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 t he surroundings from an expanding cell. A combination of a power law m odel with the Williams-Landel-Ferry (WLF) equation is used to describe the rheological behaviour of the extruded fluid. Numerical calculatio ns predict that a bubble cell first grows by rapid vaporization of the superheated moisture and subsequently shrinks. This post-extrusion sh rinkage arises from the cooling of vapour, which produces a negative p ressure difference. Qualitatively the predicted effect of initial temp erature and moisture from the model is in accord with observations tak en from the literature. Variations of the melt temperature and its gla ss transition temperature with time reveal that the structure is fixab le at a temperature of about 30 K above the glass transition temperatu re. The effect of other model parameters, including surface tension, i nitial bubble radius, rate of heat loss and constants in the power law viscosity model, are investigated. The limitations of this approach a re discussed.