Preface

Yam (also called Ñamé -Spanish and Igname - French) belongs to the genus Dioscorea(family Dioscoreaceae). Of the estimated 300-600 species are available, there are just over half-dozen principal species that are grown for consumption, while others are grown for medicinal purposes. Yams originated in the Far East and spread westwards. They have since evolved independently in the Eastern and Western Hemispheres, and today yams are grown widely in throughout the tropics. In the West African yam zone, which is the principal producer on a global basis, D. rotundata, D. alata, and D. esculenta are the most common species. Yams have both economic and social value in many growing areas. Some traditional ceremonies are celebrated with yam as the major food item such the New Yam Festival in parts of West Africa. The production and utilisation of yam is declining in most producing areas due mainly to the high labour demand and the delicate nature of the harvested crop. The small-scale farmers who produce the majority of crop need access to innovations, which will reduce drudgery and improve productivity at all levels (on-farm and post-production operations). The objective of this chapter is to outline the techniques and procedures for harvesting, handling and storing yams. Some physico-mechanical properties of yams will also be presented to assist in the design and selection of appropriate handling and processing systems.

1. Introduction

1.1 Economic and Social Impacts of Yams

Yams are second to cassava as the most important tropical root crop. Yams are a staple crop in many parts of Africa and Southeast Asia. In the South Pacific, the yam is a significant food crop, accounting for over 20%, 8.1%, and 4.6% of the total dietary calorie intake in the Kingdom of Tonga, Solomon Islands, and Papua New Guinea, respectively. Besides their importance as food source, yams also play a significant role in the socio-cultural lives of some producing regions like the celebrated New Yam Festival in West Africa, a practice that has also extended to overseas where there is a significant population of the tribes that observe it. In some parts of Southeastern Nigeria, the meals offered to gods and ancestors consists principally of mashed yam. Yams store relatively longer in comparison with other tropical fresh produce, and therefore stored yam represents stored wealth which can be sold all-year-round by the farmer or marketer. In parts of Igboland in Southeastern Nigeria, it is customary for the parents of a bride to offer her yams for planting as a resource to assist them in raising a family.

Fig. 1. Yam and other root crops on sale in the Kingdom of Tonga at a local market.

1.2 World Production and Trade

Most of the world production of yam is from Africa (about 96%) with Nigeria alone accounting for nearly 75% of the total world production (Tables 1 and 2). World annual production was estimated to be 25 million Mt in 1974, and 24 million Mt in 1992. During the past 5 years, total world production has increased from 32.7 million Mt in 1995 to 37.5 million Mt in 2000 (Table 3). Also during this period, export quantity declined slightly while export income remained fairly steady. During the period 1975-90, total yam cultivated area increased by about 38.8% globally, while the total production increased by 45.8%. However, the importance of yam in the economy of the main producing areas appears be declining due partly to competition with other crops like cassava in Nigeria, and taro in the South Pacific (Opara, 1999). The major producing areas have also continued to experience high population growth rates. During the last four decades, the annual growth rate (%) of per capita production in the major yam zones in Africa has declined (Dorosh, 1988).

Table 1: World production of yam (Dioscorea).

Area

(10³ ha)

% of World Area

Production

(10³ Mt)

% of World Production†

Yield

(Mt.ha^-1)

% of World Yield

World

2,110

100

20,198

100

9.6

100

Africa

2,049

97.1

19,539

96.7

9.4

97.9

North & Central America

22

1.0

243

1.2

11.1

115.6

South America

10

0.5

48

0.2

4.7

0.5

Asia

15

0.7

168

0.8

11.4

118.8

Oceania

15

0.7

200

1.0

13.5

140.6

Leading countries

1. Nigeria

1,350

64.0

15,000

74.3

11.1

115.6

2. Cote d’Ivoire

200

9.5

1,700

8.4

8.5

88.5

3. Ghana

160

7.6

800

4.0

5.0

52.1

4. Togo

100

4.7

750

3.7

7.5

78.1

5. Benin

59

2.8

610

3.0

10.3

107.3

Source: Adapted from (FAO, 1975)

