MODELING DEFORMATION AND FLOW DURING VAPOR-INDUCED PUFFING

An equation describing bubble expansion in pseudoplastic fluids was modified to provide a differential equation describing vapor-induced pore expansion in foams forming in 'molten' starch. Correlations for (1) the rheological properties of molten starches as functions of shear rate, water content, temperature and prior specific mechanical energy input; (2) equilibrium water partial pvessures for such melts; (3) net latent heats; and (4) the diffusivity of water were used in conjunction with the pore expansion equation, mass and enthalpy balances, and equations describing diffusive transfer of water in shells surrounding pores, to model vapor-induced puffing of starch-based particles. Properties such as initial pore radius, popping temperature, surface tension and initial moisture contents and hypothetical correlations for the flow yield stress and wall-rupture stress were used to permit the model to conform to known puffing characteristics of popcorn. The differential equations involved were solved by finite-difference procedures. Parameters in property correlations and unknown property values were adjusted to provide computed expansion times, expansion ratios, residual moisture contents and fractions of open pores that agreed with observed values for popcorn at different initial moisture contents.