Phytate

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Phytate is a storage form of phosphorus which is found in plant seeds and in many roots and tubers (Dipak and Mukherjee, 1986). Phytic acid has the potential to bind calcium, zinc, iron and other minerals, thereby reducing their availability in the body (Davis and Olpin, 1979; O'Dell and Savage, 1960). In addition, complex formation of physic acid with proteins may inhibit the enzymatic digestion of the protein (Singh and Krikorian, 1982). Iron and zinc deficiencies occur in populations that subsist on unleavened whole grain bread and rely on it as a primary source of these minerals. Deficiencies have been attributed to the presence of phytates.

Recently Marfo and Oke (1988) have shown that cassava, cocoyam and yam contain 624 ma, 855 mg and 637 mg of phytate per 100 g respectively (Table 7.4). Fermentation reduced the phytate level by 88 percent, 98 percent and 68 percent respectively, reduction being rapid within 48 hours but very slow after 72 hours processing. Thus processing into fermented foods will reduce the phylate level of root crops sufficiently to nullify its adverse effect. The loss of phytate during fermentation is due to the enzyme phytase, naturally present in the tubers or secreted by fermentative microorganisms. Processing into nbo or kokonte resulted in a loss of only 18 percent of phytate in cassava and 30 percent each in cocoyam and yam (Table 7.5). Oven-drying has only a small reductive effect on the phytate content compared with fermentation. Cooking also has a significant effect, resulting in decrease of phytate of 62 percent, 65 percent and 68 percent respectively in yam, cocoyam and cassava.

TABLE 7.4 - Phytate content of some unfermented and fermented tubers (mg/g)

¹Percentage loss in phytate is the decrease in phytate after 96 hours fermentation expressed as a percentage of total phytate.
Source: Marfo & Oke, 1988.

TABLE 7.5 - Effect of processing on phytate In cassava, cocoyam and yam

¹Percentage loss in phytate is the decrease in phytate resulting from each processing method expressed as a percentage of total phytate content.
Source: Marfo & Oke, 1988.

8. New frontiers for tropical root crops and tubers

In this chapter agro-industrial possibilities for the expanded utilization of root crops are reviewed. The success of efforts made to increase the production of tropical root crops and to promote their use as food will depend on market demand. Farmers will not be encouraged to produce a marketable excess if this leads to glut, spoilage, and low prices. Policy makers should not only promote policies to increase consumption of root crops as human foods and as animal feeds, but should also support research that will extend the utilization of these root crops. Efforts should be made to promote new technologies, appropriate for use by the rural population, to produce a variety of processed foods from root crops. This strategy will generate employment and improve incomes in rural areas. If demand is stimulated farmers will be encouraged to produce more root crops which can be converted to animal feed and industrial uses. Demand can be stimulated by development in three main areas:

• commercial dehydration of the root crops to produce flakes and flour that can be made into other food products;
• use of root crops as sources of industrial raw materials; and
• use of root crops as animal feed.

Commercial dehydration of root crops and their use

The peeled root is rinsed to remove excess starch, then cut into slices, blanched, blended to a puree and dried. Peeling can be effected by immersion in 10 percent lye solution or by steaming at high temperatures (150°C) for short periods.

The drier, which may be a heat exchanger or drum drier, can be fired by agricultural wastes such as coconut husks which are abundant and cheap in Southeast Asia. This reduces the moisture content from 70 percent to 12 percent. Improved preservation of root crops would increase their availability and reduce post-harvest wastage. Dried products require less storage space and have a longer shelf-life. They can be quickly reconstituted and prepared for eating, a factor of particular importance to urban consumers. Composite flour incorporating cocoyam has been used in extruded products such as noodles and macaroni. Similar processes could be used in the production of flour products from other root crops.

