School Science Lessons
Cassava Project
2008-12-21
Please send comments to: J.Elfick@uq.edu.au
See: Interesting websites
Preface
Before teaching this project, discuss the content of the lessons with a field officer of the Ministry of Agriculture and get advice on planting material, planting distances, sites for planting, approved mulch, composting, and control of pests and diseases. Use only the procedures, agricultural chemicals and insecticides recommended by the local field officer of the Ministry of Agriculture. If you cannot control insects by hand picking, ask the Ministry of Agriculture to recommend a chemical spray. All insect sprays are dangerous. Show the students how to use them safely. Do not get the spray onto your hands. Do not breathe in the spray. Wash your hands well after using spray. Keep the spray container in a safe place where students cannot get it. Spray on a day of no wind but if you must spray when there is a wind, spray down wind. Make sure the spray does not blow on other people.
Table of contents
1.0 Introduction to the cassava project
2.0 Origin and distribution of cassava
3.0 Form and habit
4.0 Stems and branching patterns
5.0 Leaves
6.0 Flowers and fruit
7.0 Roots and root tubers
8.0 Varieties
9.0 Selecting the site
10.0 Spacing
11.0 Time of planting
12.0 Growth period
13.0 Prepare the land for planting
14.0 Planting material
15.0 Methods of planting
16.0 Cultivation, weeding, fertilizing
17.0 Pests and diseases
18.0 Harvest
19.0 Storage
20.0 Uses of cassava and cassava products
21.0 Toxic content of cassava and detoxification
22.0 Make cassava starch
23.0 Make tapioca
24.0 Keeping records and accounts
25.0 Future of cassava growing
1. Introduction to the Cassava project
See diagram 60.1: School cassava garden
The aim of this project is not to grow a large area of cassava, but to grow a small area so students can easily watch it growing and keep it free of weeds. A good size for a cassava garden is 10 m long and 2 m wide, with 32 cassava plants, 60 X 120 cm apart. The ground should be dug well twice. Double digging is needed to make the soil soft and to kill the weeds. Ask a field officer of the Ministry of Agriculture to recommend a suitable variety for the cassava project.
2. Origin and distribution of cassava
Cassava or tapioca has the scientific name, Manihot esculenta and is in the Family Euphorbiaceae, the spruce family that includes natural rubber (Hevea brasiliensis) castor oil (Ricinus comunis) and ornamentals, e.g. poinsettia (Euphorbia sp.). Latex occurs in all parts of the plant and a related species Manihot glaziovii was formerly used in commercial rubber plantations to make rubber. Most varieties of cassava are diploid with 2n chromosome number = 36. Some triploid and tetraploid high yielding varieties exist. The common names include cassava, manihot, manioc, mandioc tapioca, tapioka, and mandioca in Brazil and Paraguay, yuca in South America, ubi singkong in India, ubi kayu (stick yam) in Malaysia, muk shu (wood potato) in China, munsumpalung in Thailand and tapiok in Papua New Guinea. Cassava is not found in the wild so its origin is known. However, it may have come from the drier parts of Mexico or Brazil and been first cultivated by the Maya people.
Cassava is an important source of starchy food in tropical regions. Its cultivation needs little labour input so it is cheap to produce. Cassava is a benchmark for food security because it is affordable by the poor. It can be harvested six months from planting but as the harvest can be delayed for up to 3 years an unusually but effective means of food preservation exists. The yield per hectare varies from a low 1.8 tones to the average 10.2 tones and even to 27.3 tones per hectare in plantations. Cassava is usually easy to grow, does not get many diseases and people like it to eat. It can produce high yields under good conditions and yield better than many other crops under bad conditions. However, continual cropping takes much plant nutrients from the soil. Research institutions such as the International Center for Tropical Agriculture (CIAT) and the International Institute for Tropical Agriculture (IITA) are developing improved varieties of cassava with lower cyanide content, more resistance to pests and diseases, better drought resistance, earlier maturity and higher yield. Cassava responds well to better agricultural practices and to genetic improvement.
