Obtention of starchy molten phase under high thermomechanical stress

Low hydrated pea starch (moisture content equal to 30% wb) has been submitt ed to various thermomechanical treatments, similar to those performed by ex trusion, on a preshearing rheometer, the Rheoplast(R). Temperature, shear r ate and duration of the treatment are fixed in the following ranges : 95-15 0 degrees C, 15-70 s(-1), 15-30 s. Starch is heated both by conduction from the barrel, and shearing in a Couette geometry. Starch transformation is e valuated by microscopy, differential scanning calorimetry, X-ray diffractom etry and intrinsic viscosity measurement. Results show that native structur e is partially or completly damaged at various structural levels studied. S tarch transformation is governed by the levels of temperature and mechanica l energy applied. Melting of starch granules begins with the inner part : c rystalline structure is affected before granular structure. This transforma tion is due to local increases of temperature, sufficient to reach melting temperature. According to calorimetric melting temperature, two phenomena m ay govern the obtention of a molten phase : one based upon mechanical effec t, i.e. interparticular friction or viscous dissipation and another one upo n thermal conduction.