total rate of 2.5 t/ha, one third or one half before planting, another third 8-10 weeks after planting, and the balance just prior to the development of the corms, has been recommended.
Altitude - the crop may be grown at altitudes from sea level up to 1 200 m.
Material - small corms are used.
Method - the corms may be planted directly into the field or, usually in the more temperate climates, started in protected nursery beds and hand transplanted when the top growth is 20-30 cm high. If planted direct in the field, the corms are planted in rows in holes 10-12.5 cm deep. This is often done manually with a hand trowel, but in larger plantings in the USA, the furrows are opened with a plough or courter and the corms dropped in at intervals of 75 cm and then covered with a covering plough or hiller. After planting the fields are flooded for 24 hours and then allowed to drain naturally; as soon as top growth reaches 20-30 cm the fields are again flooded and the water level kept to at least 10-12.5 cm throughout the growing season. Weeds are not usually a problem provided the soil has been well-tilled just before planting. In the USA the use of preemergence herbicides has been tried with success: 2,4-D amine at 1.9 kg/ha gave good weed control and was effective for 3 months.
Field spacing - in the USA a spacing of 75 x 75 cm has been recommended. In China a triangular spacing of 45-60 cm and 45 cm between plants is common practice.
Seed rate - approximately 500 kg of corms are used to plant one hectare.
Pests and diseases
Chinese water chestnuts are not normally subject to serious attacks from pests and diseases, though when grown on acid soils, ph 5.5, the plants may be attacked by a stem fungus, Cylindrosporium spp. In Florida, the most serious insect pest is the billbug, Calendra cariosa, and the crop is also attacked by a stem nematode, Ditylenchus spp. and by the awl nematode, Dolichodorus heterocephalus. In the Philippines, the crop is sometimes attacked by a grasshopper, Aiolopus thalassinus, but this pest has been effectively controlled by spraying with 2 per cent aldrin. Rodents, especially rats, can cause considerable crop losses at harvest, unless effectively controlled.
Chinese water chestnuts require a long warm growing season and the corms usually reach maturity in about 7-8 months; in many areas of China this is after the first frost has killed the green culms.
Harvesting and handling
Harvesting normally takes place after the culms have turned brown or been killed off by frost and by this time the corms have acquired a characteristic deep chestnut-brown colour. In most areas irrigation is stopped at least 3-4 weeks before harvest so that the ground dries, and the corms are carefully dug out by hand to avoid bruising. In the USA, in the harvesting of small plots, the soil is carefully lifted on to a 0.9 cm wire mesh screen and worked over with rubber pads or paddles, when about 98 per cent of the corms are left on the screen. These are picked off and dropped into water to clean them. In larger plantings a small plough is used which turns a furrow to a depth of approximately 15 cm. The furrow is then raked with a potato rake having rubber-covered prongs. The corms are carefully picked out by hand, washed thoroughly, and all damaged ones removed before they are air-dried, preferably in the shade. Harvesting may be delayed, since the corms do not deteriorate in the soil provided that there are no severe frosts.
In the USA commercial supplies of the dried corms are usually packed in 64-litre moisture-proof containers, which are sealed but not airtight. They can be kept satisfactorily at temperatures between -1 and 4°C for up to 6 months; at a temperature of about 14°C sprouting occurs.
Corms - the edible starchy corms have a dark chestnut-brown coloured outer skin and are usually rounded or onion-shaped and from I to 4 cm in diameter. The wild forms are usually the smallest and in general only corms 3 cm or more in diameter are commercially acceptable. The flesh is crisp and white with a characteristic flavour.
Yields in China are reported to average 20-40 t/ha and in the USA 28 t/ha.
Chinese water chestnuts are eaten as a vegetable either fresh or cooked and are an important ingredient of many Chinese food dishes. They are said to smell like sweet corn when boiled.
Certain cultivars are sometimes used for the preparation of starch, while very small corms are useful for poultry feed.
Secondary and waste products
In China the corms are used in traditional medicine. The dry stems can be used for cattle feed, mulching, as a packing material for horticultural products and for making baskets, mats, etc.
The corms show considerable variation in composition; an approximate analysis of the edible portion of fresh Chinese corms has been given as: moisture 77.29 per cent; protein 1.53 per cent; fat 0.15 per cent; nitrogen-free extract 18.9 per cent; reducing sugars 1.94 per cent; sucrose 6.35 per cent; starch 7.34 per cent; fibre 0.94 per cent; ash 1.19 per cent; calcium 2-10 mg/100 g; iron 0.43-0.6 mg/100 g; phosphorus 52.2-65 mg/100 g; thiamine 0.24 mg/100 g; riboflavin 0.007 mg/100 g; niacin 0.94 mg/100 g; ascorbic acid 9.2 mg/100 g.
The starch is similar to that obtained from sweet potatoes or cassava and has large grains up to 27 microns in length which may be rounded, have regular geometric shapes or be completely irregular. The juice extracted from the corms has been shown to contain an antibiotic principle, designated puchiin.
Canning - the corms are first washed and peeled, usually by hand, and then processed in a manner similar to potatoes. The recommended processing times are 30 minutes for No. 2 cans, 35 minutes for No. 2 1/2 and 45 minutes for No. 10 at 115°C, after an initial heating before sealing to 60°C.
Quick freezing - the washed, peeled corms are blanched for 4 minutes in steam at 99-100°C in single layers on wire mesh trays; they are then cooled immediately in an air-blast, packed into cans and frozen in a blast freezer at - 32°C then held at - 18°C for periods up to 12 months.
Starch - in China starch is sometimes extracted from the corms by a very primitive method. The corms are washed, then crushed, and the resultant starchy mass put in a fine bamboo basket which is set in a filter cloth and hung over a wood fire. The basket is then placed in a pan, water added, and the contents thoroughly stirred for 15 minutes. Three parts of starch milk are collected; the first contains the largest proportion of starch and is set aside to allow the starch to separate out, the rest of the starch milk being re-used to wash more pulp. After about 5 hours the starch has separated out and is collected and dried in the sun on bamboo trays.
Production and trade
There is considerable trade in Chinese water chestnuts in Asia, and prior to the embargo on imports from China into the USA shipments averaged about I 000 tonnes a year. These have been partially replaced by domestic production in the southern states, eg Florida, but the demand is reported to be increasing.
There is a growing demand for this speciality foodstuff, particularly in the USA, but the high cost of manual harvesting and the fact that the corms must be stored at low temperature to suppress sprouting are factors handicapping the commercial development of the Chinese water chestnut. It has been stated to be one of the more important crops that will thrive under Philippine conditions and is cultivated on a large scale in Laguna and Mindro (Luzon) and in Davon (Mindanao).
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