Ultrasound treatments of tef [Eragrostis tef (Zucc.) Trotter] flour rupture starch α-(1,4) bonds and fragment amylose with modification of gelatinization properties
Antonio J. Vela, Marina Villanueva, Cheng Li, Bruce Hamaker, Felicidad Ronda
Abstract
Tef is a nutritionally-rich ancient grain gaining increasing interest in gluten-free market. Molecular and structural properties in the main biopolymers of two Spanish ecotypes of tef flour physically modified by ultrasound (US) treatments under different temperatures (20, 40, 45, 50 and 55 °C) were studied. Modifications achieved were dependent on tef ecotype, where white tef presented higher susceptibility. Size-exclusion chromatography indicated higher proportion of intermediate amylose chains (degree of polymerization 300–1600) across the whole amylose chain-length distribution and a higher amylose/amylopectin ratio, that increased up to 23% and 19% in treated samples of white and brown tef respectively. US increased the starch short-range order crystallinity and promoted the formation of random coil protein structure and the disappearance of β-sheet, as confirmed from FTIR spectroscopy. XRD and 1H NMR revealed a higher fragmentation of α-(1,4) than α-(1,6) bonds due ultrasonication. The gelatinization temperature range and ΔHgel decreased after ultrasonication up to 6 °C and 13%, an effect that was reinforced or counteracted, respectively, by the treatment temperature, indicating that annealing modulates the impact of sonication on flour gelatinization properties. Tef starch structural properties were found to be significantly modified by US treatments, where temperature was a determining variable influencing the degree of modification achieved.