Processing will greatly increase the utilization of root crops. The flour can be used as a component of multimix baby foods and in composite flour for making bread. Research and development work on composite flour using root crops and other local products has advanced considerably in Colombia. Based on initial research in 1971-72, it was concluded that while rice and maize flours are preferable for use as non-wheat components in composite flours, cassava flour and starch also have good technical possibilities. The pilot work demonstrated that the production of bread from wheat flour diluted up to 30 percent with non-wheat components is possible on a commercial scale. But the large-scale introduction of such flours requires a concerted effort by both the public and private sectors to ensure the wide-spread availability at attractive prices of non-wheat raw materials. Expanded cassava production and lower prices are required if composite flour is to be economically attractive to millers and consumers (Goering, 1979).

Bread baked with composite flour from local resources would reduce the foreign exchange cost of imported wheat. This cost is particularly high in the Philippines, where a processing plant has been set up to convert 5 000 kg of fresh sweet potato into flour every day. The bread from this flour will contain more calories and a higher content of vitamin A and lysine than wheat bread and will conserve foreign exchange. If it can be marketed at a reduced cost it may help to improve the nutritional status of the population. Taylor (1982) estimated that cash benefits to farmers producing raw material for this plant could substantially improve with a guaranteed market. Since the market and the financing of the crop is guaranteed, the promotion of sweet potato as a cash crop will be more easily accepted.

Fresh root crops are rarely exported in appreciable quantities because of their high water content and perishability. Cocoyam is exported in small amounts from Fiji, Western Samoa, Tonga and Cooke Island to the United States of America, New Zealand and Australia for the Polynesian and Melanesian immigrants. Yams are also exported from Latin America and Africa for immigrants in Europe, but quantities are small and prices are high.

Use of root crops as a source of industrial raw material

Most of the world's starch supplies are derived from either grains (corn, sorghum, wheat, rice), the major root crops (potato, sweet potato, cassava, arrowroot) or the pith of the sago palm. While starches from these various plant sources vary slightly in their physical and chemical properties, they can be substituted for each other across a wide spectrum of end uses. Cassava starch must compete with other starches and relative prices, quality and dependability of supplies are basic considerations in the determination of market shares (Goering, 1979).

Cassava tubers can be processed as a source of commercial starch for use in the foodstuff, textile and paper industries. As a foodstuff the starch may be converted by acid and enzyme hydrolysis to dextrins and glucose syrups, but maize starch is often available at a lower price for these purposes. The bland flavour of cassava starch, its low amylose content, non-retrogration tendency and excellent freeze-thaw stability makes it suitable for use in food processing. Simple modification of Cassava starch by cross-bonding, or use of maize starch/cassava blends give properties ideally suited for use in a wide range of convenience foods. Starch from cocoyam has been recommended as a diluent in chemical and drug manufacturing and as a carrier in cosmetics such as face powder. It has a grain size similar to rice starch which is currently used for these purposes.

Cassava starch is manufactured in Thailand, Brazil and Malaysia and exported mainly to Japan and the United States of America. In 1975 the export level had reached about 100 000 tonnes per annum, equivalent in value to about US$30 million, with Thailand controlling about 50 percent of the market. In Thailand starch mills of various sizes have been set up including about 60 small mills with a unit capacity of two to three tonnes of starch per day, a similar number of modem mills producing 30 to 60 tonnes per day and a few industrial mills with a capacity of 100 to 150 tonnes per day. All these mills combine to give a total annual production of about 800 000 tonnes, of which about 700 000 tonnes are produced in modem mills. In Thailand a high proportion of the cassava starch produced can be utilized by local industries while the remainder is exported to other countries that have textile industries.

In several countries the traditional starch industry is an important source of starch for local users and provides a readily accessible market for tropical root crop production from small-scale farmers. A significant amount of rural nonfarm employment also is generated by the industry. Factories typically are small scale (one tonne of raw root throughput per hour), are equipped with locally fabricated machinery and use crude sedimentation processes which result in a product of variable quality. This local industry frequently finds it difficult to compete with large-scale, semi-automated factories (throughput of up to 20 tonnes per hour) or with factories using grain, sometimes imported at low prices, as the raw material. Since the price of cassava roots is normally governed by the pellet industry, only limited possibilities exist to reduce starch prices in order to make exports more competitive. The excess capacity in the Thai starch industry places that country in a good position to meet the requirements of any future new markets elsewhere. The possible export of root starches is less attractive for other developing countries that do not have established positions in the market (Goering, 1979).

