Maalekuu et al. (2014) estimated 4-40 % of losses often incurred after harvest and in storage due to injuries (31 %), pests (23 %), weight loss (4 %), sprouting (21 %) and decay (40 %). In order to prevent these post-harvest losses, the tuber can be converted into varieties of yam products and modern storage techniques may be adapted. Many researchers have studied and developed different techniques and methods of processing yam tubers into chips, flours, flakes, snacks and other products (Ekwu et al., 2005; Ezeocha et al. 2011; Hsu et al., 2003; Konan et al. 2014; Ojokoh and Gabriel, 2010; Onayemi and Potter, 1974).
12.2 Importance of Yam in Tropical Regions
The importance of yam varies in the diets of the various tribes. Yam is an important food in many tropical countries, particularly in West Africa, South Asia and the Caribbean, where it also has a social and cultural importance for about 300 million people throughout the world (Ettien et al, 2009; Manuel et al., 2005). Yam is one of the most preferred among roots and tubers by millions of people in the yam zones of Africa (Chukwu and Ikwelle, 2000) and accounts for about 32 % of farm income earned from these crops. As a source of foreign exchange, these tubers are exported from Caribbean countries to Europe and North America, primarily to meet the needs of Africans and African descendants in these regions (Chukwu and Ikwelle, 2000). Yams are increasingly used as livestock feed and raw materials for industrial purposes.
12.3 Yam Production
Yam (Figure 12.1) is the second-most important tropical crop in West Africa, next to cassava (Opara, 1999). Fu et al. (2011) reported that yam (Dioscorea species) constitute the predominant starchy staple in sub-Saharan Africa and most of the world. Africa production is concentrated in the “yam belt” stretching from the Cote d’ivoire through Ghana, Togo, Benin and Nigeria (Onwueme and Charles, 1994). Table 12.1 shows the cultivated area and yield of yam in the years 2005–2013 in the yam zone. In 2005, 48.7 million tons of yams was produced world-wide, West and Central Africa accounted for 94 %, while Nigeria (the leading producer) produced 34 million tons, which accounted for 69.8 % (FAOSTAT, 2006; IITA, 2007). In addition, in years 2007 and 2008, 46 and 52 million tons of yams were produced world-wide with the highest value in Nigeria (71 and 67.7 %, respectively) (FAO, 2010; Fu et al., 2011; IITA, 2007).
Figure 12.1 Yam tubers.
Table 12.1 Cultivated areas ('000 ha) and yield (t/ha) of yam in 2005–2013 in yam zone
Year | Nigeria (Area/yield) | Cote d'ivoire (Area/yield) | Togo (Area/yield) | Ghana (Area/yield) | Benin (Area/yield) | Africa (Area/yield) | World (Area/yield)
2005 | 2,957/11.5 | 609/8.5 | 577/10.2 | 363/10.8 | 178/11.7 | 4,221/19.7 | 4,600/10.6
2006 | 3,035/12.1 | 657/8.5 | 603/10.3 | 325/13.2 | 196/11.4 | 4,543/11.3 | 4,747/11.2
2007 | 3,123/10.0 | 692/8.0 | 585/10.6 | 324/13.5 | 166/11.6 | 4,641/9.8 | 4,855/9.7
2008 | 3,045/11.5 | 787/7.6 | 628/10.3 | 348/14.1 | 180/14.0 | 4,703/10.8 | 4,919/10.7
2009 | 2,776/10.5 | 810/6.6 | 692/10.2 | 379/15.3 | 168/14.2 | 4,513/10.1 | 4,776/9.9
2010 | 2,869/11.9 | 830/6.5 | 715/9.9 | 385/15.4 | 201/13.0 | 4,689/10.9 | 4,941/10.8
2011 | 2,889/12.9 | 834/6.6 | 712/10.2 | 404/15.6 | 182/15.0 | 4,729/11.6 | 4,961/11.5
2012 | 2,900/13.0 | 835/6.8 | 840/10.3 | 426/15.6 | 200/13.7 | 4,803/11.9 | 5,043/11.8
2013 | 2,900/13.1 | 835/6.9 | 650/10.2 | 430/15.4 | 211/15.1 | 4,807/12.0 | 5053/11.9
Source: FAOSTAT (2015)
There were reductions in yam production in the years 2007 and 2009, but production improved rapidly in the year 2010/2011 (Table 12.2). The total yield of yam per acre in Ghana and Benin were higher than Nigeria and the Cote d’ivoire, but the cultivated areas in Nigeria were more extensive than other countries in the yam zone (FAOSTAT, 2015). Increase in the area of land cultivated had been reported to cause only a slight improvement in productivity due to low soil fertility, increased pest problems and backward farming technology (FAO, 2010; Fu et al., 2011).
