A COMPUTER-SIMULATION OF THE DYNAMICS OF BUBBLE-GROWTH AND SHRINKAGE DURING EXTRUDATE EXPANSION

A model describing the dynamics of bubble growth in starchy extrudates is proposed. The model takes into account heat and moisture loss to the surroundings from an expanding cell. A combination of a power law model with the Williams-Landel-Ferry (WLF) equation is used to describe the rheological behaviour of the extruded fluid. Numerical calculations predict that a bubble cell first grows by rapid vaporization of the superheated moisture and subsequently shrinks. This post-extrusion shrinkage arises from the cooling of vapour, which produces a negative pressure difference. Qualitatively the predicted effect of initial temperature and moisture from the model is in accord with observations taken from the literature. Variations of the melt temperature and its glass transition temperature with time reveal that the structure is fixable at a temperature of about 30 K above the glass transition temperature. The effect of other model parameters, including surface tension, initial bubble radius, rate of heat loss and constants in the power law viscosity model, are investigated. The limitations of this approach are discussed.