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Potassium is the major mineral in most root crops while sodium tends to be low. This makes some root crops particularly valuable in the diet of patients with high blood pressure, who have to restrict their sodium intake. In such cases the high potassium to sodium ratio may be an additional benefit (Meneely and Battarblee, 1976). However, high potassium foods are usually omitted in the diet of people with renal failure (McCay et al., 1975). As root crops are low in physic acid relative to cereals, those minerals liable to inactivation by dietary physic acid are more available than in cereals. This is especially important for iron, which has been found to be 100 percent available in banana (Marriott and Lancaster, 1983). In addition the high vitamin C concentration in some root crops may help to render soluble the iron and make it more available than in cereals and other vegetable foods. In the United Kingdom the iron supply from potato ranks third of all individual food sources, accounting for up to 7 percent of the total household dietary iron intake. True e, al. (1978) found that 150 g of potato will supply 2.3 to 19.3 percent of the dietary requirement for iron recommended by the Food and Nutrition Board of the National Research Council of America. there is some doubt about the availability of calcium and phosphorus in cocoyam owing to the oxalate content.
An important, often unrecognized, mineral contribution that potato can make is in the appreciable amount of iodine it contains. This could be significant in goitrous areas of Africa and Asia where iodine intake is low or marginal. Since over 96 percent of the zinc in potato is available, again due to low levels of phytate, potato can also significantly contribute this mineral. Yam can supply a substantial portion of the manganese and phosphorus requirement of adults and to a lesser extent the copper and magnesium. As indicated in Table 4.12, a hectare of sweet potato will provide the calcium requirement for 60 times as many people and 12 times the requirement of iron as the same area of land planted with rice.
Apart from the yellow variety of sweet potato, which contains a high amount of beta-carotenes (up to 30 mg retinol equivalent percent) most of the other root crops contain only negligible amounts. However, their leaves contain a substantial amount of beta-carotenes that could contribute significantly to the daily requirement of vitamin A, especially for children, thereby helping to eradicate the ocular diseases affecting about six to eight million children from Asia, Africa and Latin America. Dietary retinol obtained from the consumption of animal foods is relatively expensive and contributes about 14 and 20 percent to the vitamin A intake of people in Asia and Africa respectively. Beta-carotenes from leaves such as sweet potato or cassava, which contain about 800 mg/100 g and which is about the same as beef liver, contribute as much as 86 percent in Asia and 80 percent in Africa.
The quantity of root crop leaves required to meet the average daily requirement of retinol differs considerably, with Cassava requiring only 50 g, dark green vegetable leaves 73 g, sweet potato leaves 78 g and taro leaves 133 g.
Cassava leaves have a crude protein content of 20-35 percent on a dry weight basis. The quality of the leaf protein is generally good though it is deficient in methionine. Cassava leaves are low in crude fibre and relatively high in calcium and phosphorus. Cassava varieties in which the tuber contains cyanogenic glycosides usually show a similar content in their leaves.
Like many other foods, roots and tubers are rarely eaten raw. They normally undergo some form of processing and cooking before consumption. The methods of processing and cooking range from simple boiling to elaborate fermentation, drying and grinding to make flour, depending on the varieties of roots and tubers.
The basic purpose of these methods is to make roots and tubers and their products more palatable and digestible and to make them safe for human consumption. Processing also extends the storage life of roots and tubers, which are often highly perishable in their fresh condition. Processing also provides a variety of products which are more convenient to cook, prepare and consume than the original raw materials.
Women play a very active role in all the stages involved in the production and processing of root crops. Assessment from five states in Nigeria indicated that in cassava production women on average complete 34 percent of the field preparation and 77 percent of the planting of cassava, 86 percent of the weeding and 77 percent of the harvesting. The post-harvest activities of processing, storage and marketing, are undertaken mainly by women, though recent studies indicate that men are beginning to take an interest in the processing of root crops through the purchasing and management of electrically operated grinding machines.
Although raw sweet cassava is occasionally eaten in the Congo region, Tanzania and West Africa, cassava is not generally consumed raw. A large variety of processing techniques have been developed in different parts of the world resulting in a wide variety of products. Those techniques serve not only to render the root palatable, and in many cases storable, but they also have the effect of eliminating or reducing cyanide (HCN) content to acceptable levels. Many processes such as soaking and fermenting have been designed specifically to detoxify the root. Others, such as boiling and roasting are designed to make cassava products more palatable. The degree of reduction of cyanide in the final product varies greatly with the type of processing techniques used. Many of the complex techniques used around the world today originated in South America and were introduced to the other regions with the cassava plant, or in some cases at a later date. Other processes have been developed independently in the producing countries.
