School Science Lessons
2024-07-05
Sweet Potato Project
(SPotProj)
Contents
Preface
1.0 Sweet potato plant
2.0 Leaves
3.0 Stem
4.0 Tubers
5.0 Flowers
6.0 Planting material
7.0 Planting sweet potato
8.0 Crop care
9.0 Fertilizers
10.0 Insect pests, Sweet potato weevil
11.0 Diseases
12.0 Harvesting
13.0 Storing sweet potato tubers
14.0 Costs and profits
15.0 Sweet potato varieties
16.0 Food value of sweet potatoes
17.0 Chemical constituents of sweet potatoes
1.0 Sweet potato plant
See diagram 61.1: Sweet potato plant, leaves, flowers.
Ipomoea batatas, Sweet potato tubers, Kumara, Daleys Fruit Tree Nursery
Agroclavine | Sweet potato, (Sweetpotato), Ipomoea batatas, Convolvulaceae, is a climbing herbaceous perennial vine, alternate palmately lobed leaves, sympetalous trumpet-shaped purple flowers,
edible tuberous root with smooth skin, colour n yellow to orange to red brown to purple, tuber flesh white to yellow to purple.
It comes from South America, but is now grown very widely in many tropical and temperate countries, e.g. USA, and is now grown in South West Asia.
It is the chief food crop in Papua New Guinea and is very important in the Solomon Islands and New Hebrides.
In Papua New Guinea it is called kau kau, in Fiji it is called kumara and in some Polynesian islands it is called kumala.
It produces starchy underground tubers and young green leaves which can be soaked and eaten.
It can live for years, but it usually harvested after about 3 months, before it can flower.
The biological relatives are th tropical creepers, e.g. the Morning Glory, Convolvulus sp., Convolvulaceae.
There are many different varieties producing tubers with different tastes and yields.
It is probably easier to grow than any other food plant and it does not suffer from many diseases.
They are dependable, will always give a crop and are very quick in growth, with some varieties giving a crop in 3 tor 4 months.
Uses
Sweet potato is used as an important food crop after, steaming, boiling, frying, baking.
The tubers can be cut into small pieces and dried in the sun to keep for a long time, and can be ground into flour.
The tubers and leaves can also be fed to pigs even if they infected with the sweet potato weevil.
The leaves and excess growth can be added to the compost.
It may be grown just as a colonizing ground cover on poor soils to keep soil moisture high and add organic matter.
It may be used to function as a living mulch to keep the weeds down.
Climate
The sweet potato grows well in warm, sunny and humid regions, but it can grow well in dry savanna country.
It grows well in temperatures of 24oC or more with plenty of sunlight and warm nights.
The plants will not grow well when the temperature is below 10oC.
Sweet potatoes will grow well in many situations from those that are wet to those that are dry.
At sea level it can grow well, and some varieties can grow at over 1500 metres.
Soil
Sweet potato grows best in a light, well-drained, sandy loam containing rotten plant material, but useful crops can be grown in poor soils.
If the soil containing too much organic matter and high in nitrogen, e.g. chicken manure, it will produces too many stems and leaves, and few tubers.
The pH of soil should be 5.7 to 6.7.
Rainfall
Sweet potato needs regular rain to grow well, especially when the leaves are starting to grow about 50 days after planting.
It requires a well distributed rainfall of 75 -150 cm for good production.
However, but too much rain may damage the plants and at harvest time it may cause tuber rot.
2.0 Leaves
See diagram 61.2: Leaf shapes.
Leaves of different varieties vary greatly in size and shape
The leaf shape may be entire, i.e. have no lobes, but are just one shape, or heart- shaped, or deeply divided with 3, 5 or 7 mostly pointed-shape lobes.
The long thin petioles and leaf veins are green to red in colour.
The leaves of sweet potatoes have a long thin leaf stalk or petiole.
Experiment.
Collect sweet potato leaves of different shapes.
Trace the outline of 3 leaves of different shapes on to paper.
3.0 Stems
The stem of the sweet potato plant is thin, bends easily, has little lumps along it called nodes, like knots.
The leaves are attached to its younger parts.
Where leaves drop off, a leaf scar remains.
