Modifying the structure, physicochemical properties, and foaming ability of amaranth protein by dual pH-shifting and ultrasound treatments

Native amaranth protein (APN) is a nutritional ingredient with potential applications for the structuration and functionalization of food products. However, its solubility is poor, which limits its use in the food industry. The effect of ultrasonication (US) at 50% amplitude for 10 min (APUS), pH-shifting at pH 2 and 12 (APpH2 and APpH12), and the combination of both (APpH2US and APpH12US) on the structural, physicochemical, and functional properties of APN were investigated. Alkaline pH-shifting combined with US decreased the hydrodynamic diameter of APN from 1167.0 ± 15.6 nm to 438.3 ± 8.4 nm (APpH12US), whereas acidic pH-shifting increased the hydrodynamic diameter up to 2490.0 ± 5.3 nm (APpH2). Fractions of α-helix, β-sheet, β-turn, and random coil structures of APN were affected by pH12-shifting and US, but pH2-shifting did not. Exposed sulfhydryl groups of the APN diminished, while disulfide bonds increased as a result of pH-shifting alone or combined with US. Zeta potential values revealed that the isoelectric point of APN (4.0) was increased for APpH12 (4.2) and APpH12US (4.5) and decreased for APUS (3.5). The highest solubility was achieved by APpH12US. Foaming capacity and stability were significantly increased in all treatments, except APpH2. •Acid and alkaline pH-shifting improved amaranth protein (AP) solubility.•Combined pH-shifting and ultrasound treatment improved AP foaming properties.•pH-shifting and ultrasonication increased the in vitro AP digestibility.