Physicochemical properties of sorghum (sorghum bicolor l. Moench) starch as affected by drying temperature
Abstract
N. A. Aviara1, J. C. Igbeka2, L. M. Nwokocha3
(1. Department of Agricultural and Environmental Resources Engineering, University of Maiduguri, Maiduguri, Nigeria;
2. Department of Agricultural and Environmental Engineering, University of Ibadan, Ibadan, Nigeria;
3. Department of Chemistry, University of Ibadan, Ibadan, Nigeria)
Abstract: Starch was isolated from sorghum (white) grains and its proximate composition determined. Scanning electron micrograph and x-ray diffraction pattern of the starch were obtained using scanning electron microscope and x-ray diffractometer, respectively. The effect of drying temperature in a tray dryer on starch physicochemical and pasting properties was investigated. The open air dried starch had a composition of 10.73% moisture, 0.30% ash, 1.06% protein and 1.07% fat. Amylose content was 21.08% and it had an average granule size and pH of 18.59 μm and 5.45, respectively. The starch exhibited the A-type crystalline diffraction pattern, which was not altered by drying in a tray dryer at the temperature range of 40 to 60℃. Water binding capacity and swelling power of the open air dried starch were not significantly different from those of starch dried at 40℃ (p<0.05). Water binding capacity increased from 79.63 to 88.5%, while swelling power, solubility and percentage syneresis decreased from 12.01 to 8.96 g/g, 7.08 to 2.85% and 14.00 to 10.80%, respectively as the drying temperature increased from 40 to 60oC. Paste clarity was low (22.50%–26.20%) but increased with an increase in drying temperature up to 50℃ and decreased with a further increase in temperature. Pasting properties of open air dried starch differed significantly from those of starch dried at different temperatures. Peak viscosity decreased from 398.75 to 325.25 RVU as the drying temperature increased from 40 to 60℃. Setback viscosity increased with an increase in drying temperature up to a point and decreased with a further increase in temperature. Final and breakdown viscosities as well as pasting temperature and peak time had polynomial relationships of the third order with drying temperature. Regression equations that could be used to adequately express the relationships existing between the physicochemical and pasting properties of sorghum starch and drying temperature were established. These models could be used to select the drying temperature that would yield starch of desired physicochemical properties for a functional application.
Keywords: sorghum starch, degree of crystallinity, physicochemical properties, pasting properties, drying temperature, tray dryer
Citation: Aviara N. A, J. C. Igbeka, and L. M. Nwokocha. Physicochemical properties of sorghum (sorghum bicolor l. Moench) starch as affected by drying temperature. Agric Eng Int: CIGR Journal, 2010, 12(2): 85-94.
