Air-water interfacial properties and foam characteristics of yeast protein isolates modified by different treatments

In this study, four distinct yeast protein isolate (YPI) particles were fabricated through alkaline treatment, ultrasound-assisted pH-shift, thermal induction, and enzymatic hydrolysis, respectively. The interfacial adsorption kinetics, linear and nonlinear interfacial rheology, foaming property, and foam stability of the different treated YPIs at air-water interface were systematically investigated by comparing with the untreated YPI and the conventional protein foam agent whey protein isolate (WPI). It was found that alkaline-treated YPI exhibited the most negative surface charge of -19.6 mV, endowing them with excellent dispersibility and superior colloidal stability. This treatment simultaneously reduced the intrinsic hydrophobicity of YP and induced a shift from ordered to disordered secondