Pre-gelatinization Method Pre-gelatinized starches are pre-cooked starches that can be used as thickeners in cold water. Heat-treatment processes, heat-moisture and annealing treatments cause a physical modification of starch without any gelatiniza-tion, damage to granular integrity or loss of birefringence (Abbas et al., 2010). Different methods are suggested for pre-gelatinized starch samples. Oladebeye et al. (2011) increased the moisture levels of the starch sample by adding appropriate amounts of distilled water. The mixtures were stirred in a sealed jar and heated in an air oven at 100 °C for 16 h. After cooling, the jars were opened and the starch samples were air dried to a moisture content of 10 %. But Ibrahim and Achudan (2011) suspended yam starch in distilled water and heated until the temperature reached 80 °C by slow manual stirring. Then the pre-gelatinized starch was placed into stainless steel trays in the form of a thin film (1–2 mm) and dried in an oven at 40 °C for 48 h.
Cross-linking Phosphorylation Starch suspension (200 ml of 45 %) was mixed with 10 g of sodium sulphate and 4 g of trisodium trimetaphosphate. The pH of suspension was adjusted to 9.5 by adding 10 % aqueous hydrochloric acid or sodium hydroxide. The slurry was stirred for 1 h at room temperature, and washed 3 times with distilled water. The slurry was dried in an oven at 40 °C to 12 % moisture and heated in an oil bath for phosphorylation for 2 h at 130 °C. After cooling at room temperature, the starch cake was washed with distilled water and the starch was recovered by centrifugation at 1,500 rpm for 10 min. Finally, the pH of the suspension was adjusted to 6.5 and the recovered starch was dried at 40 °C in a vacuum oven. The starch obtained was ground and sieved with a 250 pm screen (Ibrahim and Achudan, 2011; Waliszewski et al., 2004).
Enzymatic Modification This involves the exposure of starch suspensions to a number of enzymes, primarily including hydrolyzing enzymes that tend to produce highly functional derivatives (Neelam et al., 2012). Enzymatic modification of starch is hydrolysis of some part of starch into a low molecular weight of starch called maltodex-trin, or dextrin using amylolytic enzymes (Abbas et al., 2010; Miyazaki et al., 2006).
Modification of starch generally widens the possible exploitation of starch in industrial products as a result of its better viscosity, higher gel strength, improved film forming capacity, clarity and lower retrogradation tendency (Jyothi et al., 2013). Modified yam starch had been studied and incorporated into drugs (Adedokun and Itiola, 2011; Adetunji et al., 2006; Bharath et al., 2012; Odeku and Picker-Freyer, 2011), hydrogel (Odeniyi and Ayorinde, 2012), aquatic feed binder (Orire et al., 2010) and food application (Abbas et al., 2010; Adewole et al., 2011; Oke et al., 2013b; Otegbayo et al., 2013).
12.7 Effects of Processing on the Quality of Yam
Effects of processing on the nutrient composition of yam tuber is shown in Table 12.10. Boiling and roasting had been reported to reduce the nutrient composition of yam. Cooking improves digestibility, promotes palatability and improves keeping quality of root crops. Bell and Favier (1981) studied the effect of charcoal grilling on the nutrient composition of yam tubers and reported this method as the most detrimental to the nutritional quality of the yam. Likewise, Olayaki et al. (2007) reported that ingestion of roasted yam could induce haemolysis in rat. Bell and Favier (1981) also suggested that boiling unpeeled tubers and tubers fried in palm oil is the most advantageous methods. Various processing techniques like blanching, boiling, baking, etc. before consumption can reduce the level of the oxalate, alkaloid, saponin and flavonoid drastically and considerably (Table 12.11) (Eka, 1998; Sakai, 1983). Boiling can reduce the soluble oxalate content of a food, if the water used for the boiling is discarded (Oscarsson and Savage, 2007). Boiling resulted in reduction of all the anti-nutritional components in the yam (Adepoju, 2012; Ezeocha et al., 2012). It may also cause considerable skin (epidermal) rupture and facilitate the leakage of soluble oxalate into the cooking water (Albihn and Savage, 2001).
Table 12.10 Effect of processing on the nutrients in yam
Species | Protein (%) | Fat (%) | Ash (%) | Crude fibre (%) | Carbohydrate(%) | Author
D. dumetorum
Raw | 11.41 | 0.71 | 2.23 | 2.03 | 77.55 | Ezeocha et al. (2012)
Boiled (30 min) | 10.3 | 0.65 | 1.91 | 1.86 | 80.09 |
D. bulbifera
Raw | 1.25 | 1.02 | 3.88 | 1.76 | 79.64 | Sanful et al. (2013)
Boiled | 1.17 | 1.17 | 3.23 | 2.17 | 81.31 |
D. rotundata
Raw | 3.35 | 0.73 | 1.99 | Adegunwa et al. (2011)
Boiled | 3.91 | 0.58 | 1.99 | — | - |
Roasted | 3.86 | 0.59 | 2.76 | — | - |
D. alata
Raw | 4.59 | 0.3 | 2.68 | — | - | Adegunwa et al. (2011)
Boiled | 3.41 | 0.5 | 1.88 | — | - |
Roasted | 3.18 | 0.46 | 2.5 | — | - |
Note: ― is Not determined
Table 12.11 Effect of processing on anti-nutrients composition in yam
Species | Phenol | Oxalate | Phytate | Alkaloid | Author
D. dumetorum (mg/100 g)
Raw | 0.32 | 4.01 | 0.91 | 0.70 | Abiodun and Akinoso (2014c)
Parboiled | 0.12 | 0.28 | 0.79 | 0.21 |
Blanched/soaked | 0.02 | 0.16 | 0.34 | 0.18 |
D. bulbifera (mg/100 g FW)
Raw | — | 67.79 | 56.74 | — | Bhandariand Kawabata (2006)
Boiled | — | 31.78 | 44.73 | — |
Baked | — | 63.72 | 55.74 | — |
D. rotundata-cayenensis (mg/100 g)
Raw | — | 0.96 | 31.32 |- | Kouassiet al. (2010)
Baked | — | 0.34 | — | 29.29 |
Boiled | — | 0.03 | — | 17.81 |
D. alata (%)
Raw | — | 1.91 | 2.77 | — | Ezeocha and Ojimelukwe (2012)
Boiled (30 min) | 1.21 | — | - | 1.91
Note: ― is Not determined
Oxalate causes an intense irritation of the skin and mucous membranes (e.g. in the mouth) when in contact with mucilage, due to the presence of calcium oxalate crystals (raphide) (Sakai, 1983). Vitamins (especially the vitamin B group) were lost during cooking, depending on the method of preparation used (Bell and Favier, 1981). Blanching, however, effectively decreases the action of amylases on D. dumetorum starch during storage, with subsequential increase in the paste viscosities of the stored tubers. Domestic cooking methods were found to be very efficient in removing bitterness, thus making the bitter yams palatable (Bhandari and Kawabata, 2005; Ogbuagu, 2008). Extrusion cooking, explosion puffing and instantization produced products are more readily digestible (Brand et al., 1985). Hydration of granules (gelatinization) and disruption of organized granule structure increases the availability of starch to amylase digestion and are more likely to occur under processing factory cooking conditions when higher temperatures and pressures are utilized (Allen et al., 2012).