12.6.11 Starch
Starches are widely available, naturally occurring carbohydrates, found in almost all organs of plants, most especially in roots, rhizomes, fruits and seeds (Bharath et al., 2012; Okunola and Odeku, 2009). Yam is a significant and highly prized starchy crop in West and Central Africa, but largely under-utilized industrially (Otegbayo et al., 2013). The high starch content of yam tubers (70–80 % dry weight) makes it potential source of starch that can be explored commercially (Bharath et al., 2012).
Starch consists primarily of two components, amylose and amylopectin (Taghvaei-Ganjali et al., 2010). Starches are used to take up water and to produce viscous fluids/pastes and gels and to give desired textural qualities (BeMiller and Whistler, 1996). There is variability in the starch granule shapes, which are oval, round, triangular, ellipsoidal triangular and polyhedral (Figure 12.9) (Otegbayo et al., 2013; Sahore and Amani, 2013). The distribution of starch granule size frequency depended on the species (Sahore and Amani, 2013). Starches from D. rotundata, D. alata, D. cayenensis, Dioscorea burkilliana and D. bulbifera had large starch granules (11.5-90 pm), while D. esculenta and D. dumetorum had small granules (0.83–12 pm) (Table 12.9) (Akinoso and Abiodun, 2013; Otegbayo et al., 2013; Sahore and Amani, 2013). Moorthy (2004) and Riley et al. (2006) observed that the granule size of starch affects some functional properties such as swelling, solubility and digestibility. However, the smaller granule sizes improve the digestibility due to greater surface area, which is more rapidly digested by amylases (Yuan et al., 2007).
Figure 12.9 A typical morphological structure of trifoliate yam starch using light microscopy (LM) (x800) (a) and scanning electron microscopy (SEM) (×3000) (b).
Table 12.9 Starch shape and average granule size
Yam species | Starch shape | Average granule size (pm) | Author
D. alata | Oval, spherical, polyhedric, Ellipsoid | 19–46 | Riley et al. (2006), Harijono et al. (2013), Otegbayo et al. (2013)
D. bulbifera | Ovo triangular | 21.8-50 | De Vizcarrondo et al. (2004), Sahore and Amani (2013) Otegbayo et al. (2013)
Dioscore aburkilliana | Ovo triangular | 11.5-90 | Sahore and Amani (2013)
D. dumetorum | Polyhedric | 0.83–12 | Sahore and Amani (2013), Akinoso and Abiodun (2013), Otegbayo et al. (2013)
D. rotundata-cayenensis | Ovo triangular, oblong, oval, polyhedral and ellipsoid | 10–70 | Hoover (2001); Moorthy (2002); Lindeboom et al. (2004); Brunnschweiler et al. (2005), Tetchi et al. (2007); Dabonne et al. (2010)
Smaller granule size is useful in some easily digestible foods like infant foods, as fillers in talcum powders, biodegradable plastics and aerosols, while larger granule size are useful in hydrolyzed products (Jyothi et al., 2013). Yam starches can be used as thickeners and gelling agents in the food industry, as well as disintegrants in tablet formulation in the pharmaceutical industry (Adetunji et al., 2006: Okunola and Odeku, 2009) and aquatic feed binder (Orire et al., 2010).
Starch is often used as a food ingredient to improve physical properties of the host foods, and to overcome undesirable changes during processing or storage. Starch can be structurally modified by various means to enhance its functions as an ingredient (Chung et al., 2008; Eliasson and Gudmundsson, 1996). Native starches, according to Odeniyi and Ayorinde (2012) and Otegbayo et al. (2013), have some limitations in their use in the food industry due to low shear resistance, thermal resistance, thermal decomposition and a high tendency towards retrogradation. Native starches are modified in order to improve their functional properties. These can be done using physical (heat-moisture, steam-pressure, microwave, irradiated), chemical (acid, oxidized, cross-linked, acetylated, succinylated, phosphorylated, hydroxypropylated and carboxymethylated modified starches) and enzymatic technique (Abbas et al., 2010; Jyothi et al., 2013). The major methods for modification of yam starches are described as follows:
Acid Modified Starches Native Dioscorea starches (dried) were hydrolyzed by incubating the starch in 600 ml, 6 % HCl solution at 23 ± 1 °C for 192 h without stirring (Atichokudomchai and Varavinit, 2003; Okunola and Akingbala, 2013). The suspension was neutralized with 10 % (w/v) NaOH solution, and the starch slurry was washed 5 times with distilled water and dried in a hot air oven at 40 °C for 24 h. The starch was milled and passed through a 125 pm sieve.
Alkali Modified Starches Yam starch was mixed with distilled water with constant stirring. Then 7.5 g of sodium sulphate (Na2SO4) was added while maintaining constant stirring. The pH was adjusted again to 10.5 with 2.5 % NaOH and heated in a water bath at 45 °C with constant agitation. Once this temperature was reached, 4.5 g of sodium trimetaphosphate (STMP) was added. The mixture was stirred for 3 h and neutralized to pH 7 with 2.5 % HCl. The starch obtained was centrifuged and washed 3 times with distilled water. Subsequently, the obtained starch was placed in aluminum pans, dried, sieved and milled (Gutierrez et al., 2014).
Acetylated Starch Native yam starch was dispersed in distilled water and stirred magnetically for 20 min. The pH of the slurry obtained was adjusted to 8.0 using 1.0 M NaOH. Acetic anhydride (10.2 g) was added slowly to the mixture while maintaining a pH of 8.0–8.5. The reaction proceeded for 5 min after the addition of acetic anhydride. The pH of the slurry was finally adjusted to 4.5 using 0.5 M HCl and was filtered, washed 4 times with distilled water and air dried at 30 °C for 48 h (Iheagwara, 2012; Sathe and Salunkhe, 1981).
Oxidized Starch This can be obtained by mixing native yam starch with 500 ml of distilled water and the pH of the mixture adjusted to 9.5 with 2.0 M NaOH. Ten grams (10 g) of NaOCl was added to the slurry over a period of 30 min with constant stirring while maintaining a pH range of 9.0–9.5. The reaction was allowed for 10 min after all the NaOCl had been added. The pH was adjusted to 7.0 with 1 M H2SO4 and the oxidized starch was filtered, washed 4 times with distilled water and air dried at 30 °C for 48 h (Forssel et al., 1995; Iheagwara, 2012).