Buds on the stem are just above the position of each leaf scar.
Every fifth leaf is directly in line with another leaf below it.
If a piece of stem is buried, new leaves will grow from the buds above the ground and roots will grow from the stem below the ground.
When a stem grows over the surface of the soil, some roots may grow down from some nodes that are touching the soil.
4.0 Tubers
See diagram 61.4: Sweet potato tuber.
See diagram 61.13: Sweet potato tuber with shoots and roots.
See diagram 61.14: Sweet potato tuber with shoots and roots.
Tubers are produced when the roots spreading out from the planting material turn down into the soil and accumulate food reserves.
The "crown" end may produce shoots.
An outer periderm may produce secondary roots.
The tubers of sweet potatoes vary greatly in size, weighing between 0.3 and 3 kg, shape and colour of the flesh and of the skin.
The most common varieties have long spindle-shaped tubers, but other varieties have nearly round or nearly oval tubers.
The outside of the tuber has small pits on it with a bud at the bottom of each pit, which can grow out into a secondary roots.
Tubers with smooth shapes are preferred, because tubers with long grooves in them are difficult to peel without wasting a lot of flesh.
The colour of the skin and of the flesh may be white, cream, yellow or purple, with dark orange varieties containing beta carotene.
Some tubers are very good to eat, with a pleasant and feel good in the mouth.
5.0 Flowers
See diagram 61.5: Sweet potato flower.
Flowers, usually violet, purple, white, are clustered in the leaf axils.
They are formed singly or in small clusters in the axils of the leaves.
The 5 petals are joined to make a tube.
The 5 small green sepals are under the petals.
Seeds are contained in a small capsule that splits open when it is ripe, to release 2 to 4 small brown or black seeds with sharp corners.
The seeds are not usually used for sowing sweet potatoes.
Many varieties of sweet potato in cultivation do not have time to produce flowers and fruits before the harvest.
Experiment
Collect some flowers and seeds.
Describe the colour of the flowers an seeds in the seed capsules and draw them.
6.0 Planting material
See diagram 61.6: Methods of planting, Planting material.
Cuttings
Propagation from cuttings is the quickest and easiest way, and most economic way, of increasing sweet potato plants.
However, propagation from cuttings is possible only when an established vines of sweet potatoes remain in the field all through the year.
1. The most common planting method is to plant tip cuttings.
Cut a piece of stem, 20 to 40 centimetres long, with three to five growth buds, and taken from the tips of young stems.
Remove all the leaves except for the tiny leaves at the very tip.
Plant the cutting by covering almost the whole length with soil, leaving only the leaves of the tip should sticking out of the ground.
Take the cuttings only when you are ready to plant them, and keep them in the shade until they are inserted in the soil.
In warmer climates, plant the cuttings runners straight into the prepared soil, then water in well.
In cooler climates, plant the cuttings into pots, water in well, and keep the pots in a warm place, e.g. a sunny windowsill.
2. Use pieces of stem and leaves from the middle part of the stem.
The best pieces of stem to use for planting have a new shoot growing in a leaf axil.
Leave stem cuttings for a week until the leaves have died, then plant the stem piece.
3. Leave a bundle of stem cuttings in the shade and covered with grass or banana leaves to make roots growing out from the covered nodes.
4. Cut off around 4 cm just above a leaf at the tip of a vine, remove the lower leaves and place in water until roots appear.
Slips
Slips are shoots that are grown from a mature sweet potato tuber, which can be planted 40-60 cm apart.
1. Keep tubers in weak light in the shade until they start to sprout and cut off pieces of the tuber with the attached shoots then plant the pieces.
2. When the shoots are 15 -20 cm in length, they can be broken off the tuber and planted.
3. Cut a clean healthy tuber in half and place in water, half out of the water and half submerged, or half bury a tuber in moist sand or potting mix.
Keep the half tuber in a warm place until shoots begin to grow above and roots begin to grow below.
See diagram 61.6: Methods of planting, Planting material
1. Use pieces of stem and leaves from the middle part of the stem.
The best pieces of stem to use for planting have a new shoot growing in a leaf axil.
The most common planting method is to plant tip cuttings.
