pH-Dependent Modulation of Structural and Functional Properties of Safflower (Carthamus tinctorius L.) Protein Induced by Ultrasonic Treatment

Abstract This study aimed to evaluate the effects of ultrasonic and pH shift treatments (pH 2 and 12), applied both individually and in combination, on the structural and functional properties of safflower protein. Ultrasonic treatment at pH 7 significantly reduced particle size (from 457.60 to 144.14 nm) and surface hydrophobicity (from 38.93 to 33.93 µg), while enhancing solubility (from 15.32 to 26.04%) and foaming capacity (from 22.50 to 26.67%). Acidic pH shift (pH 2) increased particle size by up to 78%, accompanied by marked improvements in water holding capacity (up to 114%) and foaming capacity (up to 59%). Conversely, alkaline pH shift (pH 12) led to a substantial reduction in particle size (up to 74%) and a pronounced increase in surface hydrophobicity (up to 50%), resulting in the highest emulsion activity (14.62 m²/g) and emulsion stability (5.50 min) indexes. The combined application of pH shift with ultrasonic treatments reduced particle size by up to 40% while increasing the absolute ζ-potential and surface hydrophobicity by 24% and 20%, respectively. These changes contributed to increases of up to 45% in solubility, 28% in water holding capacity, and 30% in foaming stability. However, reductions in emulsifying properties were observed, with the emulsion activity and emulsion stability indexes decreasing by up to 3.64 m²/g and 0.88 min, respectively. Overall, the findings demonstrate that safflower protein functionality is highly pH-dependent and that the strategic combination of ultrasonic and pH shift treatments represents an effective approach to modulate its structural and functional properties for food applications.