12.4 Consumption of Yam
Annual per capita consumption in West Africa is 61 kg. World-wide annual consumption of yams is 18 million tons, with 15 million in West Africa (IITA, 2007). A larger percentage (40–59 %) of the annual yam production world-wide and in African countries are consumed as food, while some are used as animal feeds and seeds for planting in the next rainy season (Table 12.2). The use of yam for food increased from the years 2005–2011, despite reduction in yam production in the years 2007 and 2009. About 10–12 % of the annual yam production is wasted during harvesting, transportation and storage.
Table 12.2 World-wide and African consumption (1000 ton) of yam in year 2005–2011.
Year | Production (World/African) | Feed (World/African) | Seed (World/ African) | Waste (World/African) | Food (World/African) | Processed (World/African) | Other (World/African)
2005 | 48770/47337 | 7499/7269 | 1913/1874 | 5242/5134 | 20214/19167 | ― | 13726/13725
2006 | 52606/51177 | 8007/7847 | 1917/1879 | 5663/5553 | 22561/21466 | ― | 14293/14292
2007 | 46996/45455 | 7003/6739 | 1968/1927 | 4892/4780 | 23835/22705 | ― | 9739/9738
2008 | 52466/50924 | 7770/7553 | 2017/1980 | 5695/5585 | 24565/23388 | ― | 12132/12131
2009 | 47262/45449 | 6585/6331 | 2052/2013 | 5189/5067 | 27953/26551 | ― | 5321/5320
2010 | 53116/51174 | 7634/7377 | 2029/1990 | 5792/5627 | 28275/26787 | ― | 9501/9500
2011 | 56677/54766 | 8235/7982 | 2115/2081 | 6116/5948 | 29102/27640 | ― | 11181/11180
Source: FAOSTAT (2015)
According to FAOSTAT (2015) statistics, no data had been recorded for processed yam products from the years 2005–2011. This cannot be compared to data obtained for other roots and tubers such as cassava and potatoes (FAOSTAT, 2015). This may be due to the insignificant amount of yam being processed into instant yam flour, yam flakes, etc. In fact, most of these processed yam products are experimental work or made at the small-scale level. Although, some yam species are used industrially for pharmaceutical purposes, there is a need for improvement and production of processed yam products in order to reduce post-harvest losses.
12.5 Composition of Yam
The nutritional composition of yam varies from place to place depending on the climate, soil, crop varieties, species and other factors. The moisture content ranges between 58 and 86 % (Table 12.3) (Degras, 1993). The head end of the tuber is 18 % lower in moisture content than the value at the tail end (Akanbi et al., 1996; Dje et al., 2010). The tuber is highly perishable due to its high moisture content, large size and high respiration rates (Glover-Amengor et al., 2013). Yam tubers are reportedly low in ether extract, with values ranging between 0.03 and 0.27 on a wet basis. Thus, yam tubers cannot be considered as rich sources of fats and fat soluble vitamins (Eka, 1998). The methods of preparation or processing of yam before consumption may adversely affect the vitamin content of the material (FAO, 1990).