2. Leave stem cuttings for a week until the leaves have died, then plant the stem piece.
3. Leave a bundle of stem cuttings in the shade and covered with grass or banana leaves to make roots growing out from the covered nodes.
4. Keep tubers in weak light in the shade until they start to sprout.
Cut off pieces of the tuber with the shoots attached to them, then plant the pieces.
5. When the shoots are long enough, 15 -20 cm in length, they can be broken off the tuber and planted.
7.0 Planting sweet potato
See diagram 61.7: Sweet potato gardens, Ridges, Mounds.
The sweet potato grower should know the varieties and know how long they take to form tubers, and be harvested.
If planting several times, at intervals, all the tubers do not become ready for harvest at the same time, so they can be harvested only when needed.
1. Planting on the flat
The garden is prepared by digging up the soil, burying weeds, and making the soil loose.
This method of planting is not used very much, because it does not make good drainage.
2. Planting inn ridges 25-30 cm height and 60 cm apart or in moulded beds 30 cm apart
Ridges may be made by hand or by ploughs which throw up the earth up into ridges of loose soil that allow for good drainage.
3. Planting on mounds
The best method of planting is to form small mounds and plant the cuttings into the top of the mounds.
The mounds can be made by hand or be formed from ridges first made by a plough.
A mound may 25 to 90 cm wide across the top and be heaped up until 15 to 40 cm high.
Well-buried plant material may make the soil loose and warm that will help the sweet potatoe plants to grow better.
4. Use cuttings cut about 20 cm from the growing tip of the runner.
Trim all the leaves off except for the two small emerging leaves.
or
Make cuttings from 25-30 cm pieces of stem that have 5 nodes and to bury the bottom 3 nodes and leave 2 of them above ground.
Plant the cuttings 40-60 cm apart, buried horizontally.
5. The depth of planting cuttings
In some places cuttings are buried 30 cm deep, but in other places they are only buried 6 cm deep.
Planting too deep makes it too hard to harvest the tubers.
Some people plant the cuttings so they are straight up in the soil.
Some people plant the cuttings so they are at an angle of 30o or 60o to level.
Some people plant the cuttings so that the bottom part of the cutting is buried horizontally and the top part is bent upwards.
6. Usually more than one cutting is planted at each planting hole.
Planting 2 to 5 cuttings in one place on a mound is usual.
Plant a new row every week to get a continuous supply.
Dig a sweet potato garden, planting 3 cuttings in each mound.
Use cuttings that have 5 nodes and plant them so the bottom 3 nodes are under the soil and 2 are above the soil.
7. The soil should be pH 5.5-6.5, with good levels of potassium and phosphorus to maximize tuber size.
Ridges and mounds
Sweet potatoes are usually grown on ridges or mounds after deep tilling, protect them from too much moisture, so better than growing on the flat
The ridges are made about 75 cm apart, but it is better to plant on round mounds 30 to 40 cm high, 1 m apart.
Mounds should be as narrow as possible to forces the plant to bend its roots downward quickly, build up food reserves and develop tubers.
Planting sweet potato
The best way to control pests and diseases is to always plant sweet potato in new ground and use clean planting material.
Prepare soil for sweet potato and plant it.
1. On light sandy soil plant into ridges one metre apart.
2. On heavy clay soils plant into mounds.
The soil must be well drained.
Select cuttings about 50 cm long from the growing tips of healthy plants, which are producing large tubers.
Some people cut back old vines near the roots and later use the new shoots for cuttings.
Make ridges by digging trenches one metre apart, put in rotten plant material, plant ashes, potash or a low nitrogen fertilizer, e.g. NPK 5.10.10 or NPK 6:9:15.
Then hill up over the filled furrow.
The soil should be light and fine with no clods, sticks or stones in it.
Plant the cutting five cm deep, three nodes underground, 50 cm apart.
If you plant two rows on each ridge most of the ground will be covered to control weeds, but tubers will be smaller than if planting only one row.
Press down around each cutting with the foot.
The soil should be damp when planting.
Mulching will keep the soil damp and protect the ridges from erosion.
The soil should not be very dry just, because harvested tubers can be attacked by the sweet potato weevil.
Village people have learned many interesting ways to grow sweet potato and you should learn your local method.
