Size Reduction Fresh cassava roots are subjected to size reduction operations like chipping, mincing and grating (or rasping) to enhance subsequent unit operation like dewatering (pressing), drying, fermentation and starch extraction. The old-fashion method of manually grating cassava root is no longer practiced. Mechanical graters are now available in different designs and capacity (Figure 10.1.2). Size reduction also enhances biochemical detoxification of cassava roots. It breaks up tissue to release natural enzymes that catalyze conversion of toxic cyanogenic glucosides in cassava roots (linamarin and lotaustralin) to less toxic materials (glucose and cyanohydrin) in the presence of water. The cyanohydrin is further degraded to a ketone and hydrocyanic acid. Jones et al. (1994) demonstrated that mincing of cassava root caused complete degradation, while rasping and chipping caused 70–80 % and 30 % degradation of the glycosides, respectively. Besides their different influences on root detoxification, size reduction methods are also appropriate methods to achieve subsequent operations like drying and milling. Grated or retted pulps are more easily dewatered, dried and detoxified than chips, due to larger surface areas presented by the latter than the former. Literature data on the comparative effects of chipping, mincing and grating of root on the process (or energy) efficiency and quality of dried cassava products like flour and starch is generally deplete. Doporto et al. (2012) reported that the size reduction method of cassava root caused significant difference in the color of unfermented cassava flour. Grated cassava root resulted in higher lightness than sliced (chipped) root.
Figure 10.1.2 Some mobile micro scale cassava processing units (a) mobile cassava graters; (b) mobile batch mechanical press (source: Fieldwork, 2010).
Most graters and raspers are batch motorized forms of equipment. The design of these size reduction items of equipment varies mostly in terms of the configuration of the grating unit. A manual rasper consists of a stationery grater/grinding stone against which the roots are rubbed to obtain a pulp, whereas a small-scale machine consists of a high-speed rotating wooden drum with a crushing surface fixed onto it. Nanda et al. (2004) developed a primary rasper with saw tooth blades for cassava starch extraction, which had a capacity ranging from 360–385 kg h-1. Sheriff and Balagopalan (1999) described a multipurpose starch extraction plant of lesser capacity (75-125 kg h-1) and evaluated the performance of the machine for various tuber crops. Sajeev and Balagopalan (2005) developed a multipurpose mobile starch extraction plant for the in situ starch extraction in villages for cassava, sweet potato and elephant foot yam. Capacity of the machines varied from 120–200 kg h-1 and the rasping effect from 40.32–61.10 %, depending on the type of tuber crop. In large-scale modern starch factories, the Jahn-type raspers, consisting of a rotating drum with longitudinally arranged saw tooth blades around the periphery at 10 mm apart, has been widely used (Balagopalan et al., 1988; Nanda and Kurup, 1994; Sheriff et al., 2005).
Fermentation Fermentation is the most prominent processing operation applied to make edible products from raw cassava roots. The two types of cassava root fermentation practiced are solid state (SSF) and submerged fermentation (SMF). Both SSF and SMF involve activities of lactic acid bacteria (LAB). SSF is often precluded by root grating to give a cassava mass that is heaped up in the fermenter or tied in sacks and allowed to ferment for 3–5 days. However, SMF involves soaking of whole root or its chunks in water for 3–5 days. Apart from root softening, development of flavor is a common phenomenon during SSF of wet cassava meal for producing gari ― a product commonly consumed in West Africa. The root may also be submerged in water for the purpose of retting prior to further processing. It has also been established that in both processes, the role played by microorganisms in cassava root fermentation is very significant. For example, Westby (1991) investigated the ability of important microorganisms isolated from two major classes of fermented cassava products (acidic grated roots and acidic soaked roots) to hydrolyze linamarin. LAB were the commonest organisms in each product. About 64 % of the LAB was capable of causing significant reduction of the cyanogens in the respective products. Apart from detoxification, fermentation also causes significant root softening (or retting) of cassava tissue during submerged fermentation.
Okolie and Ugochukwu (1988) studied the activities of cell wall degrading enzymes isolated from Citrobacter freundii in cassava fermentation. The activities of polygalac-torase, pectinase, cellulase, amylase and phosphorylase enzymes were monitored. It was shown that pectic enzymes were of primary importance and inhibition of alpha amylase and phosphorylase had no effect on root softening. Later, Ampe et al. (1995) discovered that root softening was due to the combined action of both endogenous pectin methyl esterases and exogenous depolymerizing enzymes-mainly lyases.
CFs generally have low protein, which necessitates protein supplementation of most CF-based diets. Fungal-fermented CFs have been reported to have enhanced protein content (Akindahunsi et al, 1999; Oboh and Akindahunsi, 2003). However, some level of hepatotoxicity and cardiotoxicity was observed in rats fed with Saccharomyces cerevisae fermented CF (Oboh and Akindahunsi, 2005).
Dewatering or Pressing Only fermented or grated cassava mash is pressed to reduce the moisture content from an initial level to less than 30 %, depending on the dewatering efficiency of the press. Following pressing, the bulkiness is reduced while subsequent handling and drying of the mass is enhanced. The press may either use the screw mechanism or hydraulic force. A simple, mobile, micro-scale hydraulic press is shown in Figure 10.1.2b. There are many designs of the presses. Their capacities vary with processing scale. Manual presses are used by micro- and small-scale processors. In somnnwabueze, medium and most large processing factories, automated presses are used. The efficiency of screw presses generally are lower than hydraulic presses.
Few research efforts have been paid to development of more efficient cassava mash dewatering systems. Olusegun and Ajiboye (2009) reported the design and fabrication of a vertical double squeeze cassava pulp dewatering machine to handle about 200 kg or 4 bags of cassava pulp per batch. The dewatering was achieved in 33.72 minutes. This machine was 7 times quicker than the IITA multi-purpose press and 40 times quicker than the local method of dewatering (IITA, 1990).
Kolawole et al. (2012) reported the development of an integrated machine capable of combining wet cassava mash conveying, dewatering, pulverizing and sifting in one machine unit. The machine was capable of reducing the moisture content of the pressed mash from 68 % to about 47 % (wet basis). However, the higher conveyor screw speed led to increased product temperature. Continuous operation of the machine could increase the temperature of the mash to a level that could lead to starch gelatinization, which could be detrimental to the product quality.