Cereal Chem 61:538 - 543. | VIEW
Extrusion Cooking and Drum Drying of Wheat Starch. I. Physical and Macromolecular Modifications.
P. Colonna, J. L. Doublier, J. P. Melcion, F. de Monredon, and C. Mercier. Copyright 1984 by the American Association of Cereal Chemists, Inc.
Native wheat starch was modified by drum drying under two industrial conditions and by twin-screw extrusion cooking under five barrel temperatures (90-180 C) and five conditions of moisture (19-44%) to produce large gradations in the severity of the treatment. By both technologies, starch granules were transformed into a continuous phase of melted starch, including variable amounts of air bubbles. In comparison with native starch, specific gravity (1.47-1.48 g/cm3)was not modified; but the surface area of the treated starches was more reduced by extrusion (230-500 cm2/g) than by drum drying (620-650 cm2/g). The primary chemical structure, linked by alpha-1,4 and alpha-1,6 (3.6-4.0%) bonds, was maintained with total recovery of D-glucose, as enzymic methods show. Extrusion cooking led to a macromolecular degradation of amylose and amylopectin, by random chain splitting, as indicated by intrinsic viscosity, gel- permeation chromatography on Sepharose CL-2B, and average-molecular-weight determinations. In contrast, drumb drying, as well as extrusion cooking at 44% moisture and 90 C, degraded starch components very slightly, rendering them less soluble (4.7-24.6%) than other extrudates (56.3-81.6%). For all treated starches, the water-soluble fractions were composed of partly depolymerized amylose and amylopectin, in the same ratio as in native starch, except for the drum-dried samples, which were enriched with amylose. During extrusion cooking, shear completely disperses starch components by decreasing molecular entanglement, whereas during drum drying, the amylose fraction is preferentially solublized by leaching.