61A-15 |
Effects of hydroxypropylation and cross-linking of wheat starch on molecular mobility as studied by pulsed 1H NMR |
S. G. CHOI, Food Science and Technology, University of Georgia, Athens, GA 30602-7610 and W. L. Kerr. Chemically modified starches have become important functional ingredients in processed foods because of their improved functional properties over unmodified starches. The altered physicochemical properties due to chemical modification are dependent on the type of modification and the degree of substitution. We hypothesize that chemical modification may cause structural changes in starch granules, which influence molecular motion of starch chain and water-binding properties, leading to altered functional properties. Our objective was to study the effects of chemical modification by hydroxypropylation and cross-linking on molecular motion of both starch and water of wheat starch suspensions at low moisture levels. Starch was hydroxypropylated with propylene oxide (molar substitution (MS) of 0.05, 0.12, and 0.18) or cross-linked with phosphorus oxychloride at the concentrations of 0.03%, 0.1%, and 0.2%. Starch samples were equilibrated over 0 ~ 0.93 aw. Pulsed 1H NMR was used to identify and quantify the molecular motion of starch chains and water. Transverse proton relaxation (T2) of the systems were analyzed using distributed exponential routine from one pulse and Carr-Purcel-Meiboom-Gill (CPMG) pulse experiments. Our results showed that proton relaxation associated with starch molecules increased with increasing water content and MS. However, it was not significantly affected by cross-linking. Transverse proton relaxation associated with water in control (alkali treated), hydroxypropylated and cross-linked starch showed two distinct populations in the aw range of 0.53 to 0.93, while unmodified starch had one population. In general, the proton relaxation in both regions increased as MS increased. In conjunction, the integrated signal intensity of water proton increased with increasing MS. These results suggest that hydroxypropylation increases molecular mobility of both starch chains and water and increases water adsorptive capacity. In addition, alkali-treatment used in chemical modification may cause structural changes of starch granules. These studies may help understanding of altered functional properties by chemical modification.
Session 61A, Carbohydrate
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