High-foaming modified walnut protein enhances milk coffee quality and volatile aroma profile

This study investigated the effects of five modification methods, including ultra-high pressure, ultrasonic, alkaline protease, ultra-high pressure-alkaline protease, and ultrasonic-alkaline protease treatment, on walnut protein (WP) and their application in milk coffee. The results demonstrated that modification effectively altered key physicochemical properties of WP, including reduced molecular weight and surface hydrophobicity. The treatments induced partial reversible cross-linking, increased pore formation and β-turn content, and significantly improved protein solubility. Notably, enzyme-assisted modification increased solubility by more than threefold. Foam stability of WP aqueous solutions over 30 min was improved due to reduced and more uniform bubble size, accompanied by enhanced rheological properties. In particular, WP modified by ultrasound combined with alkaline protease significantly enhanced the foaming capacity by 282.67%. Within 30 min, this treatment also exhibited the highest foam stability, the largest foam volume, and the smallest foam diameter. After mixing with coffee, the addition of modified WP improved product appearance and reduced zeta potential, while also enhancing the sweetness perception. In addition, electronic nose response intensity and overall volatile compound levels were decreased. However, modified WP increased the release of characteristic coffee aroma compounds, including methyl acetate.