Composition, physicochemical properties and retrogradation characteristics of native, oxidised, acetylated and acid-thinned new cocoyam ( Xanthosoma sagittifolium) starch

Abstract

New cocoyam starch was modified through oxidation (oNCS), acetylation (aNCS) and acid-thinning (atNCS). Ash content, fat content, crude fibre, protein and amylose contents were reduced following modifications. The starch granules were round and polygonal in shape. The sizes ranged from 15 to 40 μm. Modifications did not alter the granule morphology. The X-ray pattern of native starch was A type, with similar pattern in modified derivatives. Acetylation improved swelling capacity while oxidation and acid-thinning reduced it. Both oxidation and acid-thinning markedly improved solubility, whereas acetylation reduced it. Swelling power of nNCS (native new cocoyam starch), oNCS, and atNCS increased progressively as the pH increased from 2 to 12, except that atNCS had 5 g/g swelling capacity at pH 2 and 4. Swelling power of aNCS was reduced as pH increased until it reached a minimum value of 24 g/g at pH 6 after which further increase in pH resulted in progressive increase in swelling power. Oxidation markedly improved the solubility of native starch at all pH values while acetylation reduced it. Hydrophilic tendency of the starch improved after oxidation and acetylation, whereas acid-thinning reduced it. Oil absorption capacity increased after oxidation and acetylation but was reduced following acid-thinning. Oxidation and acetylation reduced the gelation capacity of native starch while atNCS had better gelating property than nNCS. The pasting temperature of nNCS (76 °C) was reduced after oxidation and acetylation but increased following acid-thinning. Set-back tendency of the native starch was reduced after oxidation and acetylation but increased following acid-thinning. Studies conducted on paste clarity revealed that percentage transmittance (650 nm) increased after chemical modifications. Differential scanning calorimetry showed that oxidation and acetylation reduced peak temperature of gelatinisation ( T p) of native starch, opposite to the increase in T p after acid-thinning. Enthalpy of gelatinisation (Δ H) was reduced after oxidation and acetylation but increased following acid-thinning. Retrogradation tendency was reduced after oxidation and acetylation but increased following acid-thinning.