Hydroxypropyl derivatives of legume starches: Functional, rheological and thermal properties

Hydroxypropyl derivatives were prepared from the native starches isolated from lima bean and jack bean. Both native and hydroxypropylated starches were evaluated for functional, rheological and thermal properties. Molar substitution (MS) value ranged from 0.24 to 0.37 while degree of substitution (DS) from 0.02 to 0.04. The values of swelling power, solubility and pasting parameters of the native starches were significantly different from the hydroxypropyl derivatives with deviations of insignificant difference obtained for lima bean starches. Non-Newtonian shear-thinning behaviour was exhibited by all the samples. Their viscosity consistency, K, deduced from Herschel–Bulkley model, increased progressively with MS. Starches at low-substitution possessed high shear viscosities compared to the native and high-substituted starches. Increase in yield stress (σo) was observed at low-substitution level and decrease at high-substitution level of hydroxypropylation. Upon hydroxypropylation, enthalpies of gelatinisation (ΔHgel) increased whereas the transition gelatinisation temperatures (To, Tp and Tc) decreased. Comparatively, lower values of transition temperature and enthalpy were obtained for retrograded starch gels than the gelatinised gels. Upon hydroxypropylation, native starches experienced increase in MW and polydispersity index (PDI). All the native starches exhibited type-A XRD patterns, which remained unchanged after hydroxypropylation.