Viscoelastic properties of amaranth starch gels and pastes. Creep compliance modeling with Maxwell model

In this work, the morphological, thermal, and crystalline properties of starches isolated from two amaranth cultivars, Amaranthus cruentus (ACr) and Amaranthus caudatus (ACa); the viscoelastic properties of their pastes and gels, through oscillatory and creep-recovery tests and the adjustment to the Maxwell mechanical model, were studied. Both starches presented type A crystalline patterns with high crystallinity degree (CD), 41.84% for ACr and 40.70% for ACa, associated to their low apparent amylose content (AAM) (1.06 g/100 g for ACr and 7.39 g/100 g for ACa). The ACr starch showed the highest values for temperature of gelatinization (T-g) (74.50 degrees C) and Delta H (16.15W/g). Amaranth gelatinized starches exhibited rheological behavior that corresponds to concentrated macromolecular solutions for ACr while ACa starches showed a gel-type behavior. The ACa gels behaved as a viscoelastic solid with smaller values of instantaneous compliance (0.006 Pa (1) < J(0) < 0.0002 Pa (1)), retarded compliances (0.0005 Pa-1 < J(1) < 0.000006 Pa-1; 0.0016 Pa-1 < J(2) < 0.00008 Pa-1) and higher values of newtonian viscosities (33.4 x 10(5) Pa . s <= eta(0) <= 357.1 x 10(5)Pa.s) than ACr, which facilitated their later structural recovery (75.8% <= final recovery <= 85.1%). The gels exhibited a small contribution of viscous flow's compliance (t/eta(0)) brought about by the dashpot of the Maxwell's simple element, while in the ACr pastes, this element acquired importance. The Maxwell model of six-parameters adjusted satisfactorily (R-2 >= 0.986; RSS <= 3 x 10(-4)) the results of creep curves.