Influence of the heating rate on the pasting properties of various flours

Viscoamylographic tests were carried out on six commercial flour samples, three wheat flours (WF), two semolinas (S) and one rice flour (RF), using the Brabender Micro Visco-Amylo-Graph (MVA). The slurries were subjected to a definite temperature profile (30 °C-95 °C, 95 °C x 30 min, 95 °C-50 °C, 50 °C x 30 min), stirring at 250 min(-1) and using a 300 cm.g(f) cartridge and recording the viscosity (in Brabender Units, BU) as a function of temperature and time. The aim of this work was to evaluate the influence of different heating rates (1.5, 3.0, 5.0, 7.5, 10.0 °C/min) on the pasting properties of the various flours. The peak viscosity of WFs and Ss increased when high heating rates were applied, while the RF showed similar pasting properties independently of the heating rate. These behaviours were mainly ascribed to the different molecular organisation of the starch granules, responsible of different swelling and gelatinising extents, and also to a different kinetic of alpha-amylase inactivation according to the heating rate applied. The key role of the alpha-amylase activity in controlling the pasting viscosity of the different samples was demonstrated by the viscoamylographic test performed in the presence of silver nitrate as enzyme inhibitor.