VI. AQUATIC PLANTS FOR HUMAN FOOD

Some aquatic plants have a long tradition as human food. Boyd (1974, p.107) writes: “Rice, Oryza sativa is an emergent aquatic plant and the only vascular hydrophyte which is a major agronomic species. It is one of the world's main crops and forms the staple diet of over half the world's population. A small but important number of other aquatic crops exist, such as water chestnut (Trapa spp), lotus (Nelumbo nucifera) and watercress (Nasturtium officinale). Ipomoea aquatica is one of the few aquatics grown as a green vegetable.” (See also Edie and Ho, 1969, below)

What is needed is knowledge as to how many other aquatio plants, especially those which grow in profusion and thus are described as weeds, may be usefully converted to human food, because this is obviously more efficient and rewarding than having first to feed the plant material to an animal for conversion into edible products.

One of the pioneers of the concept of protein extraction from aquatic plants for human food is N.W. Pirie (see Chapter V).

^*Alford, L.W., 1952 Alligator weed - a good cattle food. Chemurg.Dig., 2(9):10–2

Alligator weed (Alternanthera philoxeroides) has so much mineral content that it should be introduced for human food, as salad or garnish. Eaten in small quantities it has produced no ill effects. It tastes better than some of the common salad greens and is much improved with salt and salad dressing.

Bates, R.P. and J.F. Hentages, Aquatic weed - eradicate or cultivate? Econ.Bot., 1976 30(1):39–50

In their review of the possible uses for aquatic plants (referred to in Chapter II), the authors include a section on “Human food potential”.

Boyd , C.E., 1968 Fresh water plants: a potential source of protein. Econ.Bot., 22:359–68

Discussing the amino acid content of some aquatic plants (see Chapter III), Boyd points out that the samples were somewhat low in lysine and methionine when compared to meat proteins. Phenylalanine was present in very small quantities in Alternanthera philoxeroides and Sagittaria latifolia.. Otherwise amino acid levels were similar to those for meat proteins and crop plant leaf protein isolates; so leaf protein from aquatic plants was of sufficiently high amino acid quality to be useful as a dietary supplement.

Boyd says that for natural vegetation to be useful as food, large stands containing sufficient quantities to justify harvest must be present. Stands should also be monospecific so that the quality of the product is somewhat predictable. Many aquatic weeds fulfil these conditions. “Utilization of water weeds as food could probably alleviate protein shortages in local populaces of many developing nations, but it is doubtful that these plants could contribute greatly to the total food supply of any nation… Exploratory research to assess the food value of the native aquatic flora should be initiated in nations that presently have severe protein shortages.”

Boyd, C.E., 1968a Evaluation of some common aquatic weeds as possible feedstuffs. Hyacinth control J., 7:26–7

Discussing the nutritive value of aquatic plants (see Chapter III) which often contain “as much or more crude protein, crude fat, and mineral matter, then many conventional forage crops”, the author states that people in tropical countries “located near weed infested waters could benefit greatly if the plants were used as food. Aquatic plants could temporarily help alleviate food shortages until lasting agricultural and social solutions are found.”

Edie, H.H. and B.W.C. Ho, 1969 Impoea aquatica as a vegetable crop in Hong Kong. Econ. Bot., 23:32–6

Ipomoea aquatica, water spinach, is related to the sweet potato. It has a long history of use as a vegetable in China where it is known as “Ong Tsoi”. It is widely distributed around the world. The paper describes both dry-land and wet-land cultivation of two local varieties of this plant in Hong Kong.

I. aquatica grows satisfactorily only when the mean temperature is above aboug 25°c and is therefore available in Hong Kong only during the summer months. The young stems, complete with leaves and petioles, are picked and used as a green vegetable, usually cooked by frying in oil, the final product being tender and crisp but without any very pronounced flavour.

Annual production of water spinach in Hong Kong varies between 3 and 5 million kg, and during the peak months it supplies 15% of local vegetable output, not counting the considerable quantities used as pig food.

Dry-land cultivation. Of the two local varieties of I. aquatica ‘Ching Quat’(green stem) is more resistant to cold and is grown during spring and early summer from March to May. The system of cultivation is very similar to that used for other leaf vegetables such as cabbage, where the crop is grown on raised beds separated by irrigation ditches. Seed is either sown directly onto these beds or seedlings from a nursery are transplanted. The plants are finally spaced at approximately 12-cm intervales by the time they are 15 cm tall. Heavy applications of fertilizer are given. A typical crop might receive the equivalent of 3 100 kg/ha of night soil every two to three days once the plants are past the seedling stage. Irrigation channels between the raised beds, water is splashed over the plants. This must be done three to four times daily when the plants are young, reducing to twice daily once the crop is established . Hand weeding is required frequently, especially when the plants are young. Water spinach grown in this way is ready to harvest 50 to 60 days after sowing and the entire plant is pulled up, washed and packed in baskets for transport to market. A small amount of the other local variety ‘Pak Quat’ (white stem) is also grown by the dry-land method during the summer. This variety gives a much larger plant of better quality, having fleshier, more succulent stems and petioles. Usually several harvests are taken from a single crop by cutting the shoots above ground level and encouraging lateral growth.

