Cereal Chem. 73 (5):638-643 |
Shear-Thickening Behavior and Shear-Induced Structure in Gently Solubilized Starches (1).
F. R. Dintzis (2,3), M. A. Berhow (2), E. B. Bagley (4), Y. V. Wu (5), and F. C. Felker (6). (1) Presented at the AACC 80th Annual Meeting, San Antonio, TX, November 1995. (2) National Center for Agricultural Utilization Research, Food Physical Chemistry Research; Peoria, IL. (3) Corresponding author. Fax: 309/681-6240. (4) Retired. National Center for Agricultural Utilization Research, Food Physical Chemistry Research; Peoria, IL. (5) National Center for Agricultural Utilization Research, Biopolymer Research; Peoria, IL. (6) USDA, Agricultural Research Service, Phytoproducts Research; Peoria, IL. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. Accepted May 8, 1996. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 1996.
The shear-thickening phenomenon was observed with eight unmodified starches that had been gently dissolved and dispersed at 3.0% concentrations in 0.2N NaOH. Waxy starches (maize, rice, and barley and potato) showed this phenomenon to a greater extent than did wheat, normal rice, or normal maize starches. Phase-contrast microscopy and circular dichroism measurements confirmed the development of shear-induced structure (incipient phase separation) implied by this shear-thickening behavior. The amylopectin component was responsible for shear-thickening properties and shear-induced structure formation and alteration. These structures were rather stable in 90% dimethyl sulfoxide (DMSO) and H(2)O. In this solvent, the intrinsic viscosity was not tightly coupled with flow behavior in the shear-thickening region. Shear-thickening behavior was influenced by solvent type. The relative abilities of solvent to enhance shear-thickening properties were: 0.2N NaOH > 90% DMSO with H(2)O >= H(2)O or 0.5N KCl at neutral pH. Shear-thickening properties were extremely sensitive to sample treatment; severe treatment of the sample destroyed the ability of the starch to form shear-thickened fluids. We conclude that dispersed native amylopectins (i.e., the initial higher molecular weight species) are able to form networks that are responsible for the shear-thickening behavior and shear-induced structures observed in granule-free starch solutions and dispersions.