Roasting, boiling, frying
In Latin America roasting is the simplest technique, but it is not commonly practiced. It is used only when no cooking utensil is available. The whole roots are buried in hot ashes or placed in front of the fire until cooked through.
Sweet cassava roots are more often prepared by boiling and are eaten either hot or cold or sometimes mashed. These are general methods all over the world. In Latin America, a soup or stew called cancocho or cocido is prepared by boiling cassava roots with vegetables. The technique of deep frying cassava in fat is thought to have been introduced by Europeans. In Uganda, the roots are peeled, washed, wrapped in banana leaves and steamed in a pan (Goode, 1974). Roasting sweet cassava in ashes is widely practiced in Africa. In South Africa, bitter varieties are also roasted but are first peeled and rubbed with tobacco. In Zambia roots are often soaked before roasting. Fried cassava is prepared after peeling, by washing, slicing and then frying in oil.
Grating, pounding, baking, or boiling
In Latin America, cassava roots arc grated on spiny palm trunks or pounded into a pulp. The pulp is then squeezed by hand and cooked in a variety of ways. Several groups shape the pulp into pies or cakes which are then baked in hot ashes, sometimes being wrapped in a protective covering of leaves before baking. Some groups, such as the Nambicuara, sun dry the pulp balls, wrap them in leaves and place them in a basket or bury them in the ground, to be used at times of food shortage. After several months, they retrieve the fermented balls and cook them by baking in hot ashes. The cassava pulp is boiled either by dropping the pie or balls into the boiling water or by stirring the pulp into water to form a sort of porridge. Porridge is sometimes made as a preliminary ste in the preparation of flour. The pulp is boiled and skimmed off with a plated spoon, strained through a mat of thin sticks and finally roasted in a pan to make flour.
Steaming and fermenting peujeum. A traditional product prepared in Java is peujeum (Stanton and Wallbridge, 1969). The peeled cassava roots are steamed until tender, allowed to cool and then dusted with finely powdered ragi, a rice flour starter culture flavoured with spices. The cassava mash mixed with ragi is wrapped in banana leaves in an earthenware pot and left for one or two days to ferment. The peujeum has a refreshing acidic and slightly alcoholic flavour and is either eaten immediately or baked.
Sun drying and pounding or grinding Into flour
Cassava roots, which may be soaked in water first, are sun dried and pounded into a noun This seems to be a general method everywhere.
In the preparation of fuku in Zaire the dried roots are pounded with partially fermented corn, the quantity depending on the season. The resulting flour is roasted on a Rat tray to stop further fermentation of the mixture, which was initiated by the fermenting maize. The flour is eaten as a gruel prepared in boiling water. Cassava flour is the basis of several other foods. In the preparation of nsua the flour is mixed with water and filtered through a jute bag. After removal of the water the paste is wrapped in a leaf and eaten raw. Ntinga is prepared in a similar way except that a portion of the paste is boiled in water and mixed with the remaining uncooked paste. The mixture is wrapped up in a leaf and cooked again.
Grating, pressing and roasting or baking to make flour or bread
These methods are widely used to prepare cassava flour or cassava bread in tropical America. Details vary from one group to another but the methods fall into two main groups depending on whether or not the roots are given a preliminary soaking:
Unsoaked roots. The process is very laborious and takes two or more days. The freshly dug roots are first washed to remove excess soil and then peeled. The tubers are then reduced to a pulp, normally by grating, but sometimes by crushing in a mortar or between stones. The pulp is squeezed with a variety of devices to expel the liquid. The moist pulp is left overnight in containers. Next day it is rubbed through a sieve to remove any coarse fibres. The pulp is then cooked in one of two ways depending on whether bread or flour is needed.
