Crop Processing & Utilization (CSP 808)
Processing and Utilization of Roots and Tubers
By
Ayodele Olatunde Philip
(CRP 98\0188)
Introduction
The tropical root and tuber crops are comprised of crops covering several genera. The principal root and tuber crops of the tropics are cassava (Manihot esculenta Crantz), yam (Dioscorea spp.), sweet potato (Ipomoea batatas L.), potato (Solanum spp.) and edible aroids (Colocasia spp. and Xanthosoma sagittifolium). Root and tuber crops are second only in importance to cereals as a global source of carbohydrates. These carbohydrates are mostly starches found in storage organs, which may be enlarged roots, corms, rhizomes, or tubers. They also provide some minerals and essential vitamins. The high moisture content characteristic of roots and tubers makes them difficult to store for a long length of time. Also, they are bulky and difficult to handle and transport to distant markets. With cassava these problems are increased by compounds of cyanide in the leaves and roots which have to be removed before they can be consumed. Over many years, traditional processes have evolved which yield a more durable product and in many instances a more convenient product for domestic use. In many village communities root crops remain a staple and hence are often the main part of the meal. Village scale processing of root crops is therefore an important part of women's daily work.
Processing of Cassava Tubers
After harvesting, cassava roots are susceptible to spoilage, and without any preservation measures can only be stored for about 48 hours before they begin to deteriorate. Post-harvest deterioration of cassava is related to two separate processes: physiological changes and microbial changes. Physiological deterioration often sets in within 24 hours after harvest, while microbial deterioration usually begins within a week (Cock, 1985). Therefore the roots must be processed as soon as possible after harvest to arrest the physiological process and the subsequent deterioration.
Other factors favouring the processing of cassava are that the processed products are easier to store than raw cassava, they need less storage space and they can be stored for longer periods. For example gari, a dried cassava product, is less affected by biochemical changes during storage and in transit, and therefore does not lose its nutritional value as fast as fresh cassava does in storage. Processing is therefore undertaken primarily to detoxify the cassava product, to improve its palatability and to convert it to a storable form.
Toxicity and Detoxification of Cyanide in Cassava
It is known that cassava contains two major cyanogenic glycosides: linamarin and lotaustralin. Both glycosides are hydrolysed to produce hydrocyanic or prussic acid (HCN), a poison, when they come in contact with the enzyme linamarase, which is released when the cells of cassava roots are ruptured. The equation below illustrates the degradation of linamarin and the subsequent production of cyanide. Cyanide toxicity in humans and animals on cassava diets is a well-recognized problem (Oke, 1968; Osuntokun et al., 1969; Coursey, 1973; Erman et al., 1980).
Enzymatic degradation of linamarin
There is a great variation in toxicity between cultivars. A distinction is usually made between "sweet" cultivars with relatively low contents of cyanogenic glycosides (below 10mg/100g of fresh weight), and "bitter" cultivars with high cyanogenic glycoside content (above 20mg/100g fresh weight), although many intermediate forms exist. Traditionally, the sweet cultivars were considered non-toxic while the bitter ones were considered toxic. Although the sweet cultivars are generally less toxic there is no direct correlation between toxicity and taste (Coursey and Haynes, 1970). Cyanide levels in the range 6 to 370 mg/kg have been found depending on the particular cultivar, growing conditions, (i.e. soil type, humidity, temperature) and the age of the plant. The highest proportion of HCN is found in the peels and the cortex layer immediately beneath the peels (Hahn, 1984; Onwueme, 1978). It is for this reason that cassava root is always peeled before being processed or consumed. Peeling removes the cortex and the outer periderm layer adhering to it. The presence of cyanogenic glycosides in most cassava cultivars necessitates a certain degree of detoxification before the roots can be consumed. The prussic acid in particular is lethal if more than about 0.1 g of it is contained in the food eaten by an individual at any one time (Onwueme, 1978). In general, three methods of detoxification are employed: (a) microbial detoxification through fermentation; (b) decomposition of the glycosides by heating them above 150°C and (c) rupture of the roots to allow intimate interaction between linamarase and the glycoside, then expressing or volatilizing the resultant products of hydrolysis.
All forms of cassava processing therefore only decrease the levels of cyanogenic glycosides and prussic acid in the final product. For example, boiling is said to destroy the enzyme linamarase and eliminate the prussic acid. However, the linamarin itself is not destroyed by boiling, and its long-term ingestion may lead to cyanide toxicity in humans whose diets lack sufficient protein and iodine (Cock, 1985).
