Influence of head-moisture treatment on functional, colour and thermal properties of bambara ground-nut starch

Abstract

Heat-moisture treatment (HMT) is a physical modification that alters the physicochemical properties of starch without changing its molecular structure. The objective of the study was to investigate the influence of HMT on the functional, colour and thermal properties of bambara groundnut (BG) starch. A central composite rotatable design comprising two independent factors (temperature and time) was used for the study. The central composite rotatable design was generated using Design-Expert software version 8.0.1.0. Bambara starch extraction was done by milling BG into flour (5 Kg), suspension in 15 L, 0.3% sodium hydroxide and centrifugation followed by washing using distilled water. The starch was then HMT treated in an air oven at 80 - 120 °C for 30 - 90 min under 15 % moisture content (MC) (HMT 15), 25% MC (HMT 25) and 35% MC (HMT 35). The highest L* and WI values for HMT treated BG starch were observed at HMT 80 °C for 30 min under 15% MC; 100 °C for 60 min (25% MC); and 100 °C for 17.57 min (35% MC) while the lowest was observed in HMT 100 °C for 102.43 min (15% MC); 120 °C for 90 min (25% MC); and 120 °C for 90 min (35% MC). In HMT 15-BG starch, the gelatinisation parameters onset (To), peak (Tp) and concluding temperature (Tc) of the samples decreased as treatment time and temperature increased whereas gelatinisation enthalpy of BG starch increased with increase in HMT treatment temperature and time. In HMT 25-BG starch Tp, and gelatinisation enthalpy of the starch increased with increase in HMT treatment temperature and time. While in HMT 35-BG starch, To, Tp, Tc and gelatinisation enthalpy of the starches decreased with increase in HMT treatment temperature and time. In HMT 15-BG starch, the water absorption capacity (WAC), solubility and swelling power (SP) decreased as treatment time and temperature increased while oil absorption capacity (OAC) of the starch increased with increase in HMT treatment temperature and time. In terms of HMT 25-BG starch, the WAC and OAC increased as HMT treatment time and temperature increased while SP and solubility of the starch decreased with increase in HMT treatment temperature and time. In HMT 35-BG starch, OAC, solubility and SP decreased as treatment time and temperature increased while WAC of the starch increased with increase in HMT treatment temperature and time. The optimum HMT conditions for BG starch were found to be 80 °C for 30 min (HMT 15), 105.74 °C for 30 min (HMT 25), and 113.16 °C for 30 min (HMT 35). Desirability of the obtained optimum conditions were 0.63 (HMT 15), 0.56 (HMT 25) and 0.64 (HMT 35). Information obtained from scanning electron micrograph indicates oval and round shape for bambara starch granules, with varying sizes. The range of the granule size width was 4.2 – 4.7 mm and 10 μm for length. The modified starches showed some changes in granule morphology as they seem to disintegrate with application of HMT. Unmodified and HMT - BG starches showed characteristic FTIR bands linked with common starches. All the samples displayed complex vibrations in the region below 1000 cm-1 due to the skeletal vibrations of the glucose pyranose ring. Statistical analysis on colour, thermal and functional properties of HMT 15-BG, HMT 25-BG and HMT 35-BG starch showed that effects of temperature and treatment time had no significant (p ≥ 0.05) effect on these properties of HMT-BG starch. However, treatment time had a significant linear effect (p ≤ 0.05) on swelling power, for HMT 15-BG starch. In HMT 35-BG starch, WAC was significantly affected by quadratic effect of temperature and time while solubility was significantly affected by linear effect of time and quadratic effect of temperature.