However, you should also experiment with new methods, e.g. dry season planting.
Use some clean cuttings.
Draw a diagram to show how to make ridges and where to plant them.
Show how to hill up the soil to make ridges.
Show how to select plant cuttings.
Show how to plant cuttings in the ridges.
You may then use mulch to keep the soil damp.
Make mounds.
Build up heaps of rotten plant material and wood ash.
Cover the heaps with soil to make a round mound about one metre across and half a metre high.
Plant four cuttings around the top of the mound.
Planting
Plant cuttings at a slant, leaving 3 or 4 cm above ground, and press the soil down firmly.
If planting on mounds, plant four or five cuttings in a circle on each mound, to give a planting density of 15 000 to 30 000 plants per hectare.
Propagate from tubers, if you do not have any plants of sweet potatoes with enough leafy growth to provide cuttings.
Tubers must be made to sprout in a cool nursery bed.
If the tubers are large, cut them into several pieces.
After about a month, remove from the tubers the young shoots that are 15 to 20 centimetres long and plant them.
This method of propagation from tubers is usually done only on about one third of the area of the sweet potatoes field.
Later, cuttings from the plants thus obtained can be used to enlarge the plantation.
When cultivating, remake the mounds at the same time.
8.0 Crop care
1. Use hand weeding to pull out all the weeds every two weeks after planting and burn the weeds when dry.
For control of weeds, one or two cultivations in the early stages of growth are enough, because 4 to 6 weeks after planting, the plant's own leafy growth will closely cover the soil.
2. Water the plants if the soil becomes dry, but if heavy rains wash away soil, hill up ridges with a hoe.
3. Lift the vines to prevent secondary roots growing.
4. Look for pests and diseases.
If you find anything attacking the sweet potato put the pest or disease and the part of the plant in methylated spirits.
The caterpillars of the hawk moth and the leaf miner moth will eat the leaves.
The sweet potato weevil walks down cracks in dry ground to eat the tubers.
The vine scab disease forms spots on the tips of stems.
Rats will try to eat the tubers.
5. Prepare for harvest
Harvest when the leaves turn yellow Feel the size of some tubers in the ground, then feel the size the following weeks.
Weigh some sample developing tubers.
The tubers are ready to harvest when the skin of the tubers is firm, when a cut piece turns creamy white, not greenish black around the edges, or have a white juice that turns black.
Dig out the tubers with a stick or a fork, not a spade which will cut them.
Dry them in the sun and store in a cool dry place.
Leave the tubers in the ground to harvest only when you need them or cut off the vines at ground level and dig up the lot.
Burn all the cut vines to kill all the pests and diseases on them.
Growth studies
See diagram 61.8: Sweet potato diary.
Keep records of the growth of the crop.
1. Note how many stems come from each cutting.
2. Note how fast do the stems grow, i.e. what length of stem is grown each day.
3. Note when the tubers start to form by uncovering the roots of one plant each week.
The normal life of the plant is 4-5 months at sea level, but 5-6 months if planted high up in the mountains.
4. Note when the tubers have formed properly and the plant is near the end of its years growth.
At this stage some leaves may turn yellow.
9.0 Fertilizers
This earthy vegetable likes crumbly, loose soil, well composted with organic matter, which helps create a loose, permeable, healthy bed.
Sweet potatoes grow best in a sandy loam (approx. proportions sand 53%, silt 43%, and clay 7%) preferably at temperatures above 24 C.
Although they need water, do not soak the soil, because sweet potato can rot in damp conditions.
Sweet potatoes respond well to fertilizers, especially if containing nitrogen and potassium.
The fertilizers to use are as follows:
1. Ammonium sulfate or urea or IBDU.
2. Chloride of potash or sulfate of potash.
Do not add too much of them or it will burn the plants.
Use about one matchbox full of mixed fertilizer and spread it over one square metre.
Do not put the fertilizer too close to the plants.
Sprinkle it over the ground, but keep it 10 cm away from the plants and dig it lightly into the soil.
3. Trim excess growth to increase the size of the tubers.
Insect pests.
See diagram 61.10: Sweet potato weevil 1.
See diagram 61.3: Sweet potato weevil 2.
