The effect of chemical modification on the molecular mobility, texture, and retrogradation of starch-water suspensions
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The effects of chemical modification by hydroxypropylation and cross-linking on the molecular dynamics of starch-water suspensions at low moisture levels or in the gel state were studied. The relationships with textural properties and retrogradation were also investigated.|Wheat starch was hydroxypropylated with propylene oxide (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%, respectively. Pulsed 1H NMR was used to determine moisture content and to study molecular mobility of wheat starch suspensions at low moisture content. Moisture determination by NMR agreed well with gravimetric determinations (R2= 0.99). Proton relaxation associated with starch molecules increased with increasing water content and molar substitution of hydroxypropyl group, indicating increased mobility of starch chains. However, mobility 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, suggesting that treatment with alkaline reagents may cause structural changes in starch granules.|Gels were formed from native and hydroxypropylated starches at concentrations between 25 and 40%. For all gels, two distinct water fractions were identified using distributed exponential analysis of CPMG results. For the less mobile fraction, T2a ranged from 2~6ms while for the more mobile state, T2b ranged from 20~200ms. Values of both T2a and of T2b increased with increasing moisture content and hydroxypropyl M.S. Control experiments indicated that more mobile states could be associated with both the extragranular amylose gel, as well as intragranular amylopectin-rich gel. The less mobile state was associated only with granule remnants. Gel hardness decreased significantly with increasing moisture content, and with increasing hydroxypropylation. Cohesiveness increased with degree of hydroxypropylation. Results from multi-regression analysis between hardness and NMR parameters showed that the mobility and amount of water in the less mobile state was most related to gel hardness.|During storage of starch gels, decrease in water mobility accompanied retrogradation. Greatest decrease was observed with native starch, less with alkali treated starch, and even less with hydroxypropylated starch.