Wet-land cultivation. This is the much more common and important and important method of water spinach cultivation and provides most of the summer main-crop produce. Variety ‘Pak Quat’ is almost always used. Fields for wet-land cultivation are flat and surrounded by raised banks. It is common to find watercress (Nasturtium officinale) grown in the same fields during the cooler winter months. As germination and initial growth of I. aquatica is poor under water, seedlings are normally raised on a dry portion of a field. Some six weeks after sowing, cuttings are taken from the nursery plants for transplanting. Each cutting is a portion of stem about 30 cm long with seven or eight nodes and is planted out in the field at 40 cm spacing. Before transplanting, the field is prepared by flooding to a depth of 3–5 cm and tramping the soil into a liquid mud in which the cutting root quickly without further treatment. Water is allowed to flow gently through the field and as the crop becomes established the depth is increased to some 15–20 cm. Fertilizer application is similar to that used on dry–land water spinach, continuing as long as the crop is in the field. It is common to stop the flow of water through the field for 12 hours after each application. Overhead irrigation is not necessary, and weeds are no problem except in the nursery beds.

Growth of the crop is rapid, and 30 days after transplanting the first harvest is taken.The upper portion of the main shoot is removed at about water level, stimulating lateral growth of the plants which produce the typical horizontal shoots carrying vertical branches. In subsequent harvests, which are taken every seven to ten days throughout the summer , it is the vertical branches which are picked and marketed. During September the plants begin to flower and harvesting stops. The fields are drained to encourage the reproductive stage and eventually the ripe fruit are harvested, dried in the sun, then trodden gently to release the seeds which are stored in air-tight containers for use the flowing season.

^*India, 1952 Council of Scientific and Industrial Research, The wealth of India. Vol.3. Raw materials. New Delhi, CSIR, pp.130–4

Water hyacinth is reported to be used as a table vegetable in Formosa.

Koegel, R.G.,et al., 1973 Increasing the efficiency of aquatic plant management through processing. Hyacinth Control J. 11:24–30

Referring to their work on processing of aquatic plants for protein extraction (see Chapter V) the authors comment: “The protein concentrates may have a use for human nutrition as well as for animal nutrition.”

^* Malik, H.C., 1961 It pays to grow Singhara and Bhen. Indian Farming, 11(8):23–4

The author advocates the use of pools and low-lying areas on farms to cultivate water chestnut (Trapa natans (known locally as ‘singhara’) and lotus species (known as ‘bhen’). The culture of these plants and their uses are described.

Singhara bears fruit in summer. It is common in almost all states of Northern India but is extensively grown in Uttar Pradesh , Madhya Pradesh, Bihar and Orissa where the high rainfall results in the collection of water in ponds and other low-lying areas. The root, flowering stems, young fruits and seeds of bhen are are also used as food in different ways in different parts of India. The flowers and seeds of bhen are also used as medicine. Both these crops grow luxuriantly in standing water and mud in ponds, and are highly paying in the hands of people who have acquired the skill and technique to grow them.

Singharas are used both fresh and in the dry state; the mat is ground into flour which is used for highly nutritious and edible preparations. The fruit is rich in starch and manganese and contains more calcium, phosphorous and iron than rice. There are three varieties of singharas, green with spins, red with spine and green spineless. The red one is the sweetest. The yield of singharas varies considerably; it may be 14 000 kg/ha if the crop is good, but much less if the crop has been attacked by beetle and has not been saved in time. In 1962 it was possible to obtain a return of 1.7:1 on costs.

Bhen is a nutritious and palatable vegetable, alone or in combination with other vegetables. There are two varieties of bhen, red and white; both are equally delicious and palatable. The crop on an average yields from 3 700–4 700 kg of roots per hectare. In 1962 the author calculated that income from the lotus crop would be greater than the cost of growing it in the ratio of about 4:1. This included selling the vegetative parts, seeds and flowers, as well as the roots.

^* Oyakawa, N., W. Orlandi and E.O.L. Valente, 1965 The use of Eichhornia crassipes in the production of yeast, feeds, and forages. Proc.Int.Grasslands Congr., 9(2): 1707–10

The authors describe a method for growing yeast on water hyacinth as a substrate (see Chapter V). They say: “This yeast cream has a very agreeable odour and flavour and may even be used for human consumption.”