To prepare bread, the cassava pulp is placed on a hot clay or stone griddle, pressed down into a thin layer and toasted on each side. The large, flat circular cakes are known as cassava bread, cassava casabe, beigu or couac depending on the locality. When fresh, the bread is soft inside and some people prefer to prepare it daily. More commonly, cassava bread is sun cried for several days during which time it hardens through. It can be stored in this form for several months. Cassava bread is normally eaten dipped into gruel or stew, which serves to soften it (Jones, 1959). Other types of bread can be made by adding additional ingredients to the cassava, for example in Brazil a special bread is prepared by adding pounded or grated Brazil nut to the cassava pulp.
To prepare flour, the cassava pulp is stirred continuously while cooking on the griddle in order to prevent lumps forming. The resulting flour also stores well and is variously known as farinha de mandioca, farinha seca, farinha surruhy, kwak or koeak. It may be eaten dry, mixed with hot or cold water to make a paste or gruel or mixed with other foods. Various modifications and other methods, both simple and complex, are also used.
A traditional Philippine dish based on the cassava root is known as cassava rice or landang. Freshly dug roots are peeled and grated, the grated mass is put into jute sacks and pressed between two wooden blocks to squeeze out the juice. It is then put into a winnowing basket and whirled until pellets are formed. At intervals the pellets are sieved and those not passing through are put back and whirled again. The pellets are dried on a mat and then steamed in a coconut shell on a screen mesh placed over a vat of boiling water. The cooked pellets are placed in the winnowing basket and separated by hand. In an alternative process the peeled roots are submerged in fresh, clean water in an earthenware jar or wooden vessel for five to seven days until soft. They are then macerated, the fibres are removed and the remainder is dried and made into pellets as described. The pellets from both methods are dried thoroughly in the sun for three to five days and stored until needed. Cassava rice can be eaten without further cooking.
Soaked roots. In Latin America the cassava tubers, either peeled or unpeeled, are soaked in water usually for three to eight days but sometimes even longer IO allow some fermentation to occur. After removal from the water the peels are removed where necessary and the softened roots are either crushed by hand or grated to m eke a pulp and processed as for farinha seca. This method is also used to prepare cassava bread but more often the end product is cassava flour. Many variations of this basic process exist.
In West Africa, after fermentation the cassava is pounded or ground to produce a paste which is consumed or stored depending on the country. In parts of Nigeria, the paste is boiled for 20 minutes and then removed for more pounding. In Cameroon, the wet paste is divided into two portions and wrapped in leaves before cooking. In Mozambique cassava paste is mixed with flavouring ingredients including onion and salt, before being wrapped in leaves and boiled.
The preparation of pastes by pounding cassava is a peculiarly African process not practiced in South America. The pastes are consumed in a variety of forms, the best known being fufu. The term fufu and its variants are widely used in West Africa to refer to a sticky dough or porridge prepared from any pounded starchy root including yam, cocoyam and cassava.
In the preparation of fufu the roots are peeled, washed, grated and left to ferment for two to three days. To ferment the cassava the grated mass is either simply left to stand (Doku, 1969) or put into sacks and weighed with stones to squeeze out the juice. The resulting dough is used at once for cooking or it is stored in basins covered with cold water which is changed daily. The resulting product is consumed in different ways in different countries accompanied by stew or soup.
Gari the most popular cassava product consumed in Africa. To prepare gari cassava roots are washed, peeled and grated. The pulp is then placed in cloth bags or sacks made from jute and left to ferment, the fermentation time varying from three to six days. It is the fermentation process that gives gari its characteristic sour flavour, which distinguishes it from Brazilian farinha. During this stage pressure is applied to squeeze out the cassava juice. The cassava pulp, at about 50 percent water content, is taken out of the sacks and sieved to remove any fibrous material. It is then heated or "garified" in shallow pans and stirred continuously until it becomes light and crisp.
Gari is eaten in a variety of forms. It is sometimes eaten dry or made into a paste. More commonly it is steeped in cold water, thus causing the particles to swell and soften but retaining their granular form. Alternatively, gari is mixed with cold water to make a thin gruel which is drunk with milk. A popular way of preparing gari in Nigeria is to soak it in boiling water to form a thick paste, eba, sometimes known as fufu.
Products essentially similar to gari are known by various names and are made throughout West Africa with minor variations to the processing. Recently gari processing has been mechanized in Nigeria.
