Polyphenol removal with ultrasound‐assisted ethanol extraction from defatted walnut powder: optimization of conditions and effect on functional properties of protein isolates

Walnut kernels are high in polyphenols (PPs), which cause low protein solubility, limiting the use of walnut protein in the food industry. To obtain the best technical parameters of the dephenolization treatment, the defatted walnut powder was dephenolized using ultrasound-assisted ethanol extraction (UAE), and the response surface optimization was performed on the basis of single factor. On this basis, the effects of dephenolization on the solubility, emulsifying properties and foaming properties of walnut protein isolates (WPIs) were compared to those of defatted walnut powder without dephenolization.The results showed that PP extraction in the UAE could significantly increase PP yield. The optimal process parameters were as follows: 51% (v/v) ethanol concentration, 140 W ultrasound power, 10 min extraction time, 30 °C ultrasound temperature, and a material-liquid ratio of 1:30 (w/v). The results revealed that the UAE dephenolization treatment significantly improved the functionality of WPI and that the functionality of the dephenolized WPI by UAE was superior to that of the protein without dephenolization, and that the functionality of both walnut proteins was the worst at pH 5, with solubility of 5.31% and 4.86%, emulsifying activity index (EAI) of 24.95 and 19.91 m2 /g, and foaming capacity (FC) of 3.66% and 2.94%, respectively; and the best at pH 11, with solubility of 82.35% and 73.55%, EAI of 46.35 and 37.28 m2 /g, and FC of 35.85% and 18.87%, respectively.The study found that dephenolization by UAE can significantly improve the functionality of WPI, and this method should be promoted and used in walnut and walnut protein processing industries. © 2023 Society of Chemical Industry.