Jerusalem artichoke (Helianthus tuberosus), Girasole, Topinambour
Botanical name
Helianthus tuberosus L.
Family
Compositae.
Other names
Aardpeer (Nether.); Aguaturma (Sp.); Brahmokha (Beng.); Canada potato (N. Am.); Carciofo di Gerusalemme or di terra (It.); Cotufa (Philipp.); Elianto tuberoso (It.); Erdartischocke (Ger.); Hathipick, Hatichuk (Ind.); Kiku-imo (Japan); Knollensonnenblume (Ger.); Pera di terra (It.); Pseudokolokasia (Cy.); Root artichoke; Sunroot.
Botany
An erect, hardy tuberous perennial, but normally cultivated as an annual. It attains a height of 0.6-1.2 m in the tropics and 1.8-3 m in more temperate areas. The leaves are approximately 10-20 cm long, the lower leaves opposite and the upper alternate: they are finely pubescent beneath. The number of flowers varies greatly according to climate and cultivar, but the flower heads normally have a diameter of 6-7.5 cm with yellow florets.
The plant produces a number of small tubers which somewhat resemble potatoes, but have much larger eyes and are often knobbly. There is a limited range of cultivars, one of which was developed in France and has long fusiform tubers of good quality and flavour and is often designated as H. tuberosus var. fusiformis.
Origin and distribution
The Jerusalem artichoke originated in North America and was cultivated by the Indians in the north-eastern part of the continent in pre-Columbian times. It was introduced into Europe in the early 17th century and is now widely grown in both hemispheres, but in general is regarded as of rather minor importance.
Cultivation conditions
Although Helianthus tuberosus is of temperate origin it can be grown in the tropics. The yield is normally higher under long-day conditions, but the lower yield of short-day plants is largely compensated by the rapid growth rate under high temperatures (24-30°C), and moderate yields are therefore attainable in the tropics, though the cultivar is important (see Day-length). However, it is believed that there is sufficient variability in genotype to permit the breeding of high-yielding cultivars for tropical conditions.
Temperature - most cultivars require a growing season of at least 125 frost-free days and optimum yields are obtained where there is an equable temperature in the range of 18-26°C.
Rainfall - a fairly evenly-distributed rainfall of 125 cm or less is required. In dry areas irrigation may be necessary to start germination.
Soil - the plant is tolerant of soil conditions provided that the drainage is good; yields are poor on heavy clays, particularly if there is a danger of waterlogging. The Jerusalem artichoke is successfully grown in France on soils too poor for potatoes. For optimum yields a light or medium, well-cultivated loam is required and the application of 500 kg/ha of super-phosphate before planting, followed by a side dressing of 250 kg/ha of ammonium sulphate when the top growth reaches 0.3-0.6 m, has been recommended.
It has been reported that when Jerusalem artichokes are grown on nematode-infested soils a reduction of 45 per cent in the nematode population may be brought about.
Altitude - in the tropics the crop does best at elevations of 300-750 m but it is grown in India at elevations up to 1 200 m.
Day-length - short day-lengths (photoperiods of around 12 hours) are reported to favour tuber formation, but the increased time for photosynthesis under the longer days of more temperate climates gives somewhat greater yields, though, as noted earlier, this is partially compensated for by the more rapid growth under the high temperatures of the tropics.
Planting procedure
Material - propagation is vegetative, by using setts, which are sound, disease-free small tubers, or pieces of tubers: these should weigh approximately 50 g and have at least two or three eye buds. In some areas of the tropics it has been found that the tubers require a period of dormancy of up to 7 months.
Method - the setts are normally planted in rows, on the level, in small hills, or in ridges, about 7.5-10 cm deep, and are earthed up in the same manner as potatoes when the plants are about 0.3 m tall. Early weeding may be necessary, but the plants grow and spread rapidly, and later weeding should be avoided as the developing tubers are easily damaged. Once established, the crop is difficult to eliminate from the soil as tubers or parts of tubers are frequently left in the ground, and often replanting for subsequent crops is unnecessary.
Field spacing - for maximum yields in the USA a planting distance of 1-1.3 m between the rows and 60 cm between the plants has been recommended. In the tropics, eg India, a closer spacing is often used, 0.3-0.5 m between the plants and 0.6-1 m between the rows, while various intermediate spacings are noted in the literature.
Seed rate - in India approximately 550 kg of setts are used to plant one hectare; in many other areas the seed rate is approximately 320 kg/ha.
Pests and diseases
The Jerusalem artichoke is relatively free from serious attacks of pests and diseases in the field, but where drainage is poor root rot, due to Corticium rolfsii, can be troublesome. Powdery mildew (due to Erysiphe chicorianum f. sp. helianthi) has been reported from Bulgaria. Common diseases of sunflowers (Helianthus annuus) such as downy mildew, septoria leaf spot, rust and sclerotinia wilt are regarded as potential problems. The tubers are susceptible to the development of rots during storage. Botrytis cinerea, Rhizopus stolonifer and Sclerotinio sclerotiorum are reported to be fairly common; the last two can cause severe losses even when the tubers are stored at low temperatures. In addition, the tubers are sometimes infected by a fusarium rot, probably caused by Fusarium acuminatum.
Growth period
The crop usually reaches maturity in 4-6 months, although some cultivars mature in 2 1/2 - 3 months.
Harvesting and handling
The tubers are ready for harvesting when the leaves begin to wither and die. They are usually lifted manually with a fork only as required, since they can be 'field-stored' without any deterioration in their quality or flavour. When grown for pig feed the animals are often turned loose on the plot and root out the tubers. When cultivated on a large-scale the tops are frequently cut off before harvesting and sometimes the tubers are ploughed out, but this is not as efficient as manual harvesting owing to the irregular shape, small size, and distribution of the tubers.