Gari
Gari can be considered to be the most popular form in which cassava is consumed in West Africa. Cassava is processed into gari in the following way. The harvested roots are peeled, washed and then grated. The resulting pulp is put into a cloth bag and subjected to pressure by heaping stones on the bag. After pressing, the bag is left for 2—4 days, during which time the pulp ferments. Most of the juice from the cassava pulp is expressed from the bag during this period. The fermented pulp is then removed from the bag, sieved and roasted or fried in wide, shallow metal pans, until they are dried.
The processing of cassava into gari involves several unit operations: peeling, washing, grating, pressing and fermenting, sieving, roasting and drying. Traditional gari production is laborious and time-consuming; on average it takes about 90 hours to process 100kg of gari. About 65 per cent of the total time is spent on peeling and 25 per cent on roasting (Williams, 1979). Gari occupies an important place in the diets of the people of West Africa. It is customarily consumed in the form of meal, which is prepared by soaking the gari in water to swell the starch, and by making the swollen meal into dough. The dough is then made into a ball with the fingers and is dipped into a stew containing ingredients such as palm oil, vegetables, meat or fish. Gari may also be eaten without a stew or soup by soaking it in cold water and adding sugar or milk
Peeling of cassava tubers
Peeling of cassava tubers presents a considerable problem in cassava processing. Traditionally, peeling is accomplished by hand; the roots are cut longitudinally and transversely to a depth corresponding to the thickness of the peel, which can then easily be removed. The structure of the root; the irregular shape and size does not permit easy mechanical peeling. However, in larger factories, the so-called washer-peeler, as described by Edwards (1974) is used. This equipment is divided into two sections; in the first section the tubers are cleaned of any adherent dirt and sand, while second section they are peeled. A machine of this type consists principal tank with a concrete base and a wooden casing. Along its length runs a rotating shaft about 15cm in diameter. Iron spikes or wooden paddles project from either side of the shaft with successive pairs set at right angles to each other; these cause the tubers to turn. Jets along the shaft spray water countercurrent to the flow of tubers; the water is pumped along the shaft centrifugal pump. In the peeling section, the tubers are moved continuously along under the spray of water and are peeled by the friction and the action of the rotating spikes.
Rasping or pulping
The peeled tubers are ground to a homogeneous mixture by means of a mechanical action. This is carried out by slicing the tubers and then rasping, grating or crushing them into a fine pulp. At the village level the tubers are rasped by hand on bamboo mats. A simple but effective grater is obtained by perforating a sheet of galvanized iron with a nail and then clamping it around a wooden wheel with the sharp protruding rims of the nail openings turned outwards. The wheel may be driven by hand or by foot like a bicycle, while the worker presses the tubers from above onto the rasping surface.
Gari can be considered to be the most popular form in which cassava is consumed in West Africa. Cassava is processed into gari in the following way. The harvested roots are peeled, washed and then grated. The resulting pulp is put into a cloth bag and subjected to pressure by heaping stones on the bag. After pressing, the bag is left for 2—4 days, during which time the pulp ferments. Most of the juice from the cassava pulp is expressed from the bag during this period. The fermented pulp is then removed from the bag, sieved and roasted or fried in wide, shallow metal pans, until they are dried.
The processing of cassava into gari involves several unit operations: peeling, washing, grating, pressing and fermenting, sieving, roasting and drying. Traditional gari production is laborious and time-consuming; on average it takes about 90 hours to process 100kg of gari. About 65 per cent of the total time is spent on peeling and 25 per cent on roasting (Williams, 1979). Gari occupies an important place in the diets of the people of West Africa. It is customarily consumed in the form of meal, which is prepared by soaking the gari in water to swell the starch, and by making the swollen meal into dough. The dough is then made into a ball with the fingers and is dipped into a stew containing ingredients such as palm oil, vegetables, meat or fish. Gari may also be eaten without a stew or soup by soaking it in cold water and adding sugar or milk
Peeling of cassava tubers
Peeling of cassava tubers presents a considerable problem in cassava processing. Traditionally, peeling is accomplished by hand; the roots are cut longitudinally and transversely to a depth corresponding to the thickness of the peel, which can then easily be removed. The structure of the root; the irregular shape and size does not permit easy mechanical peeling. However, in larger factories, the so-called washer-peeler, as described by Edwards (1974) is used. This equipment is divided into two sections; in the first section the tubers are cleaned of any adherent dirt and sand, while second section they are peeled. A machine of this type consists principal tank with a concrete base and a wooden casing. Along its length runs a rotating shaft about 15cm in diameter. Iron spikes or wooden paddles project from either side of the shaft with successive pairs set at right angles to each other; these cause the tubers to turn. Jets along the shaft spray water countercurrent to the flow of tubers; the water is pumped along the shaft centrifugal pump. In the peeling section, the tubers are moved continuously along under the spray of water and are peeled by the friction and the action of the rotating spikes.
