Effect of acid hydrolysis on rheological and thermal characteristics of lentil starch slurry

A comparative rheological and thermal study was carried out between acid hydrolyzed and unhydrolyzed (control) lentil starch dispersions (25-33.3 g starch per 100 g water) as function of temperature. After acid hydrolysis, the peak gelatinization temperature (T_p) shifted to higher temperature than the corresponding starch without hydrolysis whereas the gelatinization enthalpy remained unaffected by hydrolysis. The starch gelatinization kinetics was evaluated by a non-isothermal technique as function of elastic modulus (G′) and G′ vs. time (t) data up to the gelatinization peak value was considered for rate estimation. A 2nd-order reaction kinetics described well the starch gelatinization process and the process activation energy was ranged between 241 and 434 kJ/mol. Acid hydrolysis strongly affected the rheological properties by lowering gel strength compared to unhydrolyzed starch. The creep analysis further revealed that starch gel was significantly affected by hydrolysis and exhibited less resistant to the stress. A 4-parameters Burgers model well-described creep curves and supported oscillatory rheological data.     

Elastic properties of extruded starchy melts containing wheat bran using on-line rheology and dynamic mechanical thermal analysis

Addition of wheat bran reduces the expansion volumes of extruded cereals. This may be explained by the influence wheat bran has on the elastic properties of the melt. The elastic properties of extruded refined wheat flour supplemented with wheat bran were investigated using a complementary on-line and an off-line rheological technique. The Bagley pressure (end pressure), reflecting extensional properties of the melt, was measured on-line using the orifice die method (dies with different lengths). The storage modulus of the extruded samples was measured using dynamic mechanical thermal analysis (DMTA). Increasing the bran concentration to an intermediate level significantly increased the Bagley pressure likely due to a reduced mobility of starch molecules. Further increasing the bran concentration significantly decreased the Bagley pressure. This might be caused by physical rupture of the melt at the die exit due to the low adhesion properties between starch and bran particles. This change in Bagley pressure indicates that the elastic properties of the composite melt may be reduced by bran particles. The storage modulus measured by DMTA was decreased when increasing the bran concentration, further supporting a potential decrease of the elastic properties of the melt with bran.