Cassava plants photosynthesize and store solar energy in a form of carbohydrate, mainly as starch in edible, underground roots. The roots are very moist having the water content around 59-79% w/w (Table 4). On dry solid basis, starch is a major component of cassava roots, accounting upto 77-94% w/w, the rests are protein (1.7-3.8% w/w), lipid (0.2-1.4% w/w), fiber (1.5-3.7% w/w as crude fiber, i. e. cellulose and lignin) and ash (1.8-2.5% w/w) (Table 4). Some sugars, i. e. sucrose, glucose and fructose are also found in storage roots at 4-8% w/w (dry basis). In addition to cellulosic fiber, the roots also contain non-starch

n. a. = not available

Source: Food and Agriculture Organization of the United Nations [FAO], 2011

Table 2. World average root productivity (tons/ hectare) and those of major producers.

	oo
Thailand Indonesia1 India2 Vietnam3 China Cassava area (hectare/ fanner) 2-3 0.3-1.0 0.5-1.0 0.2-0.9 0.2-0.4 Intercrops None (95%), Maize (5%) Maize+rice — soybean/ peanut None/vegetables None/maize None/peanut Land preparation Tractor (3 + 7 disc) Manual / animal / tracto r Tractor Animal/tractor Manual/animal Fertilizer use — Organic (ton/hectare) -Inorganic (kgN+P20+K20/hectare) Little 30-120 Low, 3-10 Medium, N-only 10-20 High 0-5 0-60 3-5 NPK Planting time March-May (70%) Sept-Nov Oct-Dec (90%) Jan-Mar (90%) Sept-Oct Feb-May (80%) Oct-Nov Feb-Apr (90%) Harvest time Dec-May Aug-Dec Jul-Sept Oct-Jan Feb-Mar Sept-Oct Nov-Jan Planting space (m) 0.8×1.2,0.8×0.8 l. OxO.8,2.0×0.5 l. Oxl. O 1.2×0.8,0.8×0.8 l. Oxl. O, 0.8×0.8 Planting method Vertical Vertical Vertical Horizontal Horizontal Weed control Hoe 2-3x small tractor/ Paraquat Hoe l-2x Hoe 4-5x Hoe 2-3x Hoe 2-3x Harvest method Hand/tractor Hand Hand Hand Hand Main varieties KU 50, Rayong 90, Rayong 60, Rayong 5 Adira 4, local varieties H-226, H-165, local varieties KM 94, KM 60,34, HL 23 SC 205, SC 201, SC 124 Labor use (m-days/hectare) 50-60 150-300 200-350 100-200 90-180 Yield (tons/ hectare) 23.40 20 404 25 20 Production cost -Variable costs (USD/hectare) (Labor cost) (Other costs: Fertilizers, chemicals, cuttings, transportation) 365.91 (167.18) (198.73) 265.92 (185.37) (80.55) 663.85 (421.70) (242.15) 384.67 (213.60) (171.07) 427.62 (167.40) (260.22) — Fixed costs (USD/hectare) 48.89 46.67 236.50 60.00 94.94 — Total production costs USD/hectare 414.80 312.59 900.35 444.67 520.56 USD/ ton fresh roots 17.73 27.335 15.63 22.51 17.79 26.03

Table 4. Chemical composition of starch-accumulating edible parts of various starch crops.

polysaccharides, i. e. hemicellulose and pectic substances as evidenced by a presence of monosaccharide including rhamnose, fucose, arabinose, xylose, mannose, galactose, glucose in hydrolyzed cell wall materials (Kajiwara & Maeda, 1983; Menoli & Beleia, 2006; Charles et al., 2008). Some minerals such as sodium, calcium, potassium, magnesium, iron, copper, zinc, manganese and phosphorus are detected in fresh roots as well (Balagopalan et al., 1988; Rojanaridpiched, 1989; Charles et al., 2005).

Unlike grains of cereals having low moisture content (11-15%), cassava roots contain very high moisture contents and are very perishable. This is a constraint for cassava utilization as roots are subjected to deterioration and spoilage by microorganism attacks during storage. Fresh roots can be stored only a few days and should be transformed to products as soon as they are harvested. To prolong their shelf-life, the roots can be simply chopped and sun — dried; the final product is named as cassava chip with the moisture content approximately 14% (Table 4). Cassava roots also contain much lower protein contents than cereals.

The starch content of mature roots can range significantly, depending on genetic traits and environmental factors during plant development, as well as harvest time or ages after planting. Roots collected from crops being planted with the drought during initial state of growth have much lower starch contents and root yields than those from crops without the drought (Pardales and Esquibel, 1996; Santisopasri et al., 2001; Sriroth et al., 2001). Immature or young roots (less than 8 months) provide low starch yields due to low starch contents and root yields. The genetic and environmental growth condition can also influence starch qualities in term of starch composition (amylose and amylopectin content), ease of cooking as indicated by gelatinization or pasting temperature and cooked paste viscosity (Moorthy and Ramanujam, 1986; Asaoka et al., 1991;1992; Defloor et al., 1998; Sriroth et al., 1999; Santisopasri et al., 2001).