Table 12.3 Composition of yam tuber
Nutrient | Range
Moisture (%) | 58-86
Carbohydrate (%) | 11-38
Ash (%) | 0.50-2.56
Fat (%) | 0.03-0.27
Protein (%) | 0.94-3.10
Crude fibre (%) | 0.18-1.57
Starch (%) | 60-89
Amylose (%) | 14–31.56
Magnesium, mg/100 g | 0.49-1.10
Calcium, mg/100 g | 1.20-2.00
Potassium, mg/100 g | 0.39-1.00
Sodium, mg/100 g | 0.14-0.22
Phosphorus mg/100 g | 0.16-0.29
Alkaloid, mg/100 g | 0.29-1.68
Phenol, mg/100 g | 0.00-0.05
Tannin, mg/100 g | 0.00-0.06
Oxalate, mg/100 g | 0.20-0.63
Phytate, mg/100 g | 1.02-4.16
Source: Degras (1993), Muthukumarasamy and Panneerselvam (2000), Okwu and Ndu (2006), Wireko-Manu et al. (2011), Polycarp et al. (2012)
Yam tubers are good sources of energy, which is derived mainly from carbohydrate (Eka, 1985). Some yam species have been found to contain sugars such as rhamnose, galactose, arabinose, xylose, maltose, glucose, fructose and sucrose (Degras, 1993; Ketiku and Oyenuga, 1973). The content of sugars in the yam tuber is influenced by variety, age, location and cultural treatment (Ketiku and Oyenuga, 1973; Osagie, 1992). The protein content in yam varies between 0.94 and 3.10 % on a wet basis (Degras, 1993). Yam is not a rich source of protein, therefore deficiency diseases, especially kwashiorkor, are prevalent in yam-growing areas, especially those that are below average in the population (FAO, 1990).
Crude fibre of yam tuber ranged from 0.18-1.51 % (Degras, 1993), whereas the crude fibre content of the water yam flour ranged from 4.4–5.7 % (Harijono et al., 2013). D. armata and D. minunflora have been reported to contain exceptionally high crude fibre content (Eka, 1985). Fibre increases satiety with appetite reduction, slow rate of glucose absorption, reduced bile salt re-absorption, alters colonic microflora composition, and increases water, sodium and mineral absorption (Harijono et al., 2013). Calcium, potassium and magnesium are the major mineral constituents of yam tubers, while alkaloid, phytate and oxalate are the major anti-nutrients in yam tuber. The levels of anti-nutrients in the yam tuber depend on the species, cultivars and maturity. Most of the anti-nutrient compositions in yam are eliminated during processing into meal.
12.6 Yam Processing and Utilization
12.6.1 Boiled and Roasted Yam
Boiling of yam tuber is the easier and common method of preparing yam in most households. Boiled yam tuber is prepared by peeling, slicing, washing and boiling in water at 100 ± 2 °C. The yam tuber may be boiled with or without peel. Nutrient losses depend on the degree/extent of peeling. Boiling with peel gives the best nutritional balance, while peeling after boiling makes available a large part of the edible portion (Bell and Favier, 1981).
Roasting is another method of cooking yam tubers and recently has become a popular street or fast food in urban areas (Oke, 2013). The tubers are roasted whole or cut up into various sizes and roasted on charcoal. The traditional way of roasting is stressful but involves regular blowing of air to ensure a constant heat supply from the charcoal and regular changing of the position of the food item to prevent burning (Oke, 2013).
12.6.2 Yam Porridge
Yam porridge “asaro” is a popular dish in Nigeria (Figure 12.2). Processing of a typical porridge is shown in Figure 12.3. It is made by peeling the yam tuber, washing, dicing into cubes and boiling with other ingredients such as pepper, tomatoes, onions, vegetable, meat, palm oil, salt, crayfish, etc. The cooked spiced yam cubes are then mashed with a wooden spoon. This type of yam porridge is usually made from the white yam (Dioscorea rotundata) species. Another type of porridge which is common among the Ijebu’s of the South-west region of Nigeria is known as “ikokore” The Dioscorea alata yam tuber is used for this type of delicacy and is also used by the Efik/Ibibio people of the south region of Nigeria to prepare a local dish known as “ekpang nkukwo” (Baba-jide and Olowe, 2013; Lawal et al., 2012). Processing yam tubers into porridge brought a highly significant improvement in its nutrient and energy content due to the contribution of the added ingredients (Adepoju, 2012). Yam porridge is still prepared locally according to the ethnicity. The major improvement on this product is the fortification of yam tubers with other ingredients such as legumes, which are added to improve the nutritional quality of the product (Echendu et al., 2009; Tortoe et al., 2014b).
Figure 12.2 Yam porridge.
Figure 12.3 Flow chart of processing of yam tuber into porridge.
12.6.3 Pounded Yam
Pounded yam is a very popular food product in West and Central Africa. It is a glutinous dough made by peeling the yam, cutting to pieces, boiling, pounding and kneading (Figure 12.4) (Adeyeye and Oluwatola, 2014; Otegbayo et al., 2006). For consumption, the dough is usually cut with the fingers, moulded in the palm, dipped into a stew and swallowed without mastication (Otegbayo et al., 2006). Onwueme and Charles (1994) and Asiedu (1989) observed that the consistency of pounded yam depends on the species or cultivar of yam used and the characteristic of the starch grains in the yam. Dioscorea rotundata and Dioscorea cayenensis tubers produced better characteristic pounded yam dough than other yam species.
Figure 12.4 Processing of yam tuber into pounded yam.
Some yam tubers, such as D. alata and D. dumetorum tubers, have been reported to be unacceptable for pounded yam production (Abiodun, 2008; Otegbayo et al., 2006). The indigenous process of producing pounded yam is very laborious and tedious to obtain a smooth consistency (Mayaki et al., 2003; Odior and Orsarh, 2008). It requires physical pounding by two or more men or women, depending on the quantity, in mortars with pestles (Odior and Orsarh, 2008). The composition of pounded yam is given in Table 12.4.
Table 12.4 Composition of pounded yam
Parameter | Value
Moisture (%) | 62.30
Ash (%) | 2.80
Crude fibre (%) | 1.60
Protein (%) | 1.80
Fat (%) | 0.90
Carbohydrate (%) | 30.60
Ascorbic acid (%) | 40.82
Phosphorus (mg/kg) | 1166.7
Calcium (mg/kg) | 135.2
Source: Adepoju (2012);Akin-Idowu et al. (2009)
12.6.4 Yam Chips
In some West African countries such as Nigeria, Benin and Ghana, yams are processed into chips and flour. Dry yam tubers and slices are processed by peeling, slicing, blanching in hot water (40–60 °C for 1–3 h), steeping for a day and then sun dried (Figure 12.5) (Achi and Akubor, 2000; Akingbala et al., 1995; Akissoe et al., 2001; Babajide et al., 2006a; Onayemi and Potter, 1974). The processing of fresh yam tubers into yam chips is an activity that is carried out in Nigeria all through the year, regardless of seasonal variations due to high demand of this product by indigenes of South-west, North-western Nigeria (Abiala et al., 2011). The quality of the yam chips varies from processor to processor and from location to location (Akissoe et al., 2001; Mestres et al., 2004; Ojokoh and Gabriel, 2010).