See diagram 61.15: Tuber damaged by Sweet potato weevil 1.
Sweet potato weevil, Cylas formicarius in beetle family Brentidae is the most serious pest of sweet potato, causing damage in field and storage.
It is found in Australia, the South Pacific and many other countries, and is widespread in Queensland, with losses up to 97%.
The West Indian sweet potato weevil, Euscepes postfasciatus is found in North America.
A complete life cycle occurs in 35 to 40 days and all stages can be found during the year in sweet potato tubers.
During winter adults may seek shelter and remain inactive until the weather is favourable.
The only vegetable crop they feed on is sweet potato, but they also feed on Convolvulaceae plants, including railroad vine (Ipomoea pes-caprae), and morning glory (Ipomoea panduratea).
A symptom of infestation is yellowing of the vines, but a heavy infestation is usually necessary before this is noticed, so infestation may be overlooked, because damage is not apparent until harvest.
Adult
The adult body, legs, and head are long and thin, so it looks like an ant, and measures 5.5 to 8.0 mm in length.
It is shiny and smooth, but with short hairs. The head is black, with an extremely long rostrum (beak).
The antennae, thorax and legs are orange to red-brown, and the abdomen and elytra are metallic blue.
The adult weevils can feed on the leaves, vines, roots and the outside of the tubers.
Adults usually have limited access to tubers, so damage by them is less severe than by larvae and adult feeding on foliage is limited.
However, adults may feed on the tubers, creating many small holes on the outside of the tubers.
An adult weevil is a metallic blue and orange and about 6 mm long.
It eats everything, stems, leaves and roots.
The weevils lay their eggs in the roots and the larvae tunnel through the roots and make a rotten mess of them.
The female weevils chew small holes in the surface of the stems or tubers.
They then lay their eggs in these cavities.
The eggs hatch and a small larva comes out of the egg.
In dry weather, cracks may appear in the soil which allow weevils to go down to the tubers.
Eggs
The soft white eggs are about 0.65 X 0.46 mm and are laid in the root and the larvae burrow through the tubers making a sodden rotten mess.
Larvae
The main form of damage is mining of the tubers by larvae so that the tuber is riddled with dark, spongy cavities.
Also, larvae cause damage indirectly by facilitating entry of soil-borne pathogens.
Even low levels of feeding on the roots may cause a chemical reaction that gives a bitter taste and terpene odour to the tubers, so unmarketable.
Larvae also attack the vine of the plant, causing it to darken, crack, or collapse.
The larvae bore into the stems or tubers.
The larvae have no legs, are white in colour and are about 8 mm long.
If the larvae have been living in the stem, they often go down into the tubers later.
The main form of damage is mining of the tubers by larvae so that the tuber is riddled with dark, spongy cavities.
Also, larvae cause damage indirectly by facilitating entry of soil-borne pathogens.
After 3 weeks the larvae turn into a resting stage called a pupa.
Later the adult weevil comes out of the pupa.
Damage
The edges and the surface of the leaves will appear chewed with small holes and will wilt, dry, or discolour.
The stems may show signs of damage, rotting, and distortion.
Biological Control
Do not plant any more sweet potatoes for at least a year in that place so weevils cannot go from old to new plants and tubers.
Mulching prevents soil cracks, improves soil moisture and provides a habitat for the enemies of the weevil.
Do not leave any tubers in the soil after harvest.
After harvesting all the tubers, pull out all the remaining roots and burn them.
Do not use previous crops for planting material, because it may contain weevils.
Remove alternate wild hosts from near the garden e.g. wild convolvulous vines, carrot roots, and radish roots and leaves.
Use commercial pherome traps or fungal pathogens.
Control using insecticides
Use insecticides containing Diazinon and Carbofuran.
Before planting tubers, slips and cuttings, dip them in an insecticide solution.
Granular or liquid formulations are used, and systemic insecticides are preferred.
Post planting applications may be made to the foliage for adult control, especially if nearby fields are infected.
If systemic insecticide is applied, some suppression of larvae developing in the vine may also occur.
Due to the long duration of the plant growth period, preplant or planting time applications may be followed by mid season applications.
In places where harvested sweet potatoes are stored, the soil can be fumigated.
