Table 12.7 Pasting properties of flour from Dioscorea species
Yams pecies | Peak viscosity (RVU) | Holding strength (RVU) | Break down (RVU) | Final viscosity (RVU) | Set back (RVU) | Pasting time (RVU) | Pasting temperature (RVU)
D. cayenensis | 286.63 | 136.88 | 149.75 | 364.25 | 224.88 | 4.67 | 79.15
D. dumetorum | 120.17 | 89.42 | 30.75 | 149.13 | 59.75 | 4.94 | 80.73
D. atata | 328.34 | 250.00 | 78.13 | 353.55 | 103.34 | 4.77 | 84.53
D. rotundata | 306.71 | 157.21 | 149.50 | 396.88 | 239.67 | 4.84 | 80.93
Source: Abiodun (2008)
Gelatinization and pasting of starch/flour are of importance to the food industry, in particular because they influence the texture, stability and digestibility of starchy foods and thus determine the application and use of flour/starch in various food products (Oke et al., 2013a). Starch imparts texture to a great diversity of foodstuffs such as soups, potages, sauces and processed foods (Fetoh and Salwa, 2010; Thebaudin et al., 1998).
Blanching water temperature and soaking time had a significant effect on the bulk density (loose) of the yam flour (Adejumo et al., 2013). The effect of blanching water temperature and soaking time on the protein content of yam flour was probably due to the denaturating of protein caused by the effect of heat on the yam cubes during blanching. In a study conducted by Abiodun and Akinoso (2014d) on the effect of processing and harvesting periods on the physical and functional properties of trifoliate yam, it was discovered that the type of processing methods (soaking, parboiling) affected the yam properties significantly. Parboiling and boiling methods caused significant increase in the functional properties of yam flour (Table 12.8).
Table 12.8 Effect of processing on the functional properties of yam flour
Species | WAC (ml H2O/g) | Solubility (%) | Bulk density (g/cm3) | Swelling Power | Author
D. dumetorum
Raw | 2.53 | 4.95 | 0.78 | 1.75 | Abiodun and Akinoso (2014d)
Parboiled | 3.69 | 5.96 | 1.03 | 6.57 | ―"―
D. alata
Raw | 1.68 | ― | 0.86 | 2.8 | Harijono et al. (2013)
Boiled | 2.02 | ― | 0.88 | 5.9 | ―"―
D. rotundata
Raw | 1.50 | 5.33 | 0.70 | 2.23 | Abiodun (2008)
Steamed | 2.33 | 6.63 | 0.86 | 3.40 | ―"―
Note: ― is not determined
12.6.8 Yam Flakes
In an attempt to reduce post-harvest losses of yam and provide convenience food for the ease of preparation, many researchers have investigated the use of the drum dryer in yam flakes production (Hsu et al., 2003; Konan et al., 2014; Onayemi and Potter, 1974; Rodrfguez-Sosa and Gonzalez, 1972; Rodrfguez-Sosa et al., 1972). Fresh yam is processed by peeling, washing and cutting the yam tubers into slices. The sliced yam pieces were steamed cooked, mashed and blended in a mixer. Water was added to the puree at 10 % level and dehydration was performed in a drum dryer (Konan et al., 2014; Onayemi and Potter, 1974). The resulting dried film was broken into flakes in a granulator and stored in plastic bags. The instant pastes were prepared by adding water heated to 70 °C to the flakes in the ratio of 1: 2 (g: g) and mixed gently with a fork for 4–6 min to produce a dough (Konan et al., 2014).
12.6.9 Extruded Products
Extrusion cooking had been used increasingly in the food industry for the development of new products such as snacks, baby foods, breakfast cereal foods and modified starches from cereals and tubers. Development of snack products from yam further diversify its culinary uses and increase its premium as a root and tuber crop (Tor-toe et al., 2014a). The extrusion of yam flour led to the production of snacks and pre-gelatinized flours of diverse properties. In addition, extruded yam flour can be successfully used in the preparation of “fufu” (pre-cooked compact dough), a yam-based food, popular in Western Africa (Sebio and Chang, 2000).
The use of food extruders has increased considerably, mainly because of a greater demand for convenience and snack-type foods (Kpodo and Plahar, 1992). Kpodo and Plahar (1992) observed that the application of extrusion technology for the development of extruded yam products would be an economic boost to the producing countries, as well as a means of introducing variety to the utilization of the crop. Yam flour (Dioscorea alata) can be processed in a single-screw extruder, for manufacturing snacks.
The moisture, barrel temperature and feed rate play an important role in the quality of extrudates. According to Alves and Grossmann (2002), a better yam snack was obtained when extruded at 17 % moisture, 170 °C and 4 mm die diameter. Sebio and Chang (2004) and Oke et al. (2013b) also reported a maximum expansion ratio for fine yam flour and water yam flours at the barrel temperature, 120–150 °C and moisture content, 8-18 %. Increase in yam flour level decreased the expansion ratio of a yam-corn-rice based snack significantly (Seth et al., 2015). One of the most desirable physical properties for extruded snacks is the degree of expansion, because it determines their structure and consequently their quality (Oke et al., 2013c). The degree of expansion affects the density, fragility and softness of extruded products. The textural properties of the extrudates also play a significant role in determining the ultimate utilization potential of the extrudates, either as a snack or as meal (Oluwole et al., 2011). Jyothi et al. (2013) made an acceptable pasta product from D. alata and wheat flour.
12.6.1 °Composite Flours
Composite flours are made from yam flour and other flours to improve the nutritional value of the product, improve utilization of the yam and provide varieties of yam products. The flours can be stored over longer periods and thus reduce post-harvest losses in yam production (Glover-Amengor et al., 2013). Yam flour used for these products was made by peeling, washing, dicing, drying in hot air ovens and milling into flour (Okorie et al., 2011; Udensi et al., 2010). In most cases, yam tubers are soaked in a sodium metabisulphite solution to prevent a browning reaction mediated by the enzymes in the oxido-reductase family, the polyphenol oxidase (Ngoddy and Onuoha, 1985). Bread was made by replacing up to 25 % of wheat flour with yam flour. Although the loaf volume decreased with the addition of yam flour, the results showed that up to 20 % of yam flour could be included in bread formulation without altering the sensory acceptability of the blended bread (Hsu et al., 2004).
The incorporation of yam flour in bread was reported to increase the antioxidant capacity of the bread, as tested by both free radical scavenging and total anti-oxidation tests (Hsu et al., 2004). Also, Ukpabi (2010) blended D. alata flour with wheat flour for bread production and recommended 20 % of D. alata flour for the production of bread that is comparable to those made with sole wheat bread flour in Nigeria. Siddaraju et al. (2008) likewise incorporated D. alata flour in Indian dehydrated products and the products were comparable to the control. Yam flour had also been substituted with wheat flour, sorghum, plantain, cassava flour and legumes for snacks and stiff yam paste (Abiodun and Oladapo, 2010; Abulude and Ojediran, 2006; Akingbala et al., 1995; Awoyale et al., 2010; Babajide and Olowe, 2013; Eke-Ejiofor and Owuno, 2012; Glover-Amengor et al., 2013 Oluwole and Olapade, 2011). However the production of yam flour for baking had only been practiced at laboratory and small-scale levels.