A regional standard for Africa for gari was adopted by the Codex Alimentarius Commission (1986) which classified gari into five categories according to grain size and specified their essential composition and quality factors. These include raw material cassava and characteristic colour, taste and odour of gari and specification on acidity (not less than 0.6 percent nor more than 1 percent m/m determined as lactic acid). Total hydrocyanic acid (not exceeding 2 mg/kg determined as free HCN), moisture (not exceeding 12 percent m/m), crude fibre (not exceeding 2 percent m/m), ash content (not exceeding 2.75 percent m/m) and should be practically free from extraneous matter. Optionally edible fats or oils and salt may be added and gari may be enriched with added vitamins, proteins and other nutrients but addition of food additives was not allowed.
The methods used to process cassava in the South Pacific vary from island to island although boiling or baking the tubers are fairly widespread techniques. In the Solomon Islands the roots are often grated and mixed with coconut or banana as a pudding. In the New Hebrides, cassava is grated, wrapped in banana leaves and baked in an oven.
One method peculiar to the islanders in the South Pacific is the fermentation of tuber roots in pits, a process which prolongs the shelf-life of the product indefinitely. On the island of Mango in Tonga an abandoned pit estimated to be about 100 years old was found to contain food in an edible condition. Traditionally the pit is dug to a depth depending on the size of the family and lined with leaves of coconut, giant swamp taro or banana. The prepared food, which could be cassava, banana, taro or a mixture of all three, is placed in the pit to fill it and covered with more leaves, with rocks or logs placed on top to keep it in place. Fermentation proceeds for four to six weeks, after which the whole or part of the product is removed. Sometimes the fermentation is carried out with the addition of fresh water and sea water. In a modification of this process in Fiji, the unpeeled cassava root is fermented in a basked lowered into a lagoon. When it is required, it is removed, drained and pounded to a dough. The dough is kneaded with previously grated coconut, formed into balls, wrapped in breadfruit leaves and eaten either steamed or boiled. This product keeps for several months. If fresh water is used for the fermentation, the pulp is mixed with sugar or fruit, wrapped in leaves and steamed or boiled. This is known as bila and is a favourite food in Fiji. It keeps for several days.
Extraction of starch to prepare sipipa, tapioca and pot bammie
The juice obtained from grated cassava contains a certain amount of starch which settles out on standing for a few hours. In the Americas the liquid is decanted off, the starch residue is rinsed and then processed. It may be sun dried and eaten raw, or baked into crisp cakes called sipipa, which are highly prized as a delicacy by some groups. If it is still wet the starch is heated on a griddle when the starch grains burst and form granules known as tapioca flakes or globules. In Jamaica, starch is obtained by mixing grated cassava roots with water and straining the pulp through a towel. The starch is allowed to settle out for a few hours. The water is decanted off and the starch is either dried briefly, then salted and baked into pot bammie, or dried for several days, pounded in a mortar, mixed with flour and cooked into dumplings.
In Asia the traditional methods of extracting starch are similar to those used in tropical America and Africa. The starch in the extracted cassava juice is washed and sun dried on a mat. Moist wet starch is used commercially to produce tapioca. To prepare tapioca the wet starch is gelatinized into globules and sun dried.
In the South Pacific starch is extracted from cassava roots by grating, washing and draining and is then dried in an oven to produce a granular, tapioca-like product.
In Padaids Island the pulp, from which the starch has been extracted, is also used. It is formed into balls of five to six cm in diameter and dried over a fire for about a week. When required for eating the dried cassava is grated again and mixed with coconut milk and water (Massal and Barrau, 1956).
In the Solomon Islands of Anuta and Tikopia, cassava is used to produce a fermented product called ma manioka on Anuta and masi manioka on Tikopia (Yen, 1978). On Tikopia, the cassava roots are soaked in water for five or more days until soft. They are then peeled, broken up, squeezed and ensiled in pits lined with leaves. On Anuta, where there is no suitable surface water, the roots are packed loosely in pits and left for a few weeks. They are then recovered, peeled and resumed to the same pits for a funkier period. Ma is used as an emergency food baked alone or in combination with freshly pounded starchy roots and fruits.
Processing cassava juice - cassava reep, beer
The cassava juice or yard, obtained by pressing the grated cassava, is commonly used to prepare sauces and beverages in South America and the West Indies. The yard is boiled down to a thick syrupy consistency. The soup is known as cassava reep in the West Indies. Groups inhabiting the area around the headwaters of the Amazon tributaries produce a refreshing sweet-tasting drink by boiling yard for several hours. An alcoholic drink may also be prepared by fermenting the cassava juice.