Fall armyworm (Spodoptera frugiperda) is an exotic pest that has been detected in Queensland.
Tubers with weevils in them can be fed to pigs.
11.0 Diseases
To avoid diseases, do not grow sweet potatoes on the same soil for two years in succession.
If sweet potatoes are attacked by diseases and insects, burn any remaining plant material, and wait for years before growing them again in the same field.
Signs of the disease are yellow withered leaves and black marks inside the stems and tubers.
Other fungi may cause the young plant to rot and it stops growing.
To control rot, choose clean resistant varieties, so do not use plants attached by rot for propagation cuttings or tubers.
Sweet potato feathery mottle virus is the most common and widespread virus infecting sweet potato, in most countries where the crop is grown.
The virus can reach high levels of infection in crops propagated from infected material or grown near infected crops, to allow aphid transmission
from crop to crop.
12.0 Harvesting sweet potatoes
Sweet potatoes are ready for harvesting when most of the leaves have turned yellow and have started to fall off the plant.
A tuber is ready for harvest when the juice from a cut with a knife does not go black quickly.
To avoid damaging the tubers, use a small digging tool to start digging from the side of the ridge or mound to first loosen the soil.
Then use your fingers to find the first tubers, so you know their depth in the soil.
Dig carefully to turn over the soil and expose the tubers.
13.0 Storing sweet potato tubers
1. The length of time for which sweet potatoes can be kept differs with the varieties and the harvesting season.
If they are harvested in dry weather, the tubers may be stored for 2 or 3 months.
Store only those tubers which are dry, firm to hold with brightly coloured skin, and with no cuts, holes or insect damage.
Store them loose in a cool, dark ventilated place, not in plastic bags and not in a refrigerator.
Do not wash them after harvesting, but keep them dry, and remove dirt with a dry brush to prevent rotting from too much moisture.
Put them on dry ground or on boards supported on posts, and do not heap them up too much.
Wrapping in paper can protect sweet potatoes from fungus or insects.
2. Sweet potato tubers go bad very easily, so storing them for a long time is not usual.
However, there is a way of storing them for short periods, by "curing" them first.
Select only mature tubers with flesh that is white when cut.
Start with air-dried tubers.
Curing is done by giving the tubers a high temperature of 29oC and keeping them in moist air for 4 to 7 days.
Keep them in a room for curing, but there must be air flowing through this place.
After about a week of curing, this hardens the skin of the tubers and this keeps out the fungus that makes them go bad.
However, chilling may cause a hard core condition that remains even after cooking.
3. After curing for about a week, put the tubers where it is cool and the temperature is only about 16oC.
Temperature below 10oC is too cold for the sweet potatoes and they will be damaged.
In some places the people store the tubers in pits made in the ground in a cool place.
They may keep them for times up to 4 months in the pits, but usually you can only store them for shorter periods, e.g. 4 weeks.
4. To test if the tubers have been properly cured, the skin of the tubers does not slip easily when pushed, the buds may have a purplish colour,
the skin looks dry and spongy.
14.0 Costs and profits
1.16Records
1. Establishment costs are the things you buy, which will probably last for a long time, e.g. spades, hoes, sprays.
Divide the establishment costs by the number of years you think these things will last and you add this amount to the costs, e.g. 3 years.
2. Production costs are things that you must buy each year that you grow the sweet potatoes, e.g. fertilizers and insecticides.
3. Profit = returns - establishment costs - production costs.
Yields of sweet potatoes and storing.
Depending on the varieties of sweet potato and on the way they are grown, yields vary from 4 to 7 tons per hectare on average.
On a modern and well- cared- for plantation, yields may be much higher, and may even be more than 20 tons per hectare.
15.0 Sweet potato varieties
Sweet potato, Daleys Fruit Tree Nursery
1. Sweet potato varieties sold in Australia.
Gold varieties: gold skin, orange flesh, Beauregard.
'Beauregard' variety has a gold smooth skin with orange flesh and is a good source of Vitamin A.
See diagram 61.11: 'Beauregard' variety.
'Sweet n Gold' variety is very sweet and has orange to golden flesh, and is probably the most popular variety sold in Australia.
