Physicochemical properties of sweet potato starch
Tropical root crops, of which the sweet potato is an important representative, constitute an under exploited resource of developing countries. They can be used as food for both human and animal consumption and their starch is a source of industrial raw material. This review will consider recent reports on sweet potatoes, the physicochemical properties of their starches in comparison with other starches, and the possible causes of variation in these characteristics.[1]
Characterization of Potato Starch and Its Monoglyceride Complexes
Native potato starch granules and those complexed below the gelatinization temperature with 1-(C8 to C18)-monoglycerides were compared for solubility, swelling power (SP), viscosity, heat stability and water binding capacity (WBC). X-ray diffraction analysis proved that complexes were true clathrates (inclusion) compounds. Clathrate stability was a function of monoglyceride chain length. The presence of clathrates in granules decreased solubility by up to 90% and SP up to 10-times, while WBC dropped from 0.39 to as low as 0.25 g water/g starch. Endotherm characteristics were given for gelatinization, most perfect crystallite, and clathrate fusions. Enthalpies per mole glucose units of 15.4 kcal/mole for native starch and 27.5 for clathrate were obtained irrespective of monoglyceride chain length.[2]
Effect of high pressure on the structure of potato starch
Potato starch–water suspension (10%) was subjected to high pressure treatment at 600 MPa for 2 and 3 min. Cross-polarization/magic angle spinning (CP/ MAS) 13C NMR spectra of the obtained starch preparations indicated distinct signal resonances at 81.23 and 82. 46 ppm, respectively, which corresponded to amorphous sites (C-4) within starch structure. The F.t-i.r. analysis of starch preparations showed that high pressure significantly affected the intensity of bands corresponding to the amorphous (1018 cm−1) and more ordered (1047 cm−1) part of starch structure. The DSC analysis showed a decrease in gelatinization temperatures (To, Tp, Tc) upon high pressure treatment. A substantial decrease in the total enthalpy of pressurised starches along with the time of treatment was also found. The SEM analysis indicated that in the starch structure the granule’s surface was the most resistant to high pressure treatment. The inner part of the granule was almost completely filled with gel-like network, with empty spaces growing in diameter towards the centre of the granule.[3]
The Effect of Solutes on the Gelatinization Temperature Range of Potato Starch
The gelatinization behaviour of potato starch in an excess of water or solutions of sugars, other organic hydroxy compounds, or various inorganic salts was studied. The distribution of water and solutes between the external phase and the starch granules was measured by refractometry and a dye exclusion technique. When the limited water uptake of native starch granules is taken into account, the cooperative nature of gelatinization and the effect of water content on gelatinization behaviour can be explained solely on the basis of the Flory theory of polymer melting. Following the same argument, the effect of many solutes can be approximately described by the derived relationship between initial gelatinization temperature, water activity of the system and volume fraction of water in the granules.[4]
Gelatinization mechanism of potato starch
The non-Newtonian behavior and dynamic viscoelasticity of potato starch (Jaga kids red ’90, 21.0% amylose content) solutions after storage at 25 and 4°C for 24 h were measured with a rheogoniometer. The flow curves, at 25°C, of potato starch showed plastic behavior >1.0% (w/v) after heating at 100°C for 30 min. A gelatinization of potato starch occurred above 1.0% at room temperature. A very large dynamic viscoelasticity was observed when potato starch solution (3.0%) was stored at 4°C for 24 h and stayed at a constant value with increasing temperature. A small dynamic modulus of potato starch was observed upon addition of urea (4.0 M) at low temperature (0°C) even after storage at 25 and 4°C for 24 h. A small dynamic modulus was also observed in 0.05 M NaOH solution. Possible models of gelatinization and retrogradation mechanism of potato starch were proposed.[5]
Reference
[1] Tian, S.J., Rickard, J.E. and Blanshard, J.M.V., 1991. Physicochemical properties of sweet potato starch. Journal of the Science of Food and Agriculture, 57(4), pp.459-491.
[2] Hoover, R. and Hadziyev, D., 1981. Characterization of potato starch and its monoglyceride complexes. Starch‐Stärke, 33(9), pp.290-300.
[3] Błaszczak, W., Valverde, S. and Fornal, J., 2005. Effect of high pressure on the structure of potato starch. Carbohydrate Polymers, 59(3), pp.377-383.
[4] Evans, I.D. and Haisman, D.R., 1982. The effect of solutes on the gelatinization temperature range of potato starch. Starch‐Stärke, 34(7), pp.224-231.
[5] Tako, M. and Hizukuri, S., 2002. Gelatinization mechanism of potato starch. Carbohydrate Polymers, 48(4), pp.397-401.
