Water mobility and textural properties of native and hydroxypropylated wheat starch gels

The effect of wheat starch concentration and degree of hydroxypropylation on the water dynamics and textural attributes of starch gels was studied using 1H NMR and texture profile analysis. Starches were prepared with molar substitution (MS) of hydroxypropyl groups of 0.05, 0.12, and 0.18. Gels were formed from modified starches at concentrations between 25 and 40%. For all gels, two distinct water fractions were identified using distributed exponential analysis of transverse relaxation results. For the less mobile fraction, T2a ranged from 2 to 6 ms, while for the more mobile state, T2b ranged from 20 to 200 ms. Values of both T2a and T2b increased with increasing moisture content and hydroxypropyl MS. 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.