Preparation of beverages from cassava root
In addition to cassava juice, the whole root, the sliced, grated or pounded roots and cassava bread or flour are all used as starting materials for the preparation of beverages. Both non-alcoholic and alcoholic drinks are made.
Non-alcoholic drinks. The roots are peeled, grated, squeezed by hand and cooked. When cold they are masticated for a few minutes and allowed to stand for a short period but not long enough to produce an alcoholic drink. Similar drinks are made from cassava flour or bread.
Alcoholic drinks . The preparation of cassava beers is widespread in tropical America. These are known as either kashiri or chicha. A number of different methods are used. The most common methods are the following:
Processing without mastication. The drink is usually prepared by fermenting whole cassava roots. The tubers are left for up to a week in a stream for fermentation to occur. They are then removed and m ashed. Water is added to the mash which is left to stand for three days before drinking. Other methods of preparation are also used.
Many groups use cassava bread to prepare beverages. In Guyana freshly made cassava bread is dipped into water, placed in a shallow heap in a dark corner of the house and left, covered with leaves, for three to five days while moulds develop. The broken bread is then placed in large earthenware pots and left for a further two to five days. Finally water is added and fermentation takes place to produce a mildly intoxicating beverage. Other methods are used in Brazil and in Suriname to prepare alcoholic drinks from cassava bread.
Processing with mastication. The custom of mastication in the preparation of alcoholic drinks is common in tropical America. The majority of the traditional alcoholic drinks are prepared in this way. Mastication speeds up fermentation because the salivary enzymes initiate the conversion of starch to sugar.
A variety of beverages is prepared from masticated cassava. In the Brazilian tropical forest, thinly sliced and boiled pieces of cassava are squeezed, chewed and left to ferment for one to three days. In the West Indies, a drink known as paiwari is prepared by this method. Other fruits, vegetables, maize and sweet potato may also be added as ingredients to the beer.
Beverage making from cassava is not generally practiced in Africa. Goode (1974) describes a method of preparation of beer in Uganda. The flour is mixed with water and left to ferment for a week. It is then roasted over a fire and put into a container to which water and yeast are added. After about a week the liquid is drained, sugar is added and the beer is left to ferment for four days. Cassava flour is also used to make beer in South Africa, Southwest Zambia and Angola.
By far the greater part of the world's yam crop is consumed fresh. Traditionally processed yam products are made in most yam-growing areas, usually as a way of utilizing tubers that are not fit for storage.
Usually fresh yam is peeled, boiled and pounded until a sticky elastic dough is produced. This is called pounded yam or yam fulu.
The only processed yam product traditionally made at village level is yam flour. Except by the Yoruba people in Nigeria, yam flour is regarded as an inferior substitute for freshly pounded yam because it is often made from damaged tubers. Yam flour is favoured in the Yoruba area where the reconstituted food is known as amala. To a limited extent, yam flour is also manufactured in Ghana where it is known as kokonte. The nutritional value of yam flour is the same as that of pounded yam.
Preparation of yam flour
The tubers are sliced to a thickness of about 10 mm, more or less, depending on the dryness of the weather. The slices are then parboiled and allowed to cool in the cooking water. The parboiled slices are peeled and dried in the sun to reduce the moisture content.
The dried slices are then ground to flour in a wooden mortar and repeatedly sieved to produce a uniform texture. Today small hand-operated or enginedriven corn mills or flour mills are increasingly used.
Yams have not been processed to any significant extent commercially. Dehydrated yam flours and yam flakes have been produced by sun drying. The manufacture of fried products from D. alata has also been attempted recently. Both chips and French fries have been manufactured. Preservation of yam in brine has been attempted, but with little success.
Since pounded yam has so much prestige and is the most popular way of eating yam, two attempts have been made to commercialize the process. The first was the production of dehydrated pounded yam by drum drying. This product could then be reconstituted without further processing. This production was first attempted in Côte d'Ivoire in the mid-1960s, under the trade name "Foutoupret", by air drying precooked, grated or mashed yam (Coursey, 1967). Onayemi and Potter (1974) used drum drying to produce a flake which can easily be reconstituted into pounded yarn by mixing with boiling water. This is the basis of the commercial product called "Poundo" in Nigeria, which was initially successful. To reduce wastage of raw material, peeling is done by using a 10 percent lye at 104°C with varying immersion times depending on the cultivar of yam (Steele and Sammy, 1976). Sulphite is added to prevent enzymic browning.
