CAPILLARY RHEOMETRY OF CORN ENDOSPERM - GLASS-TRANSITION, FLOW PROPERTIES, AND MELTING OF STARCH

A capillary rheometer was manufactured to study the properties of corn endosperm. Samples were tested at or near the pressures and temperatu res encountered in high-temperature, short-time extrusion. The rheomet er was designed to prevent moisture loss during testing. At a set pres sure, raising the temperature caused corn endosperm particles to softe n and change shape, resulting in a pressure drop as the voids in the s ample were reduced. The temperature at which the pressure drop occurre d was considered the glass transition temperature. Continued heating c aused the pressure to rise and drop a second time as the sample soften ed and flowed through the capillary. Thermal analysis by differential scanning calorimetry showed that complete melting of starch crystals w as not necessary to permit capillary flow. Pressure and temperature co nditions sufficient to initiate flow were measured for opaque and vitr eous corn flours and expressed as a boundary curve defining the flow r egion. The position of the curve shifted as a function of sample moist ure content. The vitreous corn sample had a rough (unstable) flow that could be eliminated by addition of a small amount (2% w/w) of vegetab le oil. When isolated corn starch was studied in the capillary rheomet er, results showed that, under certain conditions, starch crystal melt ing was affected by pressure and time. A model was developed to accoun t for the effects of pressure, temperature, time, and sample moisture on starch crystal melting.