3. Form and habit
See diagram 64.3: Cassava plant
Cassava is a perennial, but cultivated as an annual crop, woody shrub with latex in all parts, 1 to 6 m in height. Many varieties and cultivars exist but most are cultigens, i.e. known only as cultivated forms. The bitter and sweet varieties of cassavas were formerly separate species, Manihot esculenta the bitter cassava and Manihot palmata the sweet cassava. However, the bitterness depends on many factors including soil, climate and location so now cassavas are informally divided into bitter and sweet types and sometimes into short season and long season cultivars. Also, cassava formerly had the scientific name Manihot utilissima. Nowadays all varieties and cultivars of cassava have the same scientific name, Manihot esculenta.
4. Stems and branching patterns
See diagram 64.4: Leaf scar | See diagram 64.8: Branching patterns
The hairless stems vary in height, branching height, may be erect or spreading and have prominent leaf scars. The colours include green, white and grey. Examine a piece of stem at least 60 cm long. Note the nodes and internodes. Each node forms an angle with (subtends) a leaf. Note where a leaf has dropped off the stem, leaving a leaf scar that may be flat but is usually a small lump that sticks out from the stem. The central part of the leaf scar is round, but the sides extend into a ridge. Just above each leaf scar is a small bud. The bud may be too small to see. This bud can grow into a leafy stem if a cutting is made and the stem planted. Mark one leaf scar low down on the stem then go upwards counting leaf scars as you go. The scars are so placed that they make two turns around the stem before you come to the next scar that is directly above the first leaf scar where started. You will pass five scars on the way up to the next scar above. Every fifth leaf scar is directly above one below it. This spiral arrangement of leaves on the stem is shown as a phyllotaxis of 2/5. The numerator of the fraction is the number of times you have to go around the stem and the denominator is the number of nodes you pass through. The position of five leaves turns twice spirally around the stem, before the next leaf is located exactly above the beginning of the spiral. So after two turns of the stem, leaf 6 is exactly above leaf 1. This regular arrangement of leaves allows the least amount of shading by other leaves. During early growth stages, a stem may increase by one node each day. Internodes are longer under favourable conditions and  shorter under unfavourable conditions, e.g. drought stress, and very long under insufficient light.
Branching patterns
The two branching patterns are reproductive branching (forking) and lateral branching. In reproductive branching the primary stem produces secondary branches that produce tertiary branches until the apical meristem start to produce flowers. In lateral branching the branches come from nodes on the lower part of the stem. Lateral branches are usually thin with long internodes and small leaves. Branching patterns can be influenced by plant spacing and the use of intercrops.
5. Leaves
See diagram 64.5: Leaves
The leaves are large and palmate, i.e. divided so that the divisions point to the apex of the petiole. Leaves are divided into 5-7 leaflets depending on the stage of growth and the variety. When the plant starts to make flowers, the leaves change in shape to become single leaflets on short stalks. The long petioles (leaf stalks) are usually longer than the lamina and may be light or dark green, or blue-green, or red in colour. The leaf area is maximum at 4-5 months after planting.
6. Flowers and fruit
See diagram 64.6.0: Flowers | See diagram 64.6.1: Seeds and fruit
The inflorescence is a panicle, open, repeatedly branched and many flowered. The flowers are produced on stalks (peduncles) in small clusters within the axis of a branch and are monoecious, i.e. have male flowers and female flowers in the same inflorescence. Different varieties have different frequency of flowering and some are influenced in their flowering by day length and temperature. Usually the male flowers are more at the tip of the inflorescence and female flowers are more at the base. Both male and female flowers have a calyx of 5 yellow red sepals but no corolla (no petals) so we can say both flowers have 5 perianth segments. The coloured calyx is large and like a petal (petaloid). The male flowers have 10 stamens in 2 whorls with small anthers and the filaments not joined together. The larger female flowers open weeks before the male flowers (protogyny) so self-pollination is unlikely and the cross-pollination can be done by insects. The green ovary on a 10-lobed glandular disc has 3 carpels each containing 1 ovule, a 3-lobed divided stigma united to form a single style, and 6 ridges. So the floral formula for the male flower is P3 A5+5, and for the female flower is P3 G(3). The fruit matures in 2-3 months and is a 6-winged capsule containing 3 seeds about 1 cm long with 6 ridges (wings). The fruit wall is just the woody endocarp because the mesocarp and epicarp die. About 5 months after pollination the dry, woody capsule splits open along the ridges to eject the seeds explosively. The grey to blown mottled seed is oval with a large protuberance (caruncle) near the scar left where the ovule had been attached to the ovary (hilum) and a thin outer coat (testa). The first sign that the cassava plant is starting to make its flowers is that it makes some leaves that have fewer leaflets. Also the stem may become thinner and may branch. The flowers, seeds and fruit have no importance in how cassava is used for food but of course they are important for plant breeding. Propagation by seed may occur under natural conditions.