Rasping or pulping
The peeled tubers are ground to a homogeneous mixture by means of a mechanical action. This is carried out by slicing the tubers and then rasping, grating or crushing them into a fine pulp. At the village level the tubers are rasped by hand on bamboo mats. A simple but effective grater is obtained by perforating a sheet of galvanized iron with a nail and then clamping it around a wooden wheel with the sharp protruding rims of the nail openings turned outwards. The wheel may be driven by hand or by foot like a bicycle, while the worker presses the tubers from above onto the rasping surface.
Fermentation and pressing of cassava pulp
The grating or rasping of the cassava root enhances contact between the enzyme linamarase and the cyanogenic glycosides, so that most of the glycoside can be hydrolysed to prussic acid. The pressing serves to remove the juice containing prussic acid, while the toasting—drying stage is aimed at vaporizing most of the remaining prussic acid.
The micro-organisms responsible for cassava fermentation are indigenous to the roots. Collard and Levi (1959) identified Corynebacterium manihot and Geotricum candida as the micro-organisms involved in a process similar to that of gari manufacture. More recent studies have questioned the validity of Levi and Collard's results. Ngaba and Lee (1977) isolated and identified Lactobacillus sp. and Streptococcus sp. as responsible for cassava fermentation and the subsequent acidity and flavour of gari. Ejiofor and Okafor (1981) observed, among others, high numbers of Candida and Geotricum spp. in pressed cassava pulp during the later stages of fermentation.
According to Meuser and Smolnik (1980) *fermentation of cassava pulp at 37°C is complete within a maximum of 5 days. The water binding capacity of cassava mash is sufficiently changed only after about 3 days, after which the water present can be pressed out to a reasonable degree. For this reason, an accelerated fermentation would be of limited use. During fermentation, there is decomposition of soluble carbohydrate (sugar) to lactic and acetic acids and ethanol. Also, the concentrations of fructose, sucrose and glucose decrease with days of fermentation. The fermentation rate of sucrose is highest as it is completely hydrolyzed within 18 hours. Fructose increases in the first 18 hours and fall steadily after 72 hours of fermentation. Unlike sucrose and fructose, a certain amount of glucose still remains after a fermentation period of 5 days.
Roasting and drying during gari production
The sieved cassava meal is roasted over fire in the traditional method. Constant stirring is carried out over during the roasting to circumvent burning and to prevent the meal from forming lumps. In the roasting process, some of the starch granules are gelatinized and aromatic compounds are formed from saccharide and soluble nitrogenous substances, so the fermented product has a characteristic taste and aroma. Following roasting, gari is dried so that it can be stored for long period and to further reduce the hydrocyanic acid content of the product.
Flowchart of gari Processing
Cassava Root Washing-- Peeling-- Grating pulp-- Pressing and Fermenting Roasting and drying Gari
*Fermentation is the breakdown of complex organic substances into simpler ones through the action of catalysis (enzymes)
The grating or rasping of the cassava root enhances contact between the enzyme linamarase and the cyanogenic glycosides, so that most of the glycoside can be hydrolysed to prussic acid. The pressing serves to remove the juice containing prussic acid, while the toasting—drying stage is aimed at vaporizing most of the remaining prussic acid.
The micro-organisms responsible for cassava fermentation are indigenous to the roots. Collard and Levi (1959) identified Corynebacterium manihot and Geotricum candida as the micro-organisms involved in a process similar to that of gari manufacture. More recent studies have questioned the validity of Levi and Collard's results. Ngaba and Lee (1977) isolated and identified Lactobacillus sp. and Streptococcus sp. as responsible for cassava fermentation and the subsequent acidity and flavour of gari. Ejiofor and Okafor (1981) observed, among others, high numbers of Candida and Geotricum spp. in pressed cassava pulp during the later stages of fermentation.