^*Pirie, 1960 N.W., Water hyacinth: a curse or a crop? Nature, Lond., 185(4706):116–7

This frequently cited paper briefly introduces the idea that water hyacinth, and especially the protein extracted from it, could be used as fodder for animals or even for human food. Referring to the basic principles of protein extraction, the author points out that the process is very simple. “The fresh leaf is pulped and the juice, neutralised if necessary, is separated. It carries with it much or most of the protein and this is coagulated and filtered off as a dark green cake which generally has little flavour after washing with water. It can be used as it stands in various dishes that are reasonably palatable, or a stable protein can be made by extracting the chlorophyll and lipids with a solvent.”

This paper has contributed as much as any towards orienting the minds of many people towards the possibilities of utilization of vigorous weeds such as water hyacinth and thus considering them as potential crops.

Pirie, N.W., 1966 Leaf protein as human food. Science, Wash., 152(3730):1701–5

The author reviews the history of extraction of leaf protein for human food and includes the water hyacinth among a range of plants which, either as whole plants or with leaves as by-products (e.g., sugar cane, jute and the sweet potato), have been neglected as a source of human food especially in developing countries. He points out that “leaf protein judged by its amino acid content appears to be nutritionally better than the seed proteins and as good as animal proteins other than the exceptional proteins of egg and milk… However the dark green colour and faint spinach or tea-like flavour gives leaf protein little immediate appeal to many people. It is easier to get immediate acceptance if the material is decolourized by solvent extraction, but experience shows that most people accept the novel appearance in a few weeks; the extra process of decolourization seems therefore to be both unnecessary and wasteful. Furthermore dark coloured and sometimes greenish food are already part of the normal cuisine in many countries.”

Quimby, P.C. and E.L. Robinson, 1975 Calcium oxalate in alligator weed and silverhead. Proc.Annu.Meet.South.Weed Sci.Soc., 28:264–5 (Abstr.)

The authors found that the calcium oxalate content of Alternanthera philoxeroides (alligator weed) was a maximum of 4.4% of the dry weight of nitrogen-fertilized terrestrial shoots. They state that: “This is well below the amount of soluble oxalate (about 20%) found in poisonous plants such as Halogeton. Nevertheless the level is high enough to be of interest to nutritionists and medical personnel in those countries, e.g. Burma, where alligator weed is being used for human food. About 4 g of oxalic acid is the minimum fatal dose for an average sized man. Thus 100 g of fertilized terrestrial alligator weed (on a dry weight basis) could potentially provide a fatal dose if the toxic principle were not solubilized and removed by boiling before eating. Diets with high oxalate-containing vegetables and low calcium have sometimes been implicated in the incidence of kidney stones.”

Sculthorpe, C.D., The biology of aquatic vascular plants. London, Edward Arnold, 610 p. 1967

This book has a section entitled “Hydrophytes as sources of food” (pp.514–7). The author begins by citing historical references to the use of the dried and ground seeds of water lilies Nymphaea caerulea and N. lotus in ancient Egypt for bread; the Egyptians also ate “the sweet, quince-like rhizomes of N. lotus which when cooked resembled egg-yolks”. The culms of Cyperus papyrus were also chewed raw, boiled or roasted. He says that in the Orient Nelumbo nucifera has been widely cultivated and its fruit and rhizomes used in a variety of cooked and fresh dishes for many centuries, as also have the rhizomes of Sagittaria trifolia, the floury seeds of the lily Euryale ferox, and the starch- and fat-laden horned fruits of Trapa spp. The fruits of T. bicornis, T. natans and T. incisa form a staple food in continental Asia, Malaysia and India.

Cultivated rice, Oryza sativa, is an aquatic food crop throughout the world. Its wild relative O. perennis is a grain food in parts of Africa. Canadian wild rice, Zizania aquatica, has been eaten by North American Indians for over 300 years and is now grown commercially in the U.S.A.

Still eaten in various tropical countries, when food is scarce, are the seeds, rhizomes, petioles and peduncles of Nymphaea caerulea, N. capensis,N. lotus and other lilies; the seeds of Enhalus acoroides, Victoria amazonica and V. cruziana; the swollen tubers of Aponogeton species and Cyperus esculentus,and the rhizomes of Typha latifolia and T. angustifolia. The rhizomes of several species of Sagittaria, and the rhizomes and seeds of Nelumbo lutea and Orontium aquaticum have been an important carbohydrate source for the North American Indians. The dried and powdered rhizomes of Butomus umbellatus, Calla palustris and Menyanthes trifoliate have been extensively used to make bread by the Eskimoes and other people of northern Eurasia.

In India young leaves, stems and roots of Ipomoea aquatica are eaten, and both there and in Malaysia the foliage of Ottelia alismoides, and all the plant (except roots) of Monocharia hastata and M. vaginalis are eaten. In Java juvenile plants of Limnocharis flava are a common vegetable. Several species of Ceratopteris are cultivated as a green salad crop in parts of Africa and tropical Asia. In temperate regions watercress, Rorippa spp., is cultivated.