7. Roots and tubers
See diagram 64.7.0: Tubers growing from the base of the stem | See diagram 64.7.1: Tuber cross-section
1. Within a week of planting stem cuttings, adventitious roots grow down and horizontally from the nodes at the base of the cutting to form the fibrous root system. At about 50 days after planting 5-10 tubers develop near the stem from the adventitious roots by secondary thickening (tuberization). The flesh is white to yellow red. The tubers (root tubers, tuberous roots, storage roots) are true roots modified to function only as storage organs that cannot absorb water or plant nutrients from the soil and cannot be used for vegetative propagation or as planting material. The older root tubers become lignified.
The mature cassava storage root has 3 distinct tissues:
1.1 Outer periderm of dead cork cells (skin, bark) a few cells thick,  to seal the surface. It may be sloughed off.
1.2 Thin cortex of sclerenchyma, cortical parenchyma (with small starch grains) and phloem.
The detachable rind (peel) is the periderm and cortex (1.1 + 1.2).
1.3 Pith of parenchyma cells (flesh) containing starch (with big starch grains) as a matrix and radially distributed vascular strands of xylem. The flesh is 85% of the total tuber weight.
2. The outer skin may be brown and rough or light brown or pink colour and smooth. The pith of the tubers (flesh) is usually white, but sometimes pale yellow.
3. Examine different tubers and record their length, shape, skin colour and feel, inside colour and whether you can snap them into two pieces by hand. Under favourable conditions, the tubers of duration varieties can exceed 100 cm in length and weigh up to 15 kg. The peel may account for 20% of the tuber. It consists of an outer corky layer and an inner part that separates the peel from the flesh of the roots.
Peeled cassava tubers may contain water 60%, starch 35%, protein 2%, fat 0.5 %, fibre 2%, ash 0.5 %. Each 100 g may contain Ca 33 mg, Fe 0.7 mg, thiamine 0.06 mg, riboflavin 0.03 mg, niacin 0.6 mg, ascorbic acid 20-30 mg, and vitamin B. The main amino acids in the protein are arginine, histidine, isoleucine, leucine and lysine. If cassava is the main staple item of diet, the childhood weaning disease kwashiorkor may occur because of the low protein content of cassava tubers. However, fortification of cassava meal by the addition of maize flour or by fermentation with maize or soybean, improves the nutritional quality. The amylose content is about 17% compared with 22% for potato starch.
8. Varieties
See diagram 64.8: Single and branched stems
1. Examine different varieties of cassava and describe them by using the following features:
1.1 Root tubers to rough skinned and dark in colour or smooth skinned and light in colour
1.2 Stem to single or branched, tall or low.
1.3 Leaflets to long and narrow or more rounded
1.4 Number of leaflets in each leaf
1.5 Length and width of middle leaflets
1.6 Colour of young leaves to green, blue-green or red blue
1.7 Colour of petioles to white, green, pink, red, dark red
1.8 Colour of stems to brown, yellow or silver, colour of younger stem and older stem
1.9 Colour inside the root tuber to white, cream or yellow
1.10 Leaf scars on the stem to small and smooth or large and raised
1.11 Cook and eat the cassava and record that have the good taste
1.12 Cook and eat the cassava and record that have the good feel in the mouth
2. The characteristics important for breeding new varieties include plant height, height at first branching, angle between branches and main stem, horizontal (decumbent) branching and erect branching, time of first number of nodes where branching starts, leaf area per unit ground area, early and late maturing, resistance to disease, level of HCN.
3. Choosing the variety to plant
Take cuttings from varieties bred for eating. Get the advice of a person who has eaten the tubers and says the variety is “sweet”. Ornamental varieties of cassava should not be used for eating. Do not eat the variegated varieties.
9. Selecting the site
A light sandy soil best with good drainage but almost any soil type can be used except salty or waterlogged soils. Good yields can be obtained on heavily cropped soils so cassava is suitable as the last crop in a rotation. During drought cassava stops growing and drops its leaves but it usually recovers well after new rain.
