Functional properties of wild yam (Dioscorea spp.) starches

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1 Tropical Science Trop. Sci. 2007, 47(1), Published online in Wiley InterScience ( 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

2 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 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 D. hirtifl ora D. bulbifera tuber D. bulbifera bulbil

3 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 D. hirtifl ora 81 No peak No peak D. bulbifera bulbil 85 No peak Viscosity (B.U.) 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).

4 36 DA Sahoré et al. (%) Syneresis D. bulbifera (tuber) Time (weeks) Figure 2. Paste syneresis at 15 C. Clarity (% transmittance) 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.

5 Properties of wild yam starches 37 Acknowledgements We thank S Ketoh, CNRA Abidjan, for technical assistance. References Amani NG (1993) Contribution à l étude des tubercules de taro (Xanthosoma sagittifolium (L.) Schott). Evolution physico-chimique de l amidon au cours des traitements technologiques. Doctoral thesis, University of Cocody, Abidjan. Amani NG, Buleon A, Kamenan A, Colonna P (2004). Variability in starch physicochemical and functional properties of yam (Dioscorea sp) cultivated in Ivory Coast. Journal of the Science of Food and Agriculture 84: Banks W, Greenwood CT (1975) Starch and its Components. Edinburgh: Edinburgh University Press. Dadié A, Aboa F, Coulibaly S (1998) Caractéristiques physico-chimiques de la farine et de l amidon de la patate douce (Ipomoea batatas). Industrie Alimentaires et Agricoles 115: Eliasson AC, Kim HR (1992) Changes in rheological properties of hydroxypropyl potato starch pastes during freezethaw treatments. I. A rheological approach for evaluation of freeze-thaw stability. Journal of Texture Studies 23: Garcia E, Lajolo FM (1988) Starch transformation during banana ripening: the amylase and glucosidase behaviour. Journal of Food Science 53: Graig SAS, Maningat CC, Seib PA, Hoseney RC (1989) Starch paste clarity. Cereal Chemistry 66: Hoover R (2001) Composition, molecular structure, and physicochemical properties of tuber and root starches: a review. Carbohydrate Polymers 45: Mazurs EG, Schoch TJ, Kite FE (1957) Graphical analysis of the Brabender viscosity curves of various starches. Cereal Chemistry 34: Rasper V, Coursey DG (1969) Investigations on starches from major starch crops grown in Ghana. II. Swelling and solubility patterns: amyloelastic susceptibility. Journal of the Science of Food and Agriculture 20: Robin JP (1976) Comportement du grain d amidon à l hydrolyse acide ménagé. Etudes physico-chimiques et enzymatiques de la fraction insoluble. Contribution à la connaissance de la structure de l amylopectine. Doctoral thesis, University Pierre and Marie Curie, Paris. Sahoré DA, Amani NG, Nemlin GJ (2005) The properties of starches from some Ivory Coast wild yam (Dioscorea) species. Tropical Science 45: Schoch TJ (1968) Effect of freezing and cold storage on pasted starches. In: The Freezing Preservation of Foods. Vol.4: Freezing of Precooked and Prepared Foods (Tressler DK, Van Arsdel WB and Copley MJ, eds), pp Westport, Connecticut: AVI Publishing Co. Zheng GH, Sosulski FW (1998) Determination of water separation from cooled starch and flour pastes after refrigeration and freeze-thaw. Journal of Food Science 63: