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Macromolecular Features of Amaranth Starch

¹Instituto Tecnológico de Acapulco, AP 600, 39300 Acapulco, Guerrero, México. ²Corresponding author. E-mail: ita@acabtu.com.mx ³Institut National de la Recherche Agronomique, rue de la Géraudière, BP 71627, 44316 Nantes Cedex 03, France. ⁴Depto. de Biotecnología y Bioquímica, Centro de Investigación y de Estudios Avanzados del IPN, Apartado Postal 629, 36500 Irapuato, Gto., México.

High-performance size-exclusion chromatography (HPSEC), static light scattering (SLS) and dynamic light scattering (DLS) techniques were used for the structural characterization of amaranth starch, solubilized in water by microwave heating in a high-pressure vessel. Apparent average molar mass (M _w) gyration radius (R _G), and hydrodynamic radius (R _H) values were obtained from Berry and Zimm treatment of light-scattering data. When heating time increased from 35 to 90 sec, the M _w, R _G, and R _H decreased, demonstrating a possible polymer degradation due to temperature. Apparent M _r values from HPSEC at 35 sec (27 ± 2 × 10⁷ g/mol) and 50 sec (20 ± 2 × 10⁷ g/mol) were lower than those determined by SLS (35 sec = 69 × 10⁷ g/mol, 50 sec = 56 × 10⁷ g/mol). However, at 70 and 90 sec, the inverse pattern was obtained. The fractal dimensions (d′_f) from HPSEC study for samples dissolved for 35 (3.26), 50 (3.24), and 70 sec (3.14) are characteristic of a particle that has the internal structure of hard sphere, and for samples dissolved for 90 sec (2.19), are characteristic of a fully swollen, randomly branched macromolecule. From SLS, d′_f decreased with increasing treatment time (d′_f = 2.44, 2.18, 1.50, and 1.03 for 35, 50, 70, and 90 sec, respectively). The particle-scattering factors and Kratky plots, well-suited for studying the internal structure of a macromolecule, showed a sample degradation when treatment time increased. Results from DLS showed bimodal distributions with differences in the peak locations when treatment time increased. The ratio of R _G to R _H (ρ) for samples analyzed were between 0.88 and 1.3; these values are characteristic of a sphere or globular structure.