Effects of chemical modification of wheat starch on molecular mobility as studied by pulsed 1H NMR

Abstract

The effects of hydroxypropylation (0.05, 0.12, and 0.18 molar substitution (MS)) and cross-linking (0.03%, 0.1%, and 0.2%) on molecular mobility in low-moisture wheat starch were investigated by pulsed 1H NMR. Starch samples were equilibrated over an a w range of 0–0.93. Transverse relaxation ( T 2) times of the systems were obtained from single-pulse and CPMG experiments and were analysed by distributed exponential fitting. T 2s ∗ values associated with starch molecules increased with increasing MS of hydroxypropyl group, indicating increased mobility of starch chains. Proton relaxation was not significantly affected by cross-linking. The proton relaxation associated with water in control (alkali treated), hydroxypropylated and cross-linked starch showed two distinct populations for samples at a w between 0.43 and 0.93. In general, water mobility in both regions increased as a w or hydroxypropyl MS increased, while cross-linking at 0.03% and 0.1% produced little change in water mobility.