In the second commercial project a type of food processor resembling a blender was developed. The yam is cooked, fumed and churned in a process equivalent to pounding, to give enough pounded yam for two to four servings. Both projects appeared at first to be very successful, but later people reverted to the manual pounding of yam which gives a characteristic viscosity and firmness that is difficult to simulate mechanically.
Attempts to manufacture fried yam chips, similar to French fried potatoes have been reported from Puerto Rico.
Cocoyam is used in essentially the same way as yam. It can be eaten boiled, fried or pounded into fufu, although it is not considered as prestigious as yam. It has also been made info porridge or pottage, as well as chips and flour. Cocoyam flour has the added advantage that it is highly digestible and so is used for invalids and as an ingredient in baby foods.
Taro is the traditional staple in the Pacific Islands, where it is made into a series of food products similar to those described for cassava. Poi is a very popular Hawaiian and Polynesian dish made by pressure cooking the raw tuber, which is then peeled and mashed to a semi-fluid consistency. It is passed through a series of strainers, the final one having openings of about 0.5 mm diameter. It is then bagged and sold, or else stored at room temperature where it undergoes lactic acid fermentation. Coconut products can be added to the fermented poi before consumption.
In Nigeria cocoyam is grated, mixed with condiments and wrapped in leaves. It is steamed for about 30 minutes and served with sauce. Popularly known as ikokore, it is very common in the western Nigeria. A modification is available in Cameroon where cocoyam is made into balls and cooked with additional ingredients. This is known as epankoko.
One advantage of banana is that the dessert varieties can be eaten raw without any funkier processing. In many parts of Africa cooking banana is prepared by boiling or steaming, mashing, baking, drying or pounding to fufu. In Cameroon, green banana is boiled and served in a sauce of palm oil with fish, cooked meat, green beans, haricot beans and seasonings. In Uganda, where it is the staple, it is boiled with other ingredients including beans. Ghee is added together with pepper, salt and onions. This dish is called akatogo. Omuwumbo is prepared by wrapping the pulp in banana leaves and steaming it for about an hour. It is then pressed in the hands to a firm mass and eaten. The green form of banana is dried and stored. Known as mutere, it may be used for cooking after grinding into flour (Goode, 1974), but it is mainly used as a famine reserve. The same procedure is used in Gabon, in Cameroon, in South and Central America and in the West Indies (Fawcet, 1921).
A soup called sancocho is made in Colombia by boiling slices of green banana with cassava and other vegetables, while in the West Indies boiled green banana is served with salted fish or meat.
Mention has already been made of the fermentation of banana in pits in the Pacific. The fermented product is formed into loaves and baked.
Known as mast, it keeps for over a year while buried in the pit, and baked masi stored in air-tight baskets in a deep hole may last for generations (Cox, 1980). The starch pseudo-stem and corm of the false banana, or ensete, is prepared by similar methods in Ethiopia. The fermented product, called kocho is used to prepare a flat, baked bread. Ripe bananas are preserved by sun drying. Known as banana figs, they are eaten as a sweetmeats. This product keeps for months or even years.
In West Africa bananas are parboiled before drying. Oven drying is practiced in Polynesia. The dried product is then bound tightly in leaves and stored until it is needed (Massal and Barrau, 1956). A similar technique is practiced in India.
In Burundi where banana occupies about 25 percent of the arable land, it is mainly used for the production of beer. It has been estimated that local beer is consumed at a rate of 1.21/caput/day. Making beer from banana is common in East Africa. Green banana is buried in pits covered with leaves to ripen for about a week, at which stage it also starts to ferment. The peels are removed, the pulp is mixed with grass in a trough and the juice is squeezed out. The residue is washed and added to the bulk of the juice. Roasted sorghum flour or millet is added and the mass is fermented for one to two days, covered with fresh banana leaves. In a modification of the process, honey is added to the fermented banana pulp.
Sweet potato can also be eaten boiled, fried or roasted. When sliced, dried in the sun and ground, it gives a flour that remains in good condition for a long time. In Indonesia sweet potato is soaked in salt water for about an hour to inhibit microbial growth before drying. The flour is used as a dough conditioner in bread manufacturing and as a stabilizer in the ice-cream industry.
