Starch Extraction Particle size and purity are important quality indices for starch. Most large-scale CS processing plants employ two levels of separation using continuous centrifugal starch extractors (coarse and fine) to ensure uniformity and purity of starch. Some plants incorporate decanters to remove proteins and other impurities. The first stage of extraction is in coarse extractors with centrifugal perforated baskets. Starch is repeatedly extracted from the pulp exiting the extractor with a screen of the same aperture (355–425 pm) as the first pass, until minimal residual starch is achieved. Starch milk from coarse extractor is passed into a fine extractor, which contains a filter cloth and sieve with aperture range from 150–125 pm to remove fine fibers. A higher degree of fineness is achieved by further passing the milk through a sieve of smaller aperture of 140–200 mesh size.
Sedimentation and Decanting The starch milk is then passed into a sedimentation channel for separation of water from the starch. The starch settles while the supernatant liquor flows over a weir and is discharged. The starch is usually allowed to settle in the channel overnight to allow for effective removal of water. Thereafter, the surface of the starch is washed to remove the layers of dirt and other impurities on the surface of the starch mass.
Drying and Milling Drying of CS cake to reduce the moisture content to between 10 and 14 % is achieved in the industry through the use of pneumatic conveying flash dryers of the type shown in Figure 10.1.2. The flash dryer is the dryer type of choice in order to prevent modification of starch, which could occur from exposure to high temperatures over long periods. Cassava starch cake fed into flash dryer is blown with hot air and dried to about 12 % moisture content within 6 sec (Sriroth et al., 2000). Milling of dried CS is necessary to obtain the right particle sizes required for different applications. Studies on effects of milling on CS are scarce. The moisture content of CS and milling duration used could affect the quality of the resultant starch. Martinez-Bustos et al. (2007) studied the effect of moisture content and milling times on the physicochemical properties of CS using a ball mill. Higher moisture contents and longer milling times caused reduction in crystallinity of CS. The starch modification also caused increased water absorption index (WAI) and the water solubility index (WSI) of CS.
Packaging and Storage The choice of CS packaging material is dictated by the structural form of starch and quality requirement. Cassava starch is a highly dusty powder of low moisture content. Prevention of moisture pick-up and leakage are important considerations in the packaging and storage of CS. Dried CS is often packed in polyethylene bags or linen cloth. A double-layered polyethylene bag is often used. A low humid environment is recommended for storage, because of the negative influence of relative humidity on the flow properties of CS (Nduele et al., 1993). Bags of CS are mounted in stores on wooden or plastic pallets, away from concrete surfaces to prevent moisture migration from the surface to the packed starch and destruction of packaging materials by insects and rodents, which could lead to loss of starch and exposure to environmental factors that could affect the quality.
10.1.3.2 Cassava Starch Productivity and Quality
The productivity and quality of CS is affected by three main factors. These are seed gene; environmental factors such as rainfall, soil characteristics and temperature; and farm practices such as irrigation, fertilizer application, intercropping system and weed control. These factors affect starch yield of cassava roots (Pardales and Esquibel, 1996; Santisopasri et al., 2000; Sittibusaya et al., 1993), root starch content (Defloor et al., 1998; Santisopasri et al., 2000), granule size distribution, swelling power (Asaoka et al., 1991; Sriroth et al., 1999a), paste viscosity, pasting temperature (Santisopasri et al., 2000; Sriroth et al., 1999ab), gelatinization temperature (Asaoka et al., 1992), amylose content (Asaoka et al., 1991) and root cyanide content (CIAT, 1990).
Root cyanide content is an important quality factor in CS trade, because of its accumulated effect on the health of consumers. Availability of water during the growth of cassava roots and soil fertility influences the root cyanide content (CIAT, 1990; Santisopasri et al., 2000). Inadequate water during the latter part of the root growth period causes concentration of cyanogenic compounds. Also, depletion of potassium content in the soil increases the cyanide content in the root. Adequate water supply throughout the growth period of cassava root and application of potassium fertilizer is beneficial to the quality of CS, as it leads to decrease in the cyanogenic content of the roots, and stimulates dry matter and starch content (Sriroth et al., 2000).
Cassava starch, like commercial starch from any other source, is traded based on quality. A documented quality standard and grade of CS found in literature is the one established by the Thailand Ministry of Industry (Table 10.1.5) (Sriroth et al., 2000). The standard and grade would facilitate and promote the trade of CS in international markets. Managing the quality as classified in the standard would improve the competitiveness of CS and encourage its application in the starch-based products.
Table 10.1.5 Standard for cassava starch
Grade
Qualifications | 1 | 2 | 3
Moisture content (% maximum) | 13 | 14 | 14
Starch (% minimum by polarimetric method) | 97.5 | 96 | 94
Ash (% maximum) | 0.15 | 0.3 | 0.5
Acid insoluble ash (% maximum) | 0.05 | 0.10 | 0.15
Protein (% maximum) | 0.3 | 0.3 | 0.3
Fiber (cm3 in 50 g starch before drying) | 0.2 | 0.5 | 1.0
pH | 4.5 to 7 | 3.5 to 7 | 3.0 to 7
Residue on 150 pm sieve (% maximum) | 1 | 3 | 5
Source: Sriroth et al. (2000)
Global CS Production World starch production was 60 million tons in 2006 (FAO, 2006). Sales from starch and its derivatives stood at $51.2 billion in 2012 and were forecast to increase to $77.4 billion in 2018 through a compounded annual growth rate of 7.1 % (BCC Research, 2013). The cassava share of the global starch production was 10 % in 2006 (FAO, 2006). Despite being the third largest producer of cassava after Nigeria and Brazil, Thailand has remained a consistent global leader in the production of CS and its derivatives (FAO 2006). Thailand produced 3.5 million tons of CS in 2006,1.3 million tons (37 %) were consumed locally, while 2.5 million tons worth $800 million were exported (Sriroth, 2008).
Utilization of CS The strengths of cassava are mainly in the areas of utilization of its starch and starch-based products. Researchers continue to find new uses for CS, because of the global availability of cassava and ease of extraction of its starch (Essers, 1994). Most importantly, some properties of CS such as bland taste, low gelatinization temperature (71oC), low retrogradation tendency, good stability, high water binding capacity and good adhesive strength (Abraham, 1993; Srirothi et al., 1999b; Jyothi et al., 2005) among others, have been reported to be responsible for its suitability as a base material in various food applications (Falade and Akingbala, 2010). Modification of CS to correct one or more of its shortcomings also gives scope to fabrication of a variety of products for food and non-food applications, thereby adding value and enhancing its versatility (Thranathan, 2005).