DisplayTitle

Capillary Rheometry of Corn Endosperm: Glass Transition, Flow Properties, and Melting of Starch¹

¹Contribution 96-326-J, Kansas Agricultural Experiment Station, Manhattan. ²Graduate research assistant, professor, and professor, respectively, Department of Grain Science and Industry, Kansas State University, Manhattan. ³Present address: Earth Grains, Inc., 4649 LeBourget, St. Louis, MO 63134. ⁴Present address: R&R Research Services, Inc., 8831 Quail Lane, Manhattan, KS 66502. ⁵Corresponding author. E-mail: r_and_r@kansas.net. ⁶Present address: AACC, 3340 Pilot Knob Road, St. Paul, MN 55121.

A capillary rheometer was manufactured to study the properties of corn endosperm. Samples were tested at or near the pressures and temperatures encountered in high-temperature, short-time extrusion. The rheometer was designed to prevent moisture loss during testing. At a set pressure, raising the temperature caused corn endosperm particles to soften and change shape, resulting in a pressure drop as the voids in the sample were reduced. The temperature at which the pressure drop occurred was considered the glass transition temperature. Continued heating caused the pressure to rise and drop a second time as the sample softened and flowed through the capillary. Thermal analysis by differential scanning calorimetry showed that complete melting of starch crystals was not necessary to permit capillary flow. Pressure and temperature conditions sufficient to initiate flow were measured for opaque and vitreous corn flours and expressed as a boundary curve defining the flow region. The position of the curve shifted as a function of sample moisture content. The vitreous corn sample had a rough (unstable) flow that could be eliminated by addition of a small amount (2% w/w) of vegetable oil. When isolated corn starch was studied in the capillary rheometer, results showed that, under certain conditions, starch crystal melting was affected by pressure and time. A model was developed to account for the effects of pressure, temperature, time, and sample moisture on starch crystal melting.