10. Spacing
The area required for one meal 1.25 kg / student of cassava for 100 students 40 square metres if yield 30 tonnes / hectare. Use a planting distance of 80-140 cm in square alignment depending on local conditions. A triangular planting pattern may be preferable. In intensive cultivation, planting may be as close as 60 cm. For ridge planting the plants are 75 cm apart with 120 cm between the ridges. The planting rate may be 10 000 to 11 750 plants per hectare when the sticks are planted at 90 cm intervals in rows 90 to 120 cm apart.
11. Time of planting
Plant at the start of the rain season. However, you can plant anytime in the year except when the soil is dry. Cassava does very well if planted at the beginning of a rain season before a dry season. It requires moderate soil moisture during the establishment period.
12. Growth period
The age to first production is 9 to 12 months. The usual life span is 15 to 24 months depending on the cultivar, climate and soil conditions, but later tubers become fibrous and woody. Some quick growing cultivars can be harvested in 6-7 months so you can plant and harvest them twice a year. However, for good yields you must harvest after 9 to 12 months. If cassava is used as a vegetable the tubers are harvested within 12 months to avoid fibrous tubers but cassava used for starch processing are left to reach full maturity, often up to 18 to 24 months after planting. The indication of maturity is when you see leaves beginning  to become yellow and fall. When this happens dig up some tubers for a test tasting.
13. Prepare the land for planting
Mark out a flat place for a cassava garden. Dig the soil well down to 20 to 50 cm and bury the weeds and grass. Dig the soil again two weeks later. You can make the soil fairly flat and level but they grow better if ridges or mounds are used, especially in wet regions. For mechanical preparation, ploughing twice and harrowing twice is recommended.
14. Planting material
Cassava is usually propagated from stem cuttings (sticks). On smallholdings using hand planting, select cuttings with 2 to 3 buds, 20 to 30 cm long and 1.5 to 4.0 cm thick. Select cuttings from the lower or mid-section woody part of the stem of plants at least 10 months old. Also, you can use the stump after harvesting provided it is not damaged. Split stalks take time to recover so do not break the ends of the cuttings. Store cuttings in the shade if not planted immediately, The sticks can be stored for 2 months in cool well-ventilated conditions, but during heavy rain storage is limited to 7 days. You can also make a cassava fence. Some people sharpen the base of stem cuttings before vertical or angle planting.
15. Planting methods
See diagram 64.15: The 3 methods of planting
1. For vertical or angle planting bury the stem cuttings at the beginning of the rain season in ridges or in furrows so that only one third above ground, about 5 cm, with about 3 nodes above the ground. Plant two cuttings at each place. Water them well and the cutting will start to bud within a week. At the apex node 1-2 shoots will develop. After the first weeks of sprouting, the shoots lengthen and the roots spread. When shoots are about 50 cm long, cut out the shoots to leave only two shoots from each cutting. Within a month you can pick leaves. If you plant cassava in rows along a path, the leaves are easy to harvest. With vertical or angle planting the roots usually penetrate deeper, but in areas of low rainfall, desiccation of the cuttings may occur. Remove all the leaves. In the dry season leave the cuttings in water untill they grow roots but in the wet season plant the cuttings in straight in the ground without delay. If planted at an angle, the roots will grow mostly on the side opposite to the slope of the stem and the sticks can be knocked down by severe rainstorms.
If planted vertically, the roots form on all sides of the stem, but the plant may grow into a tall spindly bush with only a canopy of leaves at the top to be easily blown over by the wind. So, prune these plants to a metre high.
2. For flat planting, Dig a shallow trench, lay the cuttings flat in the trenches 80-140 cm apart, at a depth of 10 cm. Horizontal planting often results in getting two plants growing from each cutting. Shoots may develop from nearly all the nodes but not the middle nodes. Longer cuttings have more nodes and produce
more shoots. However, if too many stems grow up and tubers may be small. Also, it allows more efficient use of fertilizers and makes harvest easier with less damage to the tubers. However, flat planting may cause rotting in areas of high rainfall. Flat planting can become end planting of one metre lengths of sticks.
Mechanized planting buries the cuttings horizontally, 5-20 cm deep. Germination usually occurs during the first week after planting. Prepare for replanting perhaps up to 10% of original cuttings during the first month after planting where germination has failed. More shoots develop from thick, freshly-harvested cuttings. Varieties with the genotype for apical dominance develop only one shoot especially if planted vertically. Low soil fertility delays branching so branches form higher up the stem.