Also, 'Bellevue' and 'Orleans'.
Red varieties: red skin, white flesh.
'Red Garnet' is moist, sweet flavoured, soft textured.
'Northern Star' variety has purple skin with white flesh, is drier than the orange varieties and not as sweet, so suitable for fried chips.
'Kumara' variety has large tubers, red skin and ivory flesh mixed with violet, with small emerald green leaves and dark purple petioles, high levels of
anthocyanins, can be eaten fresh or steamed.
See diagram 61.9: 'Kumara' variety

Purple varieties: white skin, purple flesh.
'Sweet n Star' variety has purple skin and pale cream flesh, is less susceptible to attack by soil insects than gold types.
'WSPF' or 'White Skin Purple Flesh' variety has internal purple rings.
Also, 'Murasaki'.
See diagram 61.12: Purple variety.
White varieties: white skin white flesh.
'Kestle' variety has white skin with white flesh.
2. Where sweet potatoes are grown, there are usually many varieties with local names.
Collect some leaves and tubers of as many varieties and note the features of the different varieties:
| Shape of leaves | Colour of tuber skin | Colour of tuber flesh | Shape of tube | Time to maturity | Taste of cooked tuber.
The main varieties are:
| Dry and starchy | Red or purple, containing anthocyanin | Moist and sweet | Dark orange containing beta carotene (called "yams" in USA)|.
16.0 Food value of sweet potatoes
Eat young sweet potato shoots and leaves in stir fries and salads.
The tubers are full of sugar and starch so they are a good energy food, but they contain a very little protein food.
They are rich in iron and calcium, minerals and in vitamins, e.g. vitamin A.
During cooking, enzymes break down starch to maltose between 60oC and 75oC.
So slow baking is better than rapid boiling to allow longer enzyme action.
Some red varieties become dark during cooking caused by phenolic compounds.
Do some local varieties have special uses, e.g. for babies?
How do people cook them?
Do you need to peel them?
Sweet potato is an important energy food.
Some varieties are also healthy foods, e.g. red flesh varieties.
Some varieties are also growth foods if you eat the skins.
Make sweet potato biscuits.
Cut tubers into thin slices and dry them in the sun.
After a few days store the pieces in a tin with a tight lid.
The pieces can be broken up with a hammer to make flour.
Mix flour with margarine, baking powder and some milk.
Roll out dough cut into shapes and bake in hot oven.
You will need sweet potato leaves and tubers.
Sweet potato in the diet
Sweet potatoes have nutritional value (protein, fibre, Vitamin C and calcium), and a low GI rating.
Both young and mature leaves have significant amounts of Vitamin B6.
Leaves can be added to a stir fry like spinach.
Try frying with garlic for an added boost as they pick up other flavours.
The emerging shoots, tips, stalks and unformed leaves can be eaten in stir fry dishes or salads.
White varieties can be cooked as chips (French fries). boiled or roasted.
Grated sweet potato can be used to thicken soup and curries, and also be used as a filler in cakes, breads and fritters.
Dried sweet potatoes can be made into a flour and used is batters and pancakes.
The sweet potato plant produces starchy underground tubers and young green leaves which can be soaked and eaten.
Sweet potato can be cut into small pieces and dried in the sun.
This will keep a long time and can be ground into flour.
Make sweet potato biscuits
Cut tubers into thin slices and dry them in the sun.
After a few days store the pieces in a tin with a tight lid.
The pieces can be broken up with a hammer to make flour.
Mix flour with margarine, baking powder and some milk.
Roll out dough cut into shapes and bake in a hot oven.
An USDA Nutrient Database records 100 g of sweet potato containing 21 g of carbohydrates, mostly starch and sugars, 3 g of fat, 2 g of protein,
and vitamins, especially Vitamin A, besides Vitamin B1 to B9, Vitamin C and E, and Minerals, especially Potassium.
17.0 Chemical constituents of sweet potatoes
Sweet potatoes contain | Calystegin B C7H13NO4 | Ipomoeamaronol | Ipomoeamorone | in the leaves and tubers.
Preface
Before teaching this project, discuss the content with a field officer of the Ministry of Agriculture and get advice on planting material.
planting distances, site 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.