Viscoelastic properties of extruded cereal melts

Hydrophilic biopolymer systems reach a state of maximum plasticization at a concentration of water designated as W-g'. The thermomechanical behaviour of various extruded cereals were investigated at concentrations above and below W-g' to determine the significance of W-g' on the viscoelastic properties in the rubbery region above T-g. Cereal melts were produced via extrusion cooking using a twin-screw extruder at in-barrel moisture contents ranging from 25 to 42% (wb). Dynamic mechanical thermal analysis (DMTA) was used to determine the viscoelastic properties for melts obtained from degermed corn meal, oat flour, pregelatinized corn starch and pregelatinized waxy maize. All of the extruded melts displayed reductions in the storage modulus of about two orders of magnitude from the glassy to rubbery regimes. At temperatures greater than 320K, another transition is apparent for extruded oat flour. These losses may be attributed to a reduction in physical crosslinks and/or entanglements. With respect to the other melts investigated, differences in the oat flour melts at temperatures above T-g may be due to the increased presence of protein and lipid. Given the apparent partly crystalline nature of these melts at moisture contents <W-g', the most appropriate representation of the viscoelastic properties may be isotimic plots of the viscoelastic functions. These contour plots provide a more comprehensive guide to material behaviour than the 'traditional' state diagrams found throughout the literature. An important point is that prediction of viscoelastic properties above T-g for polymers where the internal structure changes with temperature or moisture content, cannot be predicted using the same methods of reduced variables as for amorphous polymers. Thus rather than using these methods, experimental data, such as given here, may prove to be necessary to design or understand processes involving cereal melts based on their viscoelastic properties. (C) 1997 Academic Press Limited.