16. Cultivation, weeding, fertilizing
See diagram 64.16.1: Stages in growth 1 | See diagram 64.16.2: Stages in growth 2
Cultivation
Hand cultivation during the second or third week after planting helps to control weeds and break open the soil surface if compacted by heavy rainfall. Do no further cultivation after the eighth week after planting to avoid injury to the plants.
Weeding
Early weeding, before a closed canopy forms, improves the yield so do a first hand weeding 2-3 weeks after planting and a second hand weeding 2-3 weeks after the first weeding. The weedicides diuron and atrazine are usually effective but use of preplanting weedicides may be more economic. Do the weeding before side-dressing with fertilizer.
Hilling up
After 8-12 weeks, use a hoe to earth up the plants to encourage tuber formation.
Intercropping
The smallholder cassava crop is usually a pure crop. However, but cassava can be grown mixed with vegetables, bananas, yams, lemon grass, and sweet potatoes, or as intercropped with papaya, rubber and coconuts. Avoid planting the mixed crop before planting the cassava. Cassava is frequently cultivated as a temporary shade plant in young plantations of cocoa, coffee, rubber or oil palm
Rainfall
Annual rainfall should be greater than 500 mm per year. It does best with a well-distributed rainfall of 1 000 to 2 000 mm per year. Cassava can be grown in areas with rainfall ranging from 500 mm to more than 2500 mm per year. Good rainfall increases the yield but cassava can be successfully grown in areas with less rainfall except at planting time. It can withstand prolonged drought during the latter stages of development, so it is a useful crop where rainfall is low or uncertain.
Soil
A light sandy loam of medium fertility gives the best yield but cultivars can be found that grow in different types of soils, except salty or swampy soils. The soil should be cultivated before planting to allow roots to penetrate to 50 cm. Deep well-drained friable sandy loam to loam soils allow best root development. On clay soils, stem and leaf growth occurs at the expense of root development.
Fertilizer
Cassava needs about 500 kg / ha of grade formula NPK (N:P2O5:K2O) 2:1:2, but extra potassium is needed in some soils to give a high starch content of tubers. Too much nitrogen may cause excessive leaf growth instead of tuber growth. In some regions ammonium sulfate is applied. Apply half  the fertilizer at planting and the other half two months after planting, just before the final cultivation.
Altitude
It can be grown from sea level to 1 000 m and even 2000 m, where the lower than recommended mean temperature causes slow growth and reduced yields.
Day length
Cassava is a short day plant and produces best in day lengths less than 10-12 hours so is most productive when grown between latitudes 30oN. and 30oS.
Plant care
Weeds should be cut out with a hoe or some tool that does not dig deeply. Weeds can compete for water, soil nutrients and sunlight.
Temperature
The mean temperature should be greater than 20oC. The optimum condition is a warm moist climate with temperature range 25-29oC. Growth stops below 10oC and yields are reduced above 29oC.
Mulching
If you do not harvest the leaves for animals, cut the tops off regularly and use them as mulch said to repel root knot nematodes.
17. Pests and diseases
In most countries there are no serious insect pests. The following insects cause some damage:
1. Termites, mound-building termites, mine through stems and roots causing wilt.
2. Stem borers that bore through the stems so that plant breaks off and wilts.
3. Beetles that shred the leaves.
4. Scale insects that cause mottling of the leaves and stunting of the plant. They may leave a black mould on the leaves.
5. Mealy bugs that cause leaves of young plants to become distorted and covered with white fluff.
6. Flies that cause leaf curl and yellow leaves that drop off. The plants become stunted.
7. Also, rats, monkeys and wild pigs eat the root tubers in the ground.
8. Diseases include mosaic or curly leaf disease viruses and brown streak viruses transmitted by white fly and other vectors that cause stunting and tubers with a low starch content. Other diseases include white thread disease of roots, bacterial wilts, stem rots and rusts. To control diseases, remove and burn diseased plants and always use disease free planting material. Cercospora leaf spot (Cercospora sp.) is common but can be controlled with Bordeaux mixture before it stars attacking the tubers.
9.The different fungi that cause storage rot of tubers can be treated with ethylene bromide. Some new varieties of cassava are being bred for disease resistance. However, if harvesting the tubers is impossible due to insect damage, the leaves can still be harvested and consumed or fed to animals.