Sculthorpe points out the dangers of bacterial and other diseases from eating inadequately washed vegetation from stagnant or slow-flowing waters, especially in the tropics.

Steenberg, B., 1968 Papyrus: problems in its utilization for pulp and paper making. In Pulp and paper development in Africa and the Near East. Rome, FAO, vol. 2: 865–76. FAO Accession No. 03952-68-MR

In this paper on Cyperus papyrus the author briefly refers to the use of the young roots as human food.

Taylor, K.G. and R.C. Robbins, The amino acid composition of water hyacinth (Eichhornia crassipes) and its value as a protein supplement. Hyacinth Control J., 7:24–5

Analyses of water hyacinth have been summarized in Chapter III. The paper discusses whether the amino acids of water hyacinth could serve as a supplemental source to improve the nutritional quality of grain proteins. “Lysine and tryptophan are the first two limiting amino acids in grains, therefore interest centres on the concentration of these two amino acids in water hyacinth. Since any tryptophan present would be destroyed by the acid hydrolysis, only lysine can be considered at present.”

The authors tabulate data, collected from the literature, of amino acid content of cow's milk, corn grits, leaf protein concentrate and water hyacinth compared to the FAO reference pattern:

Amino Acid Content (%)

They comment that water hyacinth protein shows deficient levels in only two of the essential amino acids, valine and methionine, as compared to the FAO reference pattern. A diet containing an adequate protein level will be balanced in lysine if it contains 4.2% of that amino acid in its protein content. Corn is deficient in lysine, containing only 0.8%. However water hyacinth contains 5.3% lysine, while milk contains 7.8%. It is evident that water hyacinth could serve to improve the lysine content of a corn diet. Leaf protein which contains 6.3% lysine has been reported to be an effective supplement for barley in pig rations. Then present evidence indicates that water hyacinth could be used to improve the lysine content of grains.

“If subsequent work on tryptophan content of water hyacinth reveals that this amino acid occurs in substantial amounts, the protein of hyacinth may be developed into useful dietary supplements for grain diets, especially in the underdeveloped countries. If a number of uses are found for the other constituents of water hyacinth so as to make processing economically feasible, the protein could be extracted. Such a protein concentrate could be used as an effective supplement to grains for animal or human consumption.”

U.S. National Academy of Sciences, National Research Council, 1976 Making aquatic weeds useful: some perspectives for developing countries. Washington, D.C., Agency for International Development, 175 p.

This book has a valuable chapter of 20 pages, including 17 illustrations (Chapter 14) devoted mainly to the use of aquatic plants as human food. The authors state that in most parts of the world there are swamp lands and shallow ponds totally unsuited to fish culture or agriculture. The present policy for such areas is to drain and “develop” them for uses not in accord with their nature. There is a need to find techniques that will allow us to use wetlands as wetlands. Cultivating aquatic plants is one possible way. The chapter highlights a few species that, with research, could become important aquacultural crop plants. Some of these plants are pernicious weeds. They should never be introduced to new locations or encouraged to spread in old ones. They are included for the benefit of those areas that already have them and would like to see them used productively.

The plant descriptions, distribution and uses which are briefly given include: water spinach (Ipomoea aquatica), watercress (Nasturtium officinale), Neptunia oleracea, Limnocharis flave, Ceratopteris thalictroides (a fern), Ottelia alismoides, floating rice (Oryza sativa), wild rice (Zizania aquatica), lotus (Nelumbo nucifera), Chinese water chestnut (Eleocharis dulcis), taro (Colocasia esculenta), swamp taro (Cyrtosperma chamissonis), arrowhead (Sagittaria trifolia), and spirulina (Spirulina platensis), an alga. Literature references to all these plants are also included.

^* Villadolid, D.V. and D.M. Bunag, 1953 New uses for water hyacinths. Philipp.Fish.Yearb., 1953:80–1, 241–2

Water hyacinths have been used as human food in the Philippines in war time. The soft white bud of the plant was eaten either raw, or as a salad, or as an ingredient in vegetable dishes. It was called ‘repollo’ (cauliflower). Under conditions of relative food abundance it is considered unlikely that it would still be used as food.

Zafar, A.R., 1976 Economic significance of certain species of Scirpus sp. In Aquatic weeds in South East Asia, edited by C.K. Varshney and J. Rzeska. Proceedings of a Regional seminar on noxious aquatic vegetation in tropics and sub-tropics, New Delhi, 12–17 December, 1973. The Hague, Dr. W. Junk, B.V., pp.387–91

Discussing various uses of species of the sedge Scirpus, the author states that “the fibrous roots of S. maritimus are said to bear high food value, while the culm can be used as fodder.”