PHYSICAL, CHEMICAL, PHYSICOCHEMICAL AND THERMAL CHARACTERISATION OF HYDROTHERMAL AND ACID-THINNED STARCHES FROM COCOYAM (Colocasia esculenta, Xanthosoma sagittifolium) AND BITTER YAM (Dioscorea dumetorum) CULTIVARS

Abstract

The huge industrial starch demand and utilisation in Nigeria are met mostly from imported sources. Intrinsic properties of starch from commodities such as cocoyam and bitter yam which have the potential to satisfy this demand, have not been studied to show their suitability for various end uses. The objective of this study was to evaluate physical, chemical, physicochemical and thermal properties of native and modified starches from cocoyam and bitter yam cultivars grown in Nigeria. Starches were extracted from nine cultivars of cocoyam (Colocasia esculenta; NCe001, NCe002, NCe003, NCe004, NCe005, NCe010, Xanthosoma sagittifolium; NXs001, NXs002 and NXs003), white and yellow bitter yams, obtained from National Root Crops Research Institute, Umudike using standard methods. Starches were modified by Heat-Moisture Treatments, HMT (18, 24 and 27% moisture), annealing (50oC for 24 hours) and acid thinning (6% w/v HCl at 25 oC, 192 hours). Physical [dry matter, yield, colour (L*a*b*), granule microstructure, bulk density], chemical (amylose, protein, phosphorus) and physicochemical properties [swelling power, solubility index (50-90oC), Water (WAC) and Oil (OAC) Absorption Capacities, pasting characteristics, gelation capacity, light transmittance and freeze-thaw stability] of starches were determined by standard procedures. The IR spectra and gelatinisation properties were determined by FTIR and DSC, respectively. Data were analysed using ANOVA at α0.05. Cocoyam and bitter yam starches had dry matter of 26.24-37.00% and 39.11-39.55%, and yield of 51.72-65.87% and 67.02-73.56%, respectively. Colour of native starches from NCe003, NCe001 and bitter yams (L*=64.32-88.22, a*=-1.49-1.09, b*=3.2-7.31) showed significant improvements with annealing (L*=75.89-94.22, a*=-0.06-0.79, b*=3.12-6.66) and acid-thinning (L*=72.74-85.35, a*=1.14-2.59, b*=5.95-10.18) but were negatively affected by HMT (L*=44.78-84.39, a*=1.36-5.61, b*=6.20-10.99). Granule sizes ranged from 15.51µm (NCe010) to 1.17µm (NCe004) with spherical or polygonal shapes being prominent. Modification did not significantly alter microstructures. Loose bulk densities were 0.31-0.75g/mL and 0.50-0.92g/mL, while packed densities were 0.33-0.47g/mL and 0.53-0.72g/mL in cocoyam and bitter yam, respectively. Amylose contents of cocoyam (10.11-37.08%) and bitter yam (8.69-18.21%) starches decreased after the modifications. Protein and phosphorus contents of cocoyam (0.24-0.43% and 0.021-0.987mg/g) and bitter yam (0.26-0.31% and 0.058-0.508mg/g) starches were low. Swelling power and solubility increased significantly with increased temperature in native and modified starches. Native starches from NCe004 (1.445g/g), NXs002 (1.675g/g) and white bitter yam (1.635g/g) showed high WAC which decreased with moisture levels of HMT. High OAC (1.285-1.750g/g) were observed in native cocoyam starches. Pasting properties of acid-thinned starches decreased significantly. Acid-thinned NXs001, NXs002, NXs003 and white bitter yam showed high gelation at 4%w/v concentration. Light transmittance of C. esculenta (15.45-39.50%), X. sagittifolium (13.50-16.15%) and D. dumetorum (16.46-47.56%) starches decreased with increased storage. Bitter yam starches showed low syneresis, acid-thinned white bitter yam (20.15%) showed lowest value. Functional groups assigned to the alkanones, carboxylates and hydroxyl groups were identified by the FTIR at different peaks in all the starches. Gelatinization enthalpy was lower in native (9.26-18.45J/g) than modified (16.17-26.31J/g) starches. The native and modified starches from cocoyam and bitter yam have properties that vary with cultivar and modification method, and showed high potential industrial application in food systems. KEYWORDS: Starch modification, Cocoyam and bitter yam starches, Starch characterisation