10. Cassava is very susceptible to herbicide drift that causes young leaves to curl and remain stunted.
18. Harvest
See diagram 64.18: Tuber harvest
Harvesting of cassava can be done throughout the year,  whenever the roots reach maturity. Harvest 10-12 months after planting when the yield of starch per hectare is highest. The tubers can be left in ground for up to 2 years if they are not detached from the plant, but they will become fibrous and woody. When the leaves begin to turn yellow and fall the tubers are ready for harvest, but harvest can be delayed without undue loss. Use a machete to cut off the parts of the plants 40-60 cm above ground by hand (topping) leaving enough of the base of the stem to serve as a handle. Loosen the soil with a fork, pull the plant up and cut the tubers off the main stem. Some people use a bamboo pole to act as a lever for uprooting. Another method ids to water the plants then pull up after 12 hours. The tubers are about 50 cm below the surface of the soil and spread out about 120 cm from the stem. If the tubers are ploughed up some tubers may be left in the ground. Harvest tubers from six months old to too old, and the centre cortex in the tuber gets large and woody. The tubers should be crisp and break easily.
If harvesting with a tractor and plough, it should have a blade with gradual curvature to lift the soil and roots slowly and not break the tubers.
19. Storage
Cassava does not keep well after it has been dug. In a few days the tubers suffer postharvest physiological deterioration (PPD) turn black and uneatable. The indication of maturity is when you see leaves beginning  to become yellow and fall. When this happens dig up some tubers for a test tasting. So you should either only dig cassava if it can be eaten or sold straight away, or dig the roots very carefully for storage and try not to damage them because damaged roots go bad very quickly. If bruised, tubers turn grey in colour within a week. Tubers soon deteriorate after harvest so usually they can be stored for only 2 days unless coated with a fungicidal wax. However untreated tubers can be stored wrapped in plastic in the fridge for many weeks and, peeled and frozen, for up to six months. Other methods of storage include coating with mud in a "soil clamp", packing in boxes of moist sawdust and drying sliced tubers in the sun.
20. Uses of cassava and cassava products
Cassava produces more starch per acre under relatively dry conditions than any other known crop. Cassava is a staple crop and easy to grow. It can survive severe dry seasons. The young leaves are a green leafy vegetable, used in cooking or fed to chickens and pigs. Cassava is added to broiler and layer rations to give a yellow colour to chicken and duck shanks, breasts and egg yolk. They can be dried and stored as a green feed for the dry season. The tuberous root can be sliced and fried like chips, boiled and added to vegetable dishes, or made into flour to be used in bread and cakes. They can also be boiled or grated for chickens and ducks or fed whole to larger animals like pigs. Grated raw cassava can be cooked and used in bread and biscuit recipes or as a thickener in gravy, soup, porridge or in desserts. Starch from the tuber makes cassava flour (tapioca). Bitter is a staple food in many countries.
Cassava was regarded as the staple food for the poorer people in tropical countries and was once known as a slave food.
The fresh peeled tubers are eaten as a vegetable after boiling or roasting or cooked in sugar syrups and eaten as a desert. Also the tubers are boiled and pounded into a paste and added to stews and soups. The fresh tubers deteriorate rapidly so the tubers they are often preserved as sun dried chips or ground into a dry flour and used as a fermented meal. The tubers are used in the industrial production of starch for use in the foodstuff, textile and paper industries and the production of ethanol for the biodiesel industry. Dried cassava roots can be fed to livestock as chips, pellets of compressed powder and cassava meal, the residue is left after the extraction of starch. Tapioca starch is used for puddings and infant foods. Cassava flour is used in biscuits, confectionery and pasta. Cassava starch can be converted into dextrin and glucose syrups
Cassava young leaves can be are eaten as a vegetable or used as a pot herb and mature leaves can be used as an animal feed when detoxified. The sweet variety of cassava is used mainly as a source of starch. To produce granulated tapioca, the roots are grated, thoroughly washed, pressed through fine meshes and heated. Cassava tubers have more carbohydrate and less protein than other root vegetables. The tuber contains very little in food value apart from starch, which is a source of concern to nutritionists in countries where this is a staple food. It has very low fat and protein content, consequently fat soluble vitamins A and D are lacking, but it may contain up to 50 mg / 100 g of calcium, 40 mg / 100g of phosphorus and up to 24 mg / 100 g of vitamin C. If used as a staple, it must be supplemented with greens, meat and seafood, fat and legumes.
Pungent foods are made from cassava tubers well boiled until tender in salted water and drained, then served with fresh grated coconut, pounded chillies, onions, lime juice and salt.
Grated fresh cassava is a popular ingredient and is available packaged in plastic bags for cakes and glutinous solid dumplings.
When buying cassava, make sure the skin is unbroken with no mouldy spots and the smell is fresh. The tubers will keep protected from light for up to 10 days. Frozen cassava is sold in plastic packets. Grated cassava will keep in the refrigerator for 5 or 6 days.
Do not try to peel a cassava tuber with a vegetable peeler in the same way as peeling a potato because the skin is too tough. Scrub the tubers clean, then cut into 5 cm short sections. Use a sharp knife to slit the skin and the underlying layer along the length of the section. Slip the blade underneath the skin and lift up one end to pull both layers of skin away from the inner flesh. The outer skin is rough but the under layer (cortex) is smooth.
Cassava is used for folk medicines is said to be useful for treating diarrhoea, headache and irritable bowel syndrome.
Industrial uses include processing as starch, industrial alcohol, biodiesel, binders and fillers, glucose and alcoholic beverages.
21. Toxic content and detoxification
See diagram 16.3.1.7a: Breakdown of linamarin
The cyanogenic glucoside linamarin occurs in the cells of cassava (Manihot esculenta) linseed (flax, Linum usitatissimum) and Lotus japonica. The cassava plant contains the linamarin in the cells. Linamarin is hydrolysed by the enzyme linamarase to glucose and acetone cyanohydrin that can breakdown in alkaline conditions to acetone and hydrogen cyanide (prussic acid). Linamarase occurs in the cell walls of cassava and in the cell walls of bacteria in the human digestive tract. Most of the HCN is produced in the plant only after the root tubers are dug up and stored. So cassava stored for long periods could be high in HCN. In sweet varieties, the major part of the acid is located in the skin and in the exterior cortical layer, while in bitter varieties the acid is uniformly distributed in all parts of the roots. Humans can neutralize low levels of cyanide by converting the cyanide to less toxic thiocyanate compounds then excreting them. A concentration of more than 100 mg per kilogram is highly poisonous. Consumption of untreated or under-treated cassava is associated with the disease konzo, a form of chronic cyanide intoxication causing weakness of the lower extremities. The HCN content of cassava depends on the variety and is highest in plants grown on low fertility soils with potassium deficiency, first year growth and in a dry season. HCN concentration is usually highest in the rind and in the fibrous core at the centre. The cyanide derivatives are volatile so processing methods for cassava preparation include roasting, squeezing out the juice, blanching, boiling and fermentation.
Both the bitter cassava and the sweet cassava have hydrocyanic acid in the juice of the roots, with the bitter type containing a higher percentage, up to 1 g / kg of CN, especially during a drought. For safety, both bitter and sweet varieties should be cooked thoroughly before eating as hydrocyanic acid is volatile and driven off by cooking. To make bitter varieties safe to eat, cut the root into pieces and soak them in water for a day before cooking. Then boil them in clean water for 10 minutes. Sweet varieties can be peeled and cooked without soaking. However, they should still be boiled for 10 minutes before being eaten or added to stews. However many people just peel sweet cassava and eat it grated raw. Bitter cassava has dark green and sometimes red leaves. Avoid tubers that smell strongly of almonds.
1. Some methods of detoxification
After the tubers are harvested, peeled:
1.1 washed, soaked in water for three days, then squeezed to remove water, shredded and dried.
1.2 cut into pieces, grated, left to ferment for three days then inoculated with previously fermented material. After one day of further fermentation, the mixture is strained, heated and dried for storage. Fermentation is said to improve the taste and aroma of the bland tasting cassava.
1.3 partly dried in the sun, wrapped in leaves and put in a hole in the ground to allow growth of a fungus. The fungus growth removes some poisonous products. The fungus is scrapped off leaving a white starchy root that can be made into flour.
To detoxify cassava flour (tapioca) mix it with water to form a paste and leave it to stand in the shade for five hours in a thin layer spread. An enzyme in the flour breaks down the cyanide compound liberating hydrogen cyanide gas into the air. Only about 1/5 of the original cyanide remains, making the flour safe for consumption.