According to Meuser and Smolnik (1980) *fermentation of cassava pulp at 37°C is complete within a maximum of 5 days. The water binding capacity of cassava mash is sufficiently changed only after about 3 days, after which the water present can be pressed out to a reasonable degree. For this reason, an accelerated fermentation would be of limited use. During fermentation, there is decomposition of soluble carbohydrate (sugar) to lactic and acetic acids and ethanol. Also, the concentrations of fructose, sucrose and glucose decrease with days of fermentation. The fermentation rate of sucrose is highest as it is completely hydrolyzed within 18 hours. Fructose increases in the first 18 hours and fall steadily after 72 hours of fermentation. Unlike sucrose and fructose, a certain amount of glucose still remains after a fermentation period of 5 days.
Roasting and drying during gari production
The sieved cassava meal is roasted over fire in the traditional method. Constant stirring is carried out over during the roasting to circumvent burning and to prevent the meal from forming lumps. In the roasting process, some of the starch granules are gelatinized and aromatic compounds are formed from saccharide and soluble nitrogenous substances, so the fermented product has a characteristic taste and aroma. Following roasting, gari is dried so that it can be stored for long period and to further reduce the hydrocyanic acid content of the product.
Flowchart of gari Processing
Cassava Root Washing-- Peeling-- Grating pulp-- Pressing and Fermenting Roasting and drying Gari
*Fermentation is the breakdown of complex organic substances into simpler ones through the action of catalysis (enzymes)
Processing of other Root and Tuber Crops
In contrast to cassava of which a large proportion is consumed in a processed form, other roots and tubers (yams, potatoes, sweet potatoes and cocoyams) are normally stored and consumed as fresh produce. But on some occasions they also are processed into dried chips with the aim of prolonging the storage life, especially for that portion of the crop which has been affected by injuries or diseases.
Traditional methods
Many simple methods have been devised based on local resources to extend the storage life of roots and tubers. At family and village levels, the fresh or precooked roots or tubers are simply peeled, cut into chips and sun-dried by being spread out in the open, on a mat or any clean surface. Drying may take up to one week or more, depending on the weather. The dried chips are ground into flour before being incorporated into the traditional foods. Yams in Nigeria, potatoes in Peru, sweet potatoes in Cameroon and Tanzania are traditionally processed in this way, the smaller tubers or those damaged or partially decayed being particularly selected for the purpose.
Improved methods of production of dehydrated chips
In order to satisfy the demand by urban consumers for a high quality product the same considerations apply to the production of high quality dried chips or flakes as in the natural drying of cassava chips. The improved methods for drying cassava chips have been applied to the drying of yams, potatoes, sweet potatoes and cocoyams. But dried chips of these roots and tubers can be affected by discolouring compounds which do not occur in cassava. The discoloration of the dried chips occurs in three ways (Straw and Booth, undated):
Enzymatic darkening
After-cooking darkening
Browning during drying and storage.
Enzymatic darkening is caused by the oxidation of phenolic compounds resulting in a brown to blue-black discoloration which affects the quality and appearance of the final product. Cooking before peeling and slicing will destroy the enzyme and prevent this type of discoloration. If it is not possible to cook the chips immediately after slicing immersing the chips in water helps to slow down the enzymatic reaction, adding salt (3% w/w) to the water will further slow down the reaction. When the freshly cut chips have to be kept for prolonged period a preferred treatment is dipping for five minutes in a 0.1-0.2% sodium bisulphite solution, or 10 minutes in a 0.5% sodium metabisulphite solution.
After-cooking darkening is the result of the oxidation of ferrous iron present in the tuber to ferric iron. Roots and tubers chips which are still warm after being boiled, parboiled or blanched should not be left exposed to air but should be cooled as quickly as possible, for example by immersion cold water. Lowering the pH also prevents the oxidation taking place; in practice this is done by dipping the chip in a solution of 0.4% citric acid.
Browning usually takes place during the drying and storage of the chips. This is the result of the reducing sugar combining with free amino acids. The reaction occurs more rapidly at temperatures above 55°C. It is, therefore, important to keep the drying temperature as low as possible
Flowchart for Processing Tuber Flour
Tubers Slicing or Cutting-- Peeling and washing --Drying
Chips --Grinding--- Sieving Flour
The flowchart for processing of cassava flour is a bit different from the above as pulping and dewatering is an integral part.
Conclusion
Processing of crop is a widely acknowledged alternatives approach which can be used as a significant means of reducing post-harvest losses. In Africa and in Central and South America the preparation of storable cassava product through fermentation is a widespread tradition. The reduction of post harvest losses and production of convenient, safe and wholesome food through processing depend on proper use of existing traditional methods and recent developments derived from a broad spectrum of scientific disciplines.
References
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