Isolation and physicochemical characterization of starches from three varieties of Ethiopian sweet potato [<i>Ipomoea batatas</i> (L.) Lam.]

Abstract

Starch is a polysaccharide that finds versatile applications in pharmaceutical, food and other industries. Despite the presence of different varieties of sweet potatoes in Ethiopia, the physicochemical properties of their starches have not yet been studied. The aim of this study was therefore to isolate and characterize starches from three varieties of Ethiopian sweet potatoes [Ipomoea batatas (L.) Lam.] - Awassa83, Kulfo and Tulla. The starches were isolated using 0.075% (w/v) sodium metabisulphite solution. The proximate composition of the sweet potato starches, namely, amylose, protein, lipid, and fiber content were in the range of 11.13 - 18.15, 0.05 - 0.22%, 0.16 - 0.41% and 0.37 - 0.43%, respectively. The moisture content of the starches varied from 12.41 to 13.72%. Scanning electron microscopy of the starch granules showed polygonal, round and cupoliform/bell shaped granules. The average particle size of Awassa83 starch granules was 20.9 µm while Kulfo and Tulla exhibited similar average particle size of 12.1 µm. The swelling power and solubility of the starches ranged from 14.5 g/g to 21.7 g/g and 3.05% to 9.8%, respectively. X-ray diffraction pattern of the starches exhibited C-type with maximum characteristic peaks at 15^o, 17º, and 23º2θ. DSC studies provided gelatinization temperatures, i.e., onset temperature (T_o:61.82 - 65.27 ºC), peak temperature (T_p:65.60 - 69.09 ºC), and end set temperature (T_e:  72.13 - 73.69 ºC); The enthalpy of gelatinization ranged from 5.45 mJ/mg to 6.85 mJ/mg). Variations in pasting characteristics were observed among the starches comprising peak viscosity (787 mPa.s to 873 mPa.s), hot paste viscosity (652 mPa.s to 723 mPa.s), breakdown viscosity (135 mPa.s to 158 mPa.s), cold paste viscosity (898 mPa.s to 997 mPa.s), and setback viscosity (111 mPa.s to 127 mPa.s). The peak time ranged from 4 min to 4.53 min and pasting temp. from 71.85 ^oC to 74.3 ^oC. Awassa83 starch exhibited relatively high peak and breakdown viscosities and low setback viscosity, and it may be suitable as a thickening and gelling agent.

Keywords: Ipomoea batatas, Sweet potato starch, Awassa83, Kulfo, Tulla