Starch can be hydrolysed to glucose and used as a sweetener. Starches from root crops are often more expensive than those obtained from cereals such as rice and maize. Increased production could possibly reduce root starch costs and make it more competitive. Fig. 8.1 presents a diagram of a potential agroindustrial system for cassava utilisation.

Yeast fermentation of the hydrolysed starch extract of cassava or other crops gives a good yield of absolute ethyl alcohol, which can be used as an extender of petroleum-based fuels by blending up to 20 percent, without altering the carburettor of most petrol engines. Brazil initiated a National Alcohol Programme in 1975 to produce ethyl alcohol from agricultural raw materials, much of it based on sugar cane. The technology is now well developed and production has started. The starch-to-alcohol conversion ratio is 1.76 kg starch for one litre of alcohol. Sugar cane is the most energy efficient crop for alcohol production, but the use of cassava starch is increasing because it can be produced under conditions unsuitable for sugar cane. The cost from cassava was estimated at $0.57/gallon in 1978. PETROBRAS of Brazil was the first to test a large-scale plant to produce alcohol from cassava, with a full production capacity of 60 000 litres per day. Early runs were hampered by inadequate supplies of raw material and the high prices of cassava roots compared to the government-controlled price of petrol. During test runs 30 000 litres of alcohol were produced to specifications. Doubling cassava yield would make the process more economical. This could lead to an increased production of cassava and its use as a renewable energy resource (Hammond, 1977).

Figure 8-1 - An agro-industrial system for cassava

Plans are under consideration in Indonesia to develop a number of commercial alcohol factories using raw material from sweet potato, cassava and sugar cane. Special attention will be given to sweet potato production because it can be harvested twice in a year compared to cassava which is harvested only once a year (Yang, 1982). Guaranteed markets are a great incentive to farmers to produce more root crops.

Fermentation of commercial starches with Clostridium acetobutylicum yields about 30 percent of the starch dry weight of mixed solvents of butanol, acetone and ethyl alcohol, from which the pure products can be obtained by distillation. A process based on high-yielding cassava cultivars as a starch source could be financially attractive.

Some root crops have considerable pharmaceutical potential not yet exploited in developing countries. Yam contains steroidal sapogenins which are useful starting materials for the preparation of cortisone and related drugs. In Mexico various wild species of Dioscorea, especially D. mexicana, contain useful quantities of these sapogenins as high percentage of the dry matter, end they can be converted into progesterone intermediates. Other species are good sources of diosgenin, a starting material for the manufacture of corticosteroids. Oral contraceptives based on progesterone are now widely used in family planning in several tropical countries. The possibility exists for their national production from local materials. This prospect has been extensively reviewed by Oke (1972).

Some species of Dioscorea grown in Southeast Asia contain toxic saponins. They are made locally into a medicinal shampoo which is used to destroy head lice. They are also used in an insecticidal powder, similar in effect to derris dust, which is used to destroy rice parasites in paddy fields in Malaysia. D. cirrhosa contains enough tannin for commercial use. Some cultivars of D. alata contain 6 to 38 percent of tannin which is used in Southeast Asia for tanning fishing nets and in Taiwan for tanning leather, to which it imparts a red colour (Coursey, 1967).

There are many traditional medicinal uses of some species of Dioscorea among Africans, Chinese and Asians which were discovered by trial and error. The Zulus use an extract of D. sylvatica for the treatment of uterine and mammary disorders in cattle. More work is needed in this area. Apart from the academic interest, the practical rewards in pharmacy and medicine could be farreaching as in the case of the Indian snake-root, Rauwolfia serpentine Beuth. This was used for many years in traditional medicine by the Indians. It contains reserpine, which has found great use in modem medicine.

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