Table 2: World’s leading yam producers in 1990.
	Area (103 ha)	% of World Area	Production (103 Mt)	% of World Production	Yield (kg.ha-1)	% of World Yield
World	2,928	100	29,447	100	10,057	100

Africa	2,789	95.3	28,249	95.6	10,127	100.7
West Indies	59	2.0	350	1.2	6,122	60.9
Oceania	18	0.6	284	1.0	15,818	157.3
Asia	15	0.5	198	0.6	12,876	128.0
Nigeria	1,900	64.9	22,000	74.7	11,579	115.1
Cote d’Ivoire	266	9.1	2,528	8.6	9,504	94.5
Benin	90	3.1	992	3.4	11,026	109.6
Ghana	200	6.8	168	2.4	3,500	34.8
Togo	40	1.4	420	1.4	10,500	104.4
Zaire	38	1.3	270	0.9	7,200	71.6
Source: Adapted from (FAO, 1991).

Table 3: Recent data on world production and trade in yams.
	2000	1999	1998	1997	1996	1995
Production, Mt	37,532,138	37,552,383	35,753,519	34,705,657	33,587,195	32,765,435
Exports- Mt	-	23,198	21,080	28,069	27,493	26,264
Exports- 1000US$	-	20,077	19,212	20,873	20,810	21,108
Source: (FAO/STAT, 2000).

1.3 Primary product

Yams are mainly grown for direct human consumption and are marketed as fresh produce in all the growing regions. Common methods of preparation include boiling, baking or frying. Boiled and baked yam can be eaten with vegetable sauce or palm oil. Boiled yam can also be pounded or mashed in mortar and eaten as ‘fufu’ or ‘utara’. Commercially food processing equipment for boiling and mashing of yam into fufu at the press of a button are now available in the market. Yam cultivars, which contain toxic substances such as dioscorene, are first sliced and soaked in salt water for several hours before further processing for consumption.

1.4 Secondary and derived product

Yam tubers are also processed into several food products such as the yam flour, which are enjoyed in many parts of the tropics (see Section 2). Industrial processing and utilisation of yam includes starch, poultry and livestock feed, and production of yam flour.

1.5 Requirements for export and quality assurance

There are no specific standards for yam export, but intending exporters must seek information on the quality and phytosanitary regulations of the importing country as well as the product specifications required by the importer. As a guide, exporters should apply the general requirements for the International OECD Quality Standard (Opara, 2001) which issues such as minimum requirements, quality requirements, sizing, tolerance and packaging. These should be interpreted to assist in meeting the specifications agreed with the importer.

1.6 Consumer Preferences

There is considerable consumer preferences for the different yam varieties among the growing regions. White-fleshed yams which have firm texture (mainly D. rotundata) are the most popular in West Africa, while in the South Pacific, D. alata cultivars (water yam, white purplish with loose watery texture) are most common (Opara, 1999). Consumer preferences might account for some of the predominance of certain cultivars in some region, in addition to agro-climatological impacts on the growing attributes of the species. In parts of West Africa, yams, which have loose texture, are often mixed with gari and pounded with gari to prepare fufu of ‘soft’ texture.

Table 1: World production of yam (Dioscorea).
	Area (10³ ha)	% of World Area	Production (10³ Mt)	% of World Production†	Yield (Mt.ha^-1)	% of World Yield
World	2,110	100	20,198	100	9.6	100

Africa	2,049	97.1	19,539	96.7	9.4	97.9
North & Central America	22	1.0	243	1.2	11.1	115.6
South America	10	0.5	48	0.2	4.7	0.5
Asia	15	0.7	168	0.8	11.4	118.8
Oceania	15	0.7	200	1.0	13.5	140.6
Leading countries
1. Nigeria	1,350	64.0	15,000	74.3	11.1	115.6
2. Cote d’Ivoire	200	9.5	1,700	8.4	8.5	88.5
3. Ghana	160	7.6	800	4.0	5.0	52.1
4. Togo	100	4.7	750	3.7	7.5	78.1
5. Benin	59	2.8	610	3.0	10.3	107.3
Source: Adapted from (FAO, 1975)

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CHAPTER XXIV YAMS: Post-Harvest Operation