Microstructure, physicochemical properties and retrogradation behaviour of Mucuna bean (Mucuna pruriens) starch on heat moisture treatments

Mucuna bean starch was subjected to hydrothermal modifications by heat moisture conditioning at 18% (HMM18), 21% (HMM21), 24% (HMM24) and 27% (HMM27) moisture levels. At the temperature range studied (60–90 °C), increasing level of moisture conditioning reduced the solubility and swelling capacity. Solubility and swelling were pH dependent, with maximum values obtained at pH 12 in both native and modified starches. Increasing degree of alkalinity increased both solubility and swelling capacity. Water absorption capacity of the starch increased with the severity of moisture treatments, while the oil absorption capacity decreased. Apart from HMM18, all other modifications improved the gel forming capacity and firmness of the native starch. Pasting temperature increased after hydrothermal modifications, whereas peak viscosity (Pv), hot paste viscosity (Hv), viscosity after 30 min holding at 95 °C (Hv30) and cold paste viscosity (Cv) all reduced after modification. Differential scanning calorimetry studies revealed that heat moisture treatment increased the onset temperature (To), peak temperature (Tp) and conclusion temperature (Tc). In addition, gelatinisation band of the native starch increased progressively from HMM18–HMM27. Heat moisture treatment reduced the gelatinisation enthalpy (ΔH), while the enthalpy of retrogradation (ΔHr) increased with the storage time of the gelatinised starch. However, ΔHr of the heat moisture conditioned starches were lower than the value obtained for the native starch. Microscopy studies revealed no change in shape and size between the native and modified starches.