Ultrasonic-aided extraction and degossypolization of cottonseed meal protein: Optimization and characterization of functional traits and molecular structure

Cottonseed meal (CM) is a useful protein resource, but the presence of gossypol (a toxic compound) limits it utilization in industry. To reduce the gossypol content, the use of ultrasonication in extracting protein from CM was explored. Following single-factor experiments, operative limits of independent variables affecting removal ratio of total-gossypol (RRTG) and extractability of CM protein (CMP) were achieved as temperature (35–55 °C), sonication power (250–350 W), time (12–24 min), and NaOH concentration (2–6 g/L). Box-Behnken’s Design centered on a 3–level-4–factor was employed to optimize ultrasonication conditions on the yield and RRTG of CMP. The predictive models were statistically adequate (p < 0.0001) to reflect the experimental results of the responses as verified by insignificant lack-of-fit (p ≥ 0.1333) and high coefficient of determination (R2 ≥ 0.976) values. The optimized sonication conditions for two responses were 53 °C, 320 W, 21 min, and 4.6 g-NaOH/L. Under optimized conditions, the observed removal ratio of free- and total-gossypol and yield values were 93.21 %, 87.56 %, and 58.23 %, which substantially increased by 15.64 %, 12.88 %, and 29.56 %, respectively over control (alkaline extraction). Furthermore, sonicated isolate showed an increase in solubility and emulsion property relative to control, buttressed by particle sizing and zeta potential outcomes. Sodium dodecyl sulfide polyacrylamide gel electrophoresis (SDS-PAGE), Fourier Transform Infrared (FTIR) spectroscopy, atomic force microscopy (AFM), and amino acid data showed notable changes in molecular weight, functional groups, surface topography, and hydrophobic amino acid content of CMP, respectively (i.e., after sonication), suggesting partial unfolding in CMP conformation. Accordingly, CMP isolates (especially the sonicated protein) could be considered edible because of the observed low gossypol content (< 0.045 %) and thus can be utilized as natural additives in food formulations.