12.8 Technological Application to Yam Processing
Over the years, there has been technological advancement to the processing of yam products. Various machines and equipment have been employed to give a more acceptable and hygienic end product. Examples of such machines employed are yam peeler, yam slicer/dicer, dryer (drum dryer, solar dryer, vacuum dryer, freeze dryer) blancher, steamer, pounding machine, grinding machine and extruder. Recently, Okafor (2014) developed a yam flour processing system with three units, namely the slicing, the drying and the grinding units. There have been improvements on the major unit operations in yam tuber processing. Apart from the use of equipment, which improves and facilitates faster yam processing, modifications in methods of processing of yam and starches (chemical, physical and enzyme hydrolysis) are reported.
12.8.1 Major Equipments used in Yam Processing
Yam Peeler The yam peeler was designed by Ukatu (2002) for industrial yam peeling processes. This machine consists of three peeler arms. Peeler blades are fixed to the peeler arms, which scrape the tuber body to a pre-set depth. It also consists of a peeler ring on which the peeler arms are mounted and driven through a chain drive and the peeler ring housing the peeler arms have toothed structures that act as an aperture, which initiates the opening of the aperture when in contact with tuber. Adetoro (2012) also improved on the existing yam peeler by considering a wide variation in the size and shape of the tubers and variations in the mechanical properties with the age of the tubers. The major parameters determined were the effectiveness of the peeler, thickness of peel, time of harvest (age), feed of the cutter, and relative speed of rotation. The machine made use of punched holes in the drum to perform the peeling action. The punched holes can be adjusted by increasing the size of the centre punch. Peeling efficiency ranges between 80 and 85 %.
Yam Slicer/Dicer Slicing involves using sharp blades to reduce the yam tuber into smaller thicknesses purposely meant to increase the surface area of the product for faster drying (Ehiem and Obetta, 2011). Ehiem and Obetta (2011) designed a motorized yam slicer which consists of powered shaft, bevel gears, blades and feeding chutes. Manual slicing machines are common and produce slices of uniform thickness for snack foods or dried root crops (UNIDO, 2004). Various sizes of kitchen slicers with adjustable stainless steel blades are also available for slicing yam. Likewise, manual dicing machines first cut the material into strips and then further cut into uniform sized cubes (UNIDO, 2004).
Yam Dryer Drying is an appropriate preservation method and important unit operation in the processing of yam tubers into flour (Ehiem and Obetta, 2011; Satimehin, 2014). Satimehin (2014) classify changes occurring during drying of yam tubers as chemical, physical and biological changes of the tuber. These include changes in shape and structure, colour, shrinkage, cracks, case hardening and denaturation of unstable components. These changes are influenced both by the external process conditions such as air temperature, humidity and air velocity, and by the mechanisms of internal moisture (Satimehin, 2014). The drying characteristics of yam tubers are controlled or regulated using artificial drying methods. Artificial drying methods had been tested and reported to be more effective in drying yam tubers (Liberty et al., 2013). This method of drying improves the quality of the product, prevents contamination, and saves time and floor space requirements.
Artificial drying allows for the continued drying during the night, especially during the peak period of harvest (Liberty et al., 2013). Abiodun and Akinoso (2014b) and Akissoe et al. (2001) reported improvement in the colour and texture of oven dried yam chips and its reconstituted dough (amala) when compared to the sun dried method. Major yam drying equipment included drum dryer, hot air oven, solar dryer, cabinet dryer, automatic batch dryer, platform dryer, flatbed dryer and tray batch dryers (Jimoh et al., 2009; Liberty et al., 2013).
Drum dryers have high drying rates and high energy efficiencies and are suitable for slurries (Fellows, 2000). Figure 12.10 showed a typical fabricated drum dryer. The use of a drum dryer in yam flakes production comparatively improves the sensory and nutritional qualities of dried food. The yam tubers are mashed or made into a slurry before drying in the dryer. The hot air oven method involves the use of heated air to dry food samples and this facilitates heat transfer through the convection process. The cabinet dryer consists of a heating chamber having one electrical coil connected directly to a centrifugal fan and drying chamber. The heater is connected to the temperature regulator, which controls the temperature of the dryer by switching the heater on and off. The dryer consist of trays and vents which help to prevent moisture condensation at the top of the drying chamber (Owa et al., 2014). The solar drier has a wide range of possible applications in the drying of foodstuffs (Otunyo, 2011) and uses radiation from the sun and heated air is trapped in the drying chamber. The use of a solar dryer for yam processing has been reported to produce better-quality yam products (Adesina et al., 2011; Agoreyo et al., 2011; Otunyo, 2011).
Figure 12.10 Fabricated drum dryer.
Various sizes of yam crushers and grinders are available. Hammer and attrition mills are the major milling machines used for converting yam chips into flour. In most cases, the yam chips are crushed into smaller sizes using the yam crusher and then milled into flour using the yam grinder.
Yam Blancher/Steamer/Parboiler Blanching and steaming of yam tubers prior to other treatments has been one of the major unit operations in yam processing. Blanching reduces microbial load, inactivates enzymes and reduces bulkiness. Many works have been generated on blanched yam tubers (Abiodun et al., 2013; Harijono et al., 2013; Okorie et al., 2011; Oyewole and Olaoye, 2013; Quansah et al., 2010). Various steaming equipment was used to steam yam tubers, among them the water bath (Ekwu et al., 2005; Iwuoha, 2004; Leng et al., 2011), the Barlett steamer (Akinwande et al., 2008) and steam blanchers, which consist of a pan and a removable mesh base to hold the roots above the boiling water (UNIDO, 2004). The yam parboiling machine was developed by Ayodeji and Abioye (2011) and Ayodeji et al. (2012) for instant pounded yam flour processing. It consists of a chamber which is properly insulated to prevent heat loss for an effective parboiling of the yams.
Roasting Machine The use of a locally-fabricated manual roaster for roasting yam has been reported (Adegunwa et al., 2011). Likewise, a multipurpose roasting machine was recently developed by Oke (2013). According to Oke (2013), the multipurpose roasting machine is made up of a roasting chamber, heating chamber, two blowers and driving (power transmission) mechanism. The roasting chamber houses ten food hangers, five on each side of the chamber. The food hanger is used to hold the food items and the hangers are subjected to continuous rotation for uniform and smooth roasting.
Pounding Machine Pounding of cooked yam tubers had been relieved with the invention of the pounding machine. A mechanical yam pounding machine has been developed for both the domestic and commercial consumers (Odior and Orsarh, 2008). Most researchers now use the pounding machine for their research work, involving pounded yam (Omonigho and Ikenebomeh, 2000; Otegbayo et al., 2006). The yam pounding machine involves the design and fabrication of some principal components, which include the shaft, pulleys, yam beaters, bowl and frame (Odior and Orsarh, 2008). The machine produces a more hygienic pounded yam, and eliminates the tedious and laborious indigenous process of preparing pounded yam (Odior and Orsarh, 2008).