PHYSICOCHEMICAL CHARACTERIZATION OF NATIVE, MODIFIED AND NANO STARCHES OF SELECTED TUBERS AND SEEDS

Abstract

The native starches of white and red cocoyam (Colocasia esculenta), white yam (Dioscorea rotundata) and yellow yam (Dioscorea cayenensis), and seeds of pigeon pea (Cajanus cajan), lima bean (Phaseolus lunatus) and jack bean (Canavalia ensiformis) were isolated and modified to obtain their hydroxypropyl and ozone-oxidized derivatives. The nanocrystals of the starches were prepared by mild acid hydrolysis at 40oC with continuous stirring for five (5) days. The native starches were evaluated for proximate and mineral compositions and selected physicochemical properties. Molar substitution (MS) and degree of substitution (DS) of the hydroxypropylated starches were determined. The amount of reacted ozone, carbonyl and carboxyl contents of the ozone-oxidized starches were determined at 5, 10 and 15 min ozone generation time (OGTs). Comparative studies of the native and modified starches were carried out in terms of their functional properties, rheological properties, thermal properties, X-ray diffraction patterns, molar mass distribution and Fourier Transform Infrared (FTIR) spectroscopy. In addition, the starch nanocrystals were characterised for X-ray patterns, solubility and their morphologies, obtained by Transmission Electron Microscopy (TEM) were compared with those of the native starches obtained by Scanning Electron Microscopy (SEM). The yields for the native starches were 31.70% (white cocoyam starch, WCS), 31.68% (red cocoyam starch, RCS), 42.66% (white yam starch, WYS), 41.72% (yellow yam starch, YYS), 20.57% (pigeon pea starch, PPS), 20.36% (lima bean starch, LBS) and 20.26% (jack bean starch, JBS) while nanocrystals were 6.67% (WCS), 5.68% (RCS), 6.22% (WYS), 6.67% (YYS), 10.54% (PPS), 10.81% (LBS) and 13.51% (JBS) after five (5) days of acid hydrolysis. The results showed that JBS was richest in protein content (7.02±0.02%), LBS richest in crude fat (1.16±0.01%) and ash content (1.26±0.01%), PPS richest in crude fibre (1.77±0.01%) and YYS had peak carbohydrate by difference (84.28±0.02%). While potassium was the most abundant mineral in all the native starches, the heavy metals, such as lead (Pb), nickel (Ni), cadmium (Cd) and mercury (Hg), were not detected. The native starches had peak values of LGCs, WAC and OAC as 8.00%, 96.23±0.01% and 4.77±0.01% respectively. The amounts of reacted ozone, carbonyl and carboxyl contents of the oxidized starches increased as OGT increased. MS of the hydroxypropylated starches ranged from 0.23 to 0.67 and DS from 0.02 to 0.04. Hydroxypropylation improved the swelling power and solubility of the native starches. All the oxidized starches exhibited lower tendency to retrograde than the native. Ozonation and hydroxypropylation enhanced the cohesiveness and resilience of the native starch gels. The data obtained from flow measurements showed that the native and modified starches exhibited non–Newtonian behaviour, which was evident by flow behaviour index, n<1. The retrogradation profiles of the starch samples were lower in value than their gelatinization profiles. ATR-FTIR spectroscopy revealed peaks of hydroxyl group, CH2 asymmetric stretching, C–H stretch of alkyl groups and C–H deformation, which became broader upon modification. All the native starches exhibited CB-type X-ray patterns, which remained unchanged upon modification whereas V-type patterns were observed for the nanocrystals. The morphological studies of the starches revealed that the granular sizes of the native starches were in the range 11.54–26.23 μm, having mixture of oval, ellipsoidal, irregular polygon, bean-like and spherical shapes. The TEM micrographs of the starch nanocrystals showed that they appeared as platelets. None of the nanocrystals was soluble in the solvents used, but LBS nanocrystals were sparingly soluble in acetic acid, ethanol and deionized (DI) water. Evidences of the nanocrystals of these starches were a step towards meeting the quest for nanotechnology in all fields. Therefore, the native starches used in this study showed extensive uses in food and non-food applications upon modification through ozone-oxidation and hydroxypropylation processes. Their nanocrystals could be potential fillers in nanocomposites.