Once harvested the tubers quickly shrink and deteriorate if kept at ambient temperatures, but if sound and disease-free can be successfully stored for 2-5 months at 0°C and 90-95 per cent RH.
Primary product
Tubers - each plant produces a number of small edible tubers, usually 10-20 cm long and 2.5-7.5 cm in diameter, 40-300 g in weight, frequently knobbly and of irregular shape, with large eyes and thin skins. The skin colour may be white, purple or red, depending upon the cultivar, but the white-skinned ones are generally the most popular.
Yield
On sandy soils in Europe yields normally average about 30 t/ha; in India yields are usually 12-25 t/ha, although under favourable conditions 37 t/ha can be obtained and yields as high as 150-160 t/ha have been reported from the northern Caucasus.
Main use
The tubers are eaten as a vegetable similarly to potatoes, though their irregular form makes then difficult to prepare; they are often used as a constituent of soups and stews.
Subsidiary uses
The tubers are grown for livestock feeding in many areas and are relished by pigs.
Secondary and waste products
The tubers can be used as a commercial source of fructose; it is claimed that the plant can produce more fructose per hectare than beet or corn. The main carbohydrate in Jerusalem artichoke is inulin, which is easily hydrolysed to fructose; in corn the carbohydrate is starch that has first to be converted to glucose and then fructose; in beet it is sucrose, that has to be hydrolysed first to glucose and fructose, the glucose then being converted to fructose. Industrial alcohol (ethanol) and 5-hydroxymethyl furfural are additional products. Flour can be prepared from the tubers, and also a beer-like beverage. A topinambur brandy has been made experimentally in Germany. Protein (1-2 t/ha) may also be obtained, and pulp suitable for animal feeding is a by-product of the fructose and ethanol processes. The green tops also are used as forage, and the stalks, when treated by a soda-chlorine process, yield about 20 per cent of pulp suitable for papermaking.
Special features
Analyses show a wide range of variation: published average figures for the edible portion are: energy 332.3 kJ/100 g; water 78.9 per cent; protein 2.44 per cent; fat 0.41 per cent; carbohydrate 15.8 per cent; fibre 0.7 per cent; ash 1.74 per cent; potassium 478 mg/100 g; calcium 10 mg/100 g; iron 3.7 mg/100 g; phosphorus 78 mg/100 g; thiamine 0.2 mg/100 g; riboflavin 0.16 mg/100 g; niacin 1.3 mg/100 g; ascorbic acid 4 mg/100 g.
The carbohydrate consists almost entirely of inulin and inulides with small amounts of starch, fructose and glucose: occasionally the starch may reach 30 per cent of the carbohydrate, but is usually considerably less. As inulin is not digestible by human beings, utilisable energy intake is much lower than suggested by the table, and allows the tubers to be used as a low energy food, which still satisfies volume intake and hunger. The tubers have also been recommended for diabetics. When used for animal feeding the nutritive value of the tubers has been quoted as: digestible protein 1.2 per cent; total digestible nutrients 15.9 per cent; nutritive ratio 12:3.
The green tops have given analytical figures of: dry matter 27.2 per cent; protein 1.4 per cent; fat 0.3 per cent; nitrogen-free extract 18.5 per cent; minerals 2.1 per cent; calcium 0.44 per cent; phosphorus 0.09 per cent; potassium 0.37 per cent; digestible protein 0.8 per cent; total digestible nutrients 18.1 per cent; nutritive ratio 21:6.
Processing
Fructose - has been prepared commercially by crushing the tubers to extract inulin and starch, which are hydrolysed with acid to fructose or glucose, respectively, in the ratio of about 75:25 (the ratio varies with the cultivar, the maturity of the tubers and the period of storage after harvesting, the fructose portion decreasing with age and length of storage). The hydrolysed juice is then neutralised with lime and the precipitate of calcium fructose derivative filtered off and treated with carbon dioxide to give fructose; average yield from fresh tubers is about 6 per cent by weight.
Recent improvements in technology involve more sophisticated extraction methods and the use of an enzyme preparation from Saccharomyces fragilis which contains a fructosidase (inulase), whilst a microbiological technique has been developed that will give complete hydrolysis of the inulin. As with acid hydrolysis the fructose: glucose ratio is about 75:25. After separation from the glucose the fructose syrup is treated with ion-exchange resins and activated charcoal to remove colouring, foreign flavours and non-carbohydrate impurities.
Industrial alcohol - fermentation of the carbohydrates in Jerusalem artichoke to produce ethanol is a long established process. Recent developments using cells of Kluyveromyces marxianus immobilised in beads of sodium alginate have greatly accelerated both the speed and efficiency of the process. Almost complete fermentation of the carbohydrate is claimed, with yields of 2 500-6 500 litres of ethanol per hectare, depending upon the size of the crop.
Major influences
The commercial development of the Jerusalem artichoke has been handicapped by the perishable nature of the tubers and the fact that the tubers are usually compared with potatoes, although very distinct as regards composition and flavour. Also, they can sometimes cause digestive disturbances. Other disadvantages are the high cost of harvesting and the fact that it is often difficult to remove all the tubers from the soil; in some areas it has become a noxious weed. Nevertheless, in some countries this root crop is of considerable importance as a livestock feed and as a source of natural sugars and alcohol; there are signs in the literature that interest could increase as new technologies are developed.
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