22. Make cassava starch
1. Peel off the skin and wash the remainder. Grate the cassava finely, wrap in coconut sheath and tie the bundle with rope. Clamp and press the package with a level to squeeze out most of the liquid. Open the package and pulverize the wet flour with a mortar and pestle. Steam the wet flour and dry it to powder form.
2. The cells of cassava roots are quite tough so you may only get a 20% yield of starch from the tubers. Wash the mature roots to remove soil. Peel the roots to remove the skin and cortex. Slice or grate the roots to make a slurry. Sieve the slurry to separate the fibrous tissue from the starch milk. Use lots of water to separate the starch granules from the slurry. Leave the starch milk to settle for six hours. When the starch sinks to the bottom, drain away the liquid. If the surface layer of the starch is a yellow green colour and contains impurities, scrape it off to leave the creamy white mass below. Stir the white mass of starch with water and leave it to settle to form a starch cake. You may have to do this washing and settling process more than once to get pure starch. Dry the starch cake in the sun. Crush the hard lumps of starch cake into a powder and sieve the powder.
3. Wash and peel the tubers. With larger roots you may need also to remove the centre cortex, no loss, it does not have as much starch anyway. Slice the flesh to a pulp with a food processor. Put the pulp in a sieve and wash the starch from the pulp into a bowl of clean water. When you have washed most of the starch from the pulp, leave the milky water to stand for six hours in the refrigerator or it will ferment. The starch will sink to the bottom of the bowl so you can carefully drain the clear liquid away. Scrape off the surface layer of the starch slurry to leave a creamy white mass. You can add more water and do the process again to wash out more starch from the pulp. The starch is then dried and made into flour.
23. Make tapioca
The yield of tapioca from fresh cassava tubers is about 25%. Tapioca is sometimes called Brazilian arrowroot, but it is not the true arrowroot from Maranta arundinacea.
1. To make tapioca flakes, rub the moist starch prepared above through a sieve to make a coarse flour then partly gelatinize it by cooking for 2 minutes in an iron frying pan smeared with coconut oil. Dry the flakes at 50oC to a 12% moisture content. To make tapioca pearls (seeds) turn the sieved damp starch into globules by shaking it in cloth bags. Grade the globules according to size and gelatinize them by roasting for 15 minutes on a frying pan smeared with coconut oil. Dry the globules at 50oC.
2. To make tapioca, push the moist cassava starch through a coarse sieve. Then partially gelatinize the coarse moist flour by cooking it for about two minutes in a hot pan. smeared with coconut oil. The flakes are then dried and can be stored to make tapioca puddings.
24. Keeping records and accounts
See also 6.9.20.0: Understanding the records
It is very important that the students keep good records of the growth of the plants. They should look at the plants on the same day each week. They should
1. Write down the date each time.
2. Make a small drawing of each growth stage.
3. When the plants grow too tall, their growth stage can be written down in words. The pictures in the diagram show some early stages that can be seen.
4. Watch the plants each week and see what happens.
5. Draw the cuttings.
6. Record the date when cuttings planted.
7. Record the growth reached in the first week.
8. Study the plants each week until harvest.
25. Future of cassava growing
A yield of 20 to 30 MT / ha can be achieved in good conditions, planting 50 000 cuttings per ha, at spacing 0.75 X 0.25 m. However, triangular spacing may increase yield. World average cassava yield is about 10 000 kg / ha (10 metric tons per hectare, 10 mt / ha, but about 14 000 kg / ha in the Asia and pacific region and up to 25 000 kg / ha in India. On plantations, yields of 30 to 40 MT / ha are possible and high yielding cultivars may yield 50 MT / ha. [1 metric ton (tonne) = 1 000 kg (2 204.6 pounds)] Yield decreases if intercrops are planted first. There is particular interest in increasing yields in plantations for the ethanol industry to service for biofuel reserves. In USA cassava produces 4 cubic metres (kilolitres) / hectare of bioethanol. Cassava is an important food crops in Africa with Nigeria the leading producer.
History
These teaching materials were originally written and illustrated by Mr J. A. Sutherland, Faculty of Education, University of New England, Armidale, Australia and later edited by Dr J. Elfick, School of Education, University of Queensland, Brisbane, Australia.