Tropical Science Trop. Sci. 2007, 47(1), 33 37 Published online in Wiley InterScience (www.interscience.wiley.com).189 Functional properties of wild yam (Dioscorea spp.) starches DA Sahoré*, NG Amani and A Kamenan Food Science and Technology Department, University of Abobo-Adjamé, Abidjan, Côte d Ivoire. *To whom correspondence should be addressed (alexissahore@yahoo.fr) Abstract The starches of the wild yam species Dioscorea praehensilis,,,, and D. bulbifera (tuber and bulbil) were isolated, and their paste viscosity and clarity, iodine binding and syneresis were investigated. Copyright 2006 John Wiley & Sons, Ltd Key words: starch, wild yam, Dioscorea spp., functional properties Introduction The structure and physico-chemical properties of the major tuber and root starches have been studied extensively (Hoover 2001). However, little attention has been given to the tubers of various species of yam (Dioscorea spp.), which still grow wild in the forests of West Africa (Rasper and Coursey, 1969) and are used as food. Sahoré et al. (2005) described the proximate composition, amylose content, morphological and gelatinisation properties, swelling power and solubility of the starches of some of these wild yams. This paper concerns the functional properties, iodine binding, paste viscosity and clarity, and syneresis of these starches. Materials and methods Tubers of the wild yam species Dioscorea praehensilis, D. hirtifl ora,, D. togoensis, and D. bulbifera were collected in the south forest zone of Côte d Ivoire and samples were identified at the Cocody University Herbarium. Freshly collected tubers of wild yam were peeled, sliced and homogenised at room temperature using a Waring blender. The paste was dispersed in 4% NaCl solution and filtered through sieves with decreasing mesh size (0, 2 and 100 µm). The removed solids were decanted, washed several times with distilled water, and the starch was dried to constant weight in thin layers in a convection oven at 35 C. The viscosity of the starch during the pasting process was monitored using a Brabender amylograph (Mazurs et al., 1957). Dry starch (40 g) was slurried in distilled water, made up to 4 ml, and heated at a rate of 1.5 C min 1 to 95 C, maintained there for 15 min, then cooled at 1.5 C min 1. The pasting temperature was that at which the viscosity started to rise. Accepted 28 November 2005
34 DA Sahoré et al. A 9% w/v gel was prepared by dissolving each starch in distilled water and heating it for 30 min whilst agitating. Portions of about 10 g were cooled to room temperature and then centrifuged at 2700 g for 30 min. The initial syneresis was evaluated and the rest of each gel was frozen at 15 C for 4 weeks. Every week, two test portions were kept at C for about 90 min and then centrifuged at 2700 g for 30 min. Syneresis was the percentage of the initial mass which separated as liquid (Schoch 1968). The procedure of Graig et al. (1989) and Zheng and Sosulski (1998) was used to determine starch paste clarity. A 2% w/v aqueous dispersion of starch was boiled for 30 min with constant stirring, and then cooled to 30 C. The transmittance was measured at 620 nm. The rest of each gel was kept at 6 C for 4 weeks: each week two portions were thawed at 30 C and the transmittance measured at 620 nm. The state of the starch during heating was characterised by its iodine-binding spectrum (Robin 1976). To 1 ml of an aqueous suspension of starch (0.2 g / ml), solubilised at 95 C, was added 0.2 ml of Lugol s solution (0.2 g I 2 in a solution of KI 2% w/v). The colour of the iodine-stained starch dispersion was measured by a spectrophotometer from 400 to 700 nm. The proportion of amylose to amylopectin was determined as the ratio of the optical density of amylose at 630 nm to that of amylopectin at 540 nm (Garcia and Lajolo 1988). Results and discussion When starch is heated in water above its gelatinisation temperature, lixiviation occurs, with amylose diffusing preferentially out of the starch granule (Banks and Greenwood 1975). Our samples had amylose/amylopectin ratios from 1.12 to 1.29 (Table 1). These values are slightly lower than those of cocoyam, 2.32 (Amani 1993), and potato, 1.57 (Dadié et al. 1998). The pasting temperatures ranged from 80 C to 87 C (Table 2). These temperatures are very high, suggesting the presence of strong bonding forces within the granules (Hoover 2001). After cooling, two different groups of wild yam starch were found: D. hirtifl ora, D. dumetorum and D. bulbifera had amylose contents below 17% d.b., whereas, and had amylose contents above 25% d.b. The viscosity increased gradually throughout the period of heating for the starches from D. hirtifl ora, and D. bulbifera (Figure 1), while the others showed a peak after 35 40 min. Table 1. Iodine-binding characteristics of starch suspension (optical density) Starch source O.D. O.D. O.D. 630 /O.D. 540 λ max (amylose) (amylopectin) (nm) at 630 nm at 540 nm 1.2277 1.0851 1.13 600 620 D. hirtifl ora 1.6003 1.2418 1.29 630 6 0.9867 0.8655 1.14 610 630 2.1024 1.8794 1.12 580 610 D. bulbifera tuber 1.1739 1.0227 1.15 600 620 D. bulbifera bulbil 1.1013 0.9642 1.14 600 620
Properties of wild yam starches 35 Table 2. The pasting characteristics of starches Starch source Pasting Viscosity (B.U.) temperature ( C) Peak On At end of On cooling viscosity attaining holding to C 95 C period 80 300 300 85 60 D. hirtifl ora 81 No peak 140 135 180 81 175 175 75 87 No peak 65 65 85 83 295 295 155 120 D. bulbifera bulbil 85 No peak 180 180 225 Viscosity (B.U.) 3 300 2 200 1 100 0 28 30 32 34 36 38 40 42 44 46 48 60 70 80 90 100 110 120 128 Time (min) Figure 1. Brabender amylograph of starches. The syneresis of gels of the starches was 1 10% at the outset. It increased considerably during the first week but then stabilised in the second week (Figure 2). The stable level reflects the resistance to freezing: starch was the most resistant to freezing (27%) whereas D. bulbifera bubil was the least (57%). Most of our results were much lower than the 60% syneresis of frozen yam starches centrifuged at 7900 g (Eliasson and Kim 1992). The paste clarity decreased slightly during the first week and then stabilised at about 25 to 40% transmittance (Figure 3). These results are similar to those of Amani et al. (2004), who reported 24.7, 37.4 and 40.7% respectively for D. esculenta, D. alata and the D. cayenensis/rotundata complex. Paste clarity is a very complex property depending on granule size and swelling power, amylose content and amylose macromolecular properties (Graig et al. 1989; Amani et al. 2004).
36 DA Sahoré et al. (%) Syneresis 70 60 40 30 20 D. bulbifera (tuber) 10 0 0 1 2 3 4 Time (weeks) Figure 2. Paste syneresis at 15 C. Clarity (% transmittance) 55 45 40 35 30 25 20 0 1 2 3 4 Time (weeks) D. bulbifera (tuber) Figure 3. Clarity at 6 C. Conclusion The starches extracted from these wild yam tubers were thermoresistant, with relatively opaque and less viscous gels. Their aqueous suspensions contained less amylose and showed a slight tendency to retrogradation. Such properties are positive quality factors for the potential use of starches from these lesser known yam species.
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