Abstract

The effects of hydration times (HT) (1, 3.5 and 6 h) and cooking times (CT) (20, 40, and 60 min) on the thermal characteristics of cooked pearl millet flour (CPMF) was investigated. The differential scanning calorimetry thermograms depicted two separate endothermic transitions identified as starch gelatinisation (29 to 30°C) and disruption of the starch-lipid complexes (90 to 120°C). The onset (T_o), peak (T_p) and completiontemperature (T_c) of uncooked pearl millet flour (UPMF) were 50.6, 72.2 and 80.2°C respectively and that for the hydrated and CPMF ranged from 28.3 to 49.1°C, 47.9 to 79.7°C and 51.3 to 86.9°C respectively. The gelatinisation temperature range (DT_r) for the UPMF was 29.6°C and that of the treated flour ranged from 3.2 to 37.7°C. HT and CT significantly (P<0.05) affected the gelatinisation properties of the millet flour. HT gradually decreased the degree of gelatinisation, whereas, CT drastically increased the degree of gelatinisation. Hydrating millet flour for 1 h and cooking for 40 min was sufficient to achieve 100% gelatinised flour. Starch-lipid complex formed after gelatinisation at hydration time (3.5 h) and cooking for 40 min. HT (3.5 min) and CT (40 min) significantly decreased the T_o, T_p, T_c temperatures and uniformity of gelatinisation (PHI). The melting properties of the formed complex T_o, T_p and T_c of uncooked millet flour were 100.58, 118.58 and 135.21°C respectively while that for the hydrated and cooked millet flour ranged from 99.39 to 106.29, 114.79 to 119.02 and 133.07 to 137.01°C respectively. Melting range (DT_r) and melting enthalpy (DH_melting), for the UPMF is 34.63°C and 1026.94 J/g respectively and 28.16 to 35.49°C, and 751.61 to 1072.44 J/g respectively for the CPMF. Increasing both the HT and CT reduced the amount of energy required to break the starch-lipid complex formed during the cooking of millet flour. The UPMF showed a typical A-type X-ray diffraction pattern, which was altered by the hydrothermal treatments (1 h hydration and 20 min cooking) resulting in a transformation into the V-hydrate form evidence of starch-lipid complex formation. The formation of this complex possibly explains why millet flour remains intact during cooking into fura.

Key words: Hydrothermal, fura, pearl millet flour, starch-lipid complexes, differential scanning calorimeter X-ray diffraction, complexing index.