Figure 12.5 Yam tuber processing into dried yam chips.
Blanched yam tubers are usually spread to dry on rock surfaces, road sides and on old yam vines (Babarinde et al., 2013) (Figure 12.5). However, the drying period is mostly much longer and frequently takes between two and three weeks (Mestres et al., 2004), depending on the thickness of the yam slices (Figure 12.5). During the long drying period, the tubers often become mouldy and fermented, thereby affecting the colour and aroma of product. Tubers laid out to dry are often soiled by rain (uncontrollable climatic conditions), dust, sand and animal excrement, which leads to losses in quality due to poor hygiene (Adams and Moss, 1999; Liberty et al., 2013).
12.6.5 Fried Yam Products
Fried yam chip is a popular, convenient and ready-to-eat snack in sub-Saharan Africa (Alimi et al., 2013) (Figures 12.6 and 12.7). The potential use of yam in producing deep-fried crisps snacks were studied by Toure et al. (2012) and Tortoe et al. (2014a). Fried yam chips are made by peeling, slicing, washing and frying in hot oil. The fried chips may be coated with whipped whole egg (Alimi et al., 2013). Another type of fried yam product is called “ojojo” (fried yam cake) eaten in the South-western part of Nigeria. The water yam (Dioscorea alata) tuber is the best species for this type of product. The product is prepared by grating peeled yam tuber, and ingredients such as pepper and salt are added to taste and fried in hot oil, as shown in Figure 12.8.
Figure 12.6 Yam chips dried at road side.
Figure 12.7 Yam chips of different thickness.
Figure 12.8 Processing of fried yam cake.
Fried yam balls are another type of product from cooked yam. They are available in most fast food establishments and well accepted by consumers. The cooked yams are mashed and added to other ingredients such as pepper, salt and egg. The mashed cooked yam is moulded into a ball shape, wrapped with breadcrumbs and fried in hot oil. Sobukola et al. (2008) and Tortoe et al. (2014a) used deep frying machine to cook yam crisps. Fried slices were drained, transferred to a colander lined with tissue paper and allowed to cool. Yam crisps were then packaged in polypropylene bags and sealed.
12.6.6 Paste/Stiff Dough
Yam flour is the major ingredient in making stiff dough/paste (amala), largely eaten by ethnic Yorubas of the South-western part of Nigeria and kokonte in Ghana (Babajide et al, 2006a; Degras, 1993). Amala is a popular starchy food prepared by reconstituting (cooking and stirring with boiling water) fermented or unfermented yam flour (elubo), produced traditionally from the processed tuber flesh of the white yam (D. rotundata or D. alata) (Awoyale et al., 2010). It is a traditional thick paste prepared from blanched dried yam flour with a particular texture (Akissoe et al., 2006). Stiff yam dough is prepared by reconstituting the flour with boiling water in the ratio of 1: 4 (flour/water, weight by volume [w/v]). The mixture is stirred using a wooden paddle until a smooth thick paste is obtained. Hot water is added to the paste, depending on the texture and left to boil for about 3 min until cooked (Ayodele et al., 2012). There is literature available on the production and preparation of amala from white yam, water yam, trifoliate yam and other yam species (Abiala et al., 2011; Abiodun and Akinoso, 2014a, 2014b, 2014c, 2014d; Babajide et al., 2006b; Jimoh et al., 2009). Abiala et al. (2011), Adejumo et al. (2013), Ojokoh and Gabriel (2010) and Sankat and Mujaffar (2004) reported that the dark colour observed in reconstituted dough using the sun drying method may be due to the inability to control the drying process and parameters, weather uncertainties, high labour costs, the requirement of a large drying area, insect infestation, and contamination with dust and other undesirable materials. The composition of the stiff dough is shown in Table 12.5.
Table 12.5 Composition and brown index of stiff dough
Composition | Value
Moisture (%) | 78.72
Fat (%) | 0.34
Fibre (%) | 1.10
Protein (%) | 5.73
Ash (%) | 1.74
Carbohydrate (%) | 12.38
Sodium (mg/100 g) | 473.95
Potassium (mg/100 g) | 435.36
Calcium (mg/100 g) | 198.72
Magnesium (mg/100 g) | 140.23
Iron (ppm) | 3.64
Brown index (oven sried) | 20.57
Brown index (sun dried) | 46.72
Source: Karim et al. (2013);Abiodun and Akinoso (2014b)
12.6.7 Instant Yam Flour
Instant yam flour is a modern invention to simplify the tedious traditional process of preparing pounded yam (Mayaki et al., 2003). According to Adeola et al. (2012), the technology of instant yam flour is simple. It involves peeling, washing, dicing, sulphit-ing, blanching, drying and milling of yam tubers. The resulting powdered yam flour is the instant pounded yam flour which only requires being stirred in boiling water to obtain the pounded yam. Akinoso and Olatoye (2013) observed that instant-pounded yam flour requires a shorter processing time and less energy. This was to preserve yam and reduce human drudgery associated with pounded yam production (Komolafe and Akinoso, 2005; Konan et al., 2014). The proximate composition of instant yam flour is presented in Table 12.6.
Table 12.6 Proximate composition of instant yam flour
Composition | Value
Fat (%) | 0.7–5.86
Fibre (%) | 1.20-3.27
Protein (%) | 2.81-4.65
Ash (%) | 0.92-1.90
Carbohydrate (%) | 72.85–83.50
Source: Olaoye and Oyewole (2012);Adeola et al. (2012)
Textural Qualities of Yam Flour Pasting properties of yam flours varied, depending on the species, cultivars, location, harvesting periods and treatments given to the yam tuber (Abiodun and Akinoso, 2014a; Akinwande etal, 2007; Wireko-Manu et al., 2011). D. alata, D. rotundata and D. cayenensis had higher peak and final viscosities than D. dumetorum. A high peak viscosity corresponds with a higher thickening power of starch (Table 12.7) (Adewole et al., 2011). Higher final viscosities were observed for yam species (Abiodun, 2008), which may be due to a higher degree of association between starch-water systems and the high ability to re-crystallize leading to higher viscosities during cooling of yam starches (Wireko-Manu et al., 2011). Peroni et al. (2006) observed higher setback viscosity in yam starch as a result of retrogradation when compared to other roots and tuber crops. This retrogradation tendency limits the use of yam starch in the food industry (Wireko-Manu et al., 2011).