Sweet potato has been processed into chips (crisps) in much the same way as potato and the product is now popular in Asia. The sugar-coated chips are popular in China, the sailed variety is popular in the United States of America, while those spiced with cayenne pepper and citric acid have been tested in Bangladesh with favourable results (Kay, 1985).
Starch is produced from sweet potato in much the same way as from the other starchy roots except that the solution is kept alkaline (pH 8.6) by using lime, which helps to flocculate impurities and dissolve the pigments. The starch shows properties intermediate between potato starch and corn/ cassava starch in terms of viscosity and other characteristics. In Japan about 90 percent of the starch produced from sweet potato is used in the manufacture of starch syrup, glucose and isomerized glucose syrup, lactic acid beverages, bread and other food manufacturing industries.
In Japan, sweet potato starch is also used for the production of distilled spirits called shochu (Sakamoto and Bouwkamp, 1985). The process is very similar to that of whisky production except that the koji, equivalent to the malt starter in whisky production, is obtained by inoculating steamed rice soaked in water overnight with Aspergillus kawachii for two days at 35 to 37°C. The koji is mixed with starch water and yeast to promote saccharification and fermentation. The filtrate is finally distilled. The yield is about 8001 from I tonne of sweet potato.
Like other root crops potato can be eaten boiled, fried or roasted. Since it is essentially a temperate root crop its use has been extensively commercialized. French fries and potato crisps are very popular snacks in the United States of America and else where. Unlike cereal starches the starch from potato sets rapidly at high temperatures and has a high hot-paste viscosity which makes it preferable for the manufacture of adhesives. It also finds applications in the textile industry, the food industry and in the production of alcohol and glucose. Most of these processes are mechanized and highly efficient. For short domestic storage, peeled potatoes are immersed in a solution of sodium metabisulphite to inhibit discolouration by enzymic action. They can then be refrigerated for several days before cooking and consumption.
The preparation of crisps is very similar to that of French fries except that the former are cut into very thin slices while the latter are cut into rods. The flour produced from potato is incorporated into bread, and is used as a thickener in dehydrated soups, gravies, sauces and baby foods. Dehydrated potato dice are ingredients in processed foods including canned meat, meat stew, frozen meat pies and salads.
Woolfe (1987) has given a detailed description of the processing of bitter potato in the Andes, especially of those varieties that contain toxic alkaloids. In the preparation of chuño blanco the potato is evenly spread on the ground on a very frosty night. If, on the following day it is not well frozen, it is exposed for another night. The successive freezing and thawing separates the tuber cells and destroys the differential permeability of the cell membrane, thereby allowing the cell sap to diffuse out from the cell into the intracellular spaces (Treadway et al., 1955). In this way by trampling the thawing tubers the liquid is squeezed out and the skins are removed. The residue is recovered and immersed in a running stream for one to three weeks, to remove toxins. After draining it is spread in the sun to dry. During the drying period a white crust forms on the tubers, which is the origin of the name of the food. Chuño blanco forms the basis of soup and stews. It is a delicacy among the inhabitants of the high Andean areas of Peru and Bolivia especially when served steaming hot with cheese.
The preparation of chuño negro is very similar to that of chuño blanco except that during trampling the skin is not removed, the soaking process is omitted, and the residue is simply sun dried after trampling. The product is dark brown-black in colour, and hence the name. It is usually soaked in water for one or two days before being cooked in order to remove any residual bitter flavours.
A more prestigious potato preparation that is popular in large cities and in Peru is papa seca. The potato is boiled, peeled, sliced and sun dried and then ground into a fine flour. The flour is normally used for a special dish called carapulca which is prepared with meat, tomato, onions and garlic, but it may also be made into soup.
These traditional techniques are particularly important for processing the bitter varieties of potato with a high alkaloid content, which would otherwise have been too toxic for food use. Christiansen (1977) found that the level of glycoalkaloids could be reduced, from 30 mg/100 g in the fresh potato, to about 4 mg in chuño blanco and 16 mg in chuño negro. In the Andean highlands, where frost, storm or drought can lead to destruction of crops, irregular yields and food shortages, it is essential to cultivate some frost-resistant bitter varieties of potato that can be processed into reserve food from year to year.
A good review of simple technologies for root crop processing is provided by the United Nations Development Fund for Women publication, Root crop processing, 1989.
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