| Physicochemical properties of sweet potato starch were studied by means of Scanning electron microscope (SEM), X-ray diffraction(X-RD), rapid visco analyzer(RVA) and differential scanning calorimetry(DSC). Also the relationships between starch properties and noodle qualities were analysed. The main results were as follows:1. Significant difference in viscosities were observed with different RVA operating conditions. The most important factors causing these variations were the heating rate, the holding time at 95℃, the final holding time and the shear thinning. In addition, the peak viscosities were kept at high level when initial holding time was zero or one minute. Otherwise the maximum peak viscosity was shown with heating for 4 min or 6 min. The cooling time and the final temperature were important factors affecting the final viscosity, but the desired conditions for them were dependent on attributes of instrument. A proposal for RVA procedure of sweet potato samples is: at least 1 min at 50℃, heat to 95℃ for 4 min, hold at 95℃ for 5. 5 min, cool to 50℃ for 4 min, and hold at 50℃ for 4 min.2. Variation of starch properties was shown among genotypes. Phosphorus content and average diameter of starch granules ranged from 2. 50 to 6.41 u mol/g and from 14. 6 to 22. 8 u m respectively. Amylose content changed between 18.4% and 24.3%, which was correlated to crystallinity (r=-0. 64, P<0. 01) and pasting temperature (r=-0. 50, P<0. 05). Most of starch samples displayed Ca-Type X-ray diffraction pattern. The crystallinity of starch granules ranged from 29.6% to 42.0%. RVA viscoamylographs of all starch showed type A pasting curves, characterized by high pasting peak followed by a high degree of shear-thining. Again, the major diference among genotypes showed pasting temperature and viscosity values. Peak viscosity was correlated to hot paste viscosity (r=0. 77, P<0. 01) and final viscosity (r=0. 81,P<0. 01). In addition, during starch gelatinization the averagesof To, Tp and Tc were 65. 7℃, 76. 0 and 82. 7 respectively. Tp was higher than pasting temperature, so gelatinization occurs before pasting in sweet potato starch. Gelatinization enthalpy changed between 11.5 and 14. 2J/g. Retrogradation percentage ranged from 30. 7% to 42. 2%, which only correlated with setback (r=-0.45, P<0. 05).3. When the growth duration of sweet potato prolonged, a large increase in average granule size (Zhi883, 4. 3 u m; Xul8, 5. 0 u m; Zhe3449,2.5 u m) was observed. Peak viscosity also rised with increasing growth duration. The variations of crystallinity and breakdown were dependent on genotypes, whereas pasting time was almost constant and ranged from 4. 7 to 5. Omin. During the forming of root tuber, pasting temperatures and thermal characteristics of starch samples all showed a decreasing tendency. This indicated that the percentage of amorphous field increased gradually during the forming of root tuber. Furthermore, longer growth duration made X-ray diffraction patterns of starch granules shift from A to the B type.4. A large difference in noodle qualities was found among genotypes. Swelling ddegree ranged from 4. 96 to 7.16g/g, cooking loss ranged from 8. 27% to 11.63%. And cooking resistance changed between 2 and 17. Swelling degree highly correlated with cooking resistance (r=0. 75, P<0. 01) In addition, swelling degree was correlated to amylose content (r=0.67, P<0.01), crystallinity (r=-0. 77, P<0.01) and granule size (r=0. 73, P<0. 01). Effects of RVA viscosities on swelling degree and cooking loss were insignificant, but gelatinization temperature highly affected the cooking resistance. Cooking loss only correlated with retrogradation percentage (r=-0. 67, P<0. 01). |
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