| Summary: | Solids puff when unbalanced forces exerted by vapor and gas in pores exceed the yield strength of solid surrounding the pores. Pressures increase as temperature rise; and matrix yield strength and flow resistance decrease. Expansion triggered by losses of matrix strength due to melting and glass-rubber transitions occurs. Water-loss and evaporative cooling induce reversal of these processes and stabilize expanded products so that they do not collapse after heating is stopped. Water sorption behaviors for corn starch and popcorn grits between 100 and 180$\sp\circ$C were measured at various moisture contents. Pressures inside popcorn at the instant of popping are roughly 110 to 120 psia. Non-condensible gases were produced by heating popcorn grits to 150 and 180$\sp\circ$C. At 180$\sp\circ$C, corn starch turns into a brown, sticky liquid. Heats of sorption for corn starch were estimated, and changed significantly with moisture content and temperature. Popping temperatures of popcorn in 200$\sp\circ$C air inversely correlated with initial moisture content. Temperatures dropped at popping in the range of 5.4-6.2$\sp\circ$C. DSC was used to detect and measure phase transition between 40 and 250$\sp\circ$C for corn starch and popcorn grits. Corn starch and popcorn grits melt between 160 and 240$\sp\circ$C, and the melting temperature decrease and amount of heat absorbed during melting increase as moisture content increase. Coating popcorn with zein slightly reduced rates of moisture loss during heating. Expansion bulk volumes increased 15% for coated popcorn and the unpopped ratio decreased slightly, but only when the popcorn popper was not preheated. Expansion volume didn't increase significantly when the popper was preheated. Zein coating did not significantly improve expansion bulk volume and unpopped ratios for samples with damaged pericarp. Expansion ratios, extents of evaporation, puffing temperatures, pore characteristics, and heating-induced enthalpy changes were measured versus temperature and moisture content for popcorn, and used to identify conditions that induce puffing, and levels of moisture loss needed to stabilize puffed structures. Puffing probably initially occurs through expansion of vapor trapped in individual grain of starch. Pore walls rupture and pores merge when excessive vaporization occurs. Pore openness of popped popcorn increases linearly with initial moisture contents.
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