MODELING DEFORMATION AND FLOW DURING VAPOR-INDUCED PUFFING

An equation describing bubble expansion in pseudoplastic fluids was mo dified to provide a differential equation describing vapor-induced por e expansion in foams forming in 'molten' starch. Correlations for (1) the rheological properties of molten starches as functions of shear ra te, water content, temperature and prior specific mechanical energy in put; (2) equilibrium water partial pvessures for such melts; (3) net l atent heats; and (4) the diffusivity of water were used in conjunction with the pore expansion equation, mass and enthalpy balances, and equ ations describing diffusive transfer of water in shells surrounding po res, to model vapor-induced puffing of starch-based particles. Propert ies such as initial pore radius, popping temperature, surface tension and initial moisture contents and hypothetical correlations for the fl ow yield stress and wall-rupture stress were used to permit the model to conform to known puffing characteristics of popcorn. The differenti al equations involved were solved by finite-difference procedures. Par ameters in property correlations and unknown property values were adju sted to provide computed expansion times, expansion ratios, residual m oisture contents and fractions of open pores that agreed with observed values for popcorn at different initial moisture contents.