Hydroxypropyl and nanocrystal derivatives of white (Colocasia antiquorium) and red (Colocasia esculenta) cocoyam starches: functional, pasting, thermal and structural properties

Hydroxypropyl starch derivatives were prepared from the reaction of propylene oxide with the native starches of white and red cocoyam under alkaline conditions while starch nanocrystal derivatives were produced via acid hydrolysis method. The native starches and their hydroxypropyl derivatives were analyzed for functional behaviours, pasting properties, thermal properties, X-ray diffractometry and morphology while the starch nanocrystals were analyzed for solubility, X-ray diffractometry and morphology. Molar substitution values of the hydroxypropyl starch derivatives ranged between 0.23 and 0.26 and their degree of substitution ranged between 0.02 and 0.03. After derivation, the swelling power and water solubility of the native starches increased from 12.74 ± 0.02 to 21.36 ± 0.04 g/g (white cocoyam starch, WCS), 12.17 ± 0.04 to 18.06 ± 0.04 g/g (red cocoyam starch, RCS) and 11.61 ± 0.03 to 21.20 ± 0.06% (WCS), 12.21 ± 0.02 to 25.36 ± 0.05% (RCS) respectively. Except breakdown, all the pasting parameters (peak viscosity, hot paste viscosity, cold paste viscosity, setback, peak time and pasting temperature) significantly decreased (p < 0.05) as the levels of substitution increased. The gelatinization temperatures and peak height index reduced with an increase in substitution levels. Both native and hydroxyproylated starches exhibited CB-type X-ray patterns while the nanocrystals had V-type patterns. The starch nanocrystals formed precipitates with chloroform, but were practically insoluble in other solvents. The morphology of their microstructures revealed that the native starches were spherical and polygonal in shape while the nanocrystal starches appeared as aggregates. The starches studied could serve as alternative sources in meeting the ever-increasing demands of starch for food and non-food applications.