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YAM

YAM – INTRODUCTION

A monocot related to lilies and grasses, yams are vigorous herbaceous vines, providing an edible tuber. They are native to Africa, Asia, and the Americas. Some yams are also invasive plants, often considered a "noxious weed", outside cultivated areas. Yam tubers vary in size from that of a small potato to over 60 kg (130 lb). Some 870 species of yams are known, and 95% of these crops are grown in Africa.

Yam tubers can grow up to 15 m (49 ft) in length and 7.6 to 15.2 cm (3.0 to 6.0 in) high. The tuber may grow into the soil up to 1.5 metres (4.9 ft) deep. The plant disperses by seed.

The edible tuber has a rough skin difficult to peel, but softens after heating. The skins vary in color from dark brown to light pink. The majority of the vegetable is composed of a much softer substance known as the "meat". This substance ranges in color from white or yellow to purple or pink in mature yams.

Cultivation

Yam crop begins when whole seed tubers or tuber portions are planted into mounds or ridges, at the beginning of the rainy season. The crop yield depends on how and where the setts are planted, sizes of mounds, interplant spacing, provision of stakes for the resultant plants, yam species, and tuber sizes desired at harvest. Small-scale farmers in West and Central Africa often intercrop yams with cereals and vegetables. The seed yams are perishable and bulky to transport. Farmers who do not buy new seed yams usually set aside up to 30% of their harvest for planting the next year. Yam crops face pressure from a range of insect pests and fungal and viral diseases, as well as nematode. Their growth and dormant phases correspond respectively to the wet season and the dry season. For maximum yield, the yams require a humid tropical environment, with an annual rainfall over 1500 mm distributed uniformly throughout the growing season. White, yellow, and water yams typically produce a single large tuber per year, generally weighing 5 to 10 kg (11 to 22 lb).

Despite the high labor requirements and production costs, consumer demand for yam is high in certain subregions of Africa, making yam cultivation quite profitable to certain farmers.

Storage

Roots and tubers such as yam are living organisms. When stored, they continue to respire, which results in the oxidation of the starch (a polymer of glucose) contained in the cells of the tuber, which converts it into water, carbon dioxide, and heat energy. During this transformation of the starch, the dry matter of the tuber is reduced.

Amongst the major roots and tubers, properly stored yam is considered to be the least perishable. Successful storage of yams requires:

  • initial selection of sound and healthy yams
  • proper curing, if possible combined with fungicide treatment
  • adequate ventilation to remove the heat generated by respiration of the tubers
  • regular inspection during storage and removal of rotting tubers and any sprouts that develop
  • protection from direct sunlight and rain

Storing yam at low temperature reduces the respiration rates. However, temperatures below 12 °C (54 °F) cause damage through chilling, causing a breakdown of internal tissues, increasing water loss and yam's susceptibility to decay. The symptoms of chilling injury are not always obvious when the tubers are still in cold storage. The injury becomes noticeable as soon as the tubers are restored to ambient temperatures.

The best temperature to store yams is between 14 and 16 °C (57 and 61 °F), with high-technology-controlled humidity and climatic conditions, after a process of curing. Most countries that grow yams as a staple food are too poor to afford high-technology storage systems.

Sprouting rapidly increases a tuber's respiration rates, and accelerates the rate at which its food value decreases.

Certain cultivars of yams store better than others. The easier to store yams are those adapted to arid climate, where they tend to stay in a dormant low-respiration stage much longer than yam breeds adapted to humid tropical lands, where they do not need dormancy. Yellow yam and cush-cush yam, by nature, have much shorter dormancy periods than water yam, white yam, or lesser yam.

Storage losses for yams are very high in Africa, with insects alone causing over 25% harvest loss within 4 months.

Yam production - 2018

Country

Production
(millions of tonnes)

 Nigeria

45.0

 Ghana/p>

7.1

 Ivory Coast

5.8

 Benin

3.2

 Ethiopia

1.4

 Togo

0.8

 Cameroon

0.6

World

68.1

Source:UN Food & Agriculture Organization


Nutritional value

Yam, raw

Nutritional value per 100 g (3.5 oz)

Energy

494 kJ (118 kcal)

 

Carbohydrates

27.9 g

Sugars

0.5 g

Dietary fiber

4.1 g

 

Fat

0.17 g

 

Protein

1.5 g

 

Vitamins

Quantity%DV

Vitamin A equiv.

1%

7 μg

Thiamine (B1)

10%

0.112 mg

Riboflavin (B2)

3%

0.032 mg

Niacin (B3)

4%

0.552 mg

Pantothenic acid (B5)

6%

0.314 mg

Vitamin B6

23%

0.293 mg

Folate (B9)

6%

23 μg

Vitamin C

21%

17.1 mg

Vitamin E

2%

0.35 mg

Vitamin K

2%

2.3 μg

 

Minerals

Quantity%DV

Calcium

2%

17 mg

Iron

4%

0.54 mg

Magnesium

6%

21 mg

Manganese

19%

0.397 mg

Phosphorus

8%

55 mg

Potassium

17%

816 mg

Zinc

3%

0.24 mg

 

·         Units

·         μg = micrograms • mg = milligrams

·         IU = International units

Percentages are roughly approximated using US recommendations for adults. 
Source: USDA Nutrient Database


Raw yam has only moderate nutrient density, with appreciable content (10% or more of the Daily Value, DV) limited to potassium, vitamin B6, manganese, thiamin, dietary fiber, and vitamin C (table). Yam supplies 118 Calories per 100 grams. Yam generally has a lower glycemic index, about 54% of glucose per 150 gram serving, compared to potato products.

The protein content and quality of roots and tubers is lower than other food staples, with the content of yam and potato being around 2% on a fresh-weight basis. Yams, with cassava, provide a much greater proportion of the protein intake in Africa, ranging from 6% in East and South Africa to about 16% in humid West Africa

As a relatively low-protein food, yam is not a good source of essential amino acids. Experts emphasize the need to supplement a yam-dominant diet with more protein-rich foods to support healthy growth in children.

Yam is an important dietary element for Nigerian and West African people. It contributes more than 200 calories per person per day for more than 150 million people in West Africa, and is an important source of income. Yam is an attractive crop in poor farms with limited resources. It is rich in starch, and can be prepared in many ways. It is available all year round, unlike other, unreliable, seasonal crops. These characteristics make yam a preferred food and a culturally important food security crop in some sub-Saharan African countries.