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

Native amaranth protein (AP(N)) 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 (AP(US)), pH-shifting at pH 2 and 12 (AP(pH2) and AP(pH12)), and the combination of both (AP(pH2US) and AP(pH12US)) on the structural, physicochemical, and functional properties of APN were investigated. Alkaline pH-shifting combined with US decreased the hydrodynamic diameter of AP(N) from 1167.0 +/- 15.6 nm to 438.3 +/- 8.4 nm (AP(pH12US)), whereas acidic pH-shifting increased the hydrodynamic diameter up to 2490.0 +/- 5.3 nm (AP(pH2)). Fractions of alpha-helix, beta-sheet, beta-turn, and random coil structures of AP(N) were affected by pH12-shifting and US, but pH2-shifting did not. Exposed sulfhydryl groups of the AP(N) 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 AP(N) (4.0) was increased for AP(pH12) (4.2) and AP(pH12US) (4.5) and decreased for AP(US) (3.5). The highest solubility was achieved by AP(pH12US). Foaming capacity and stability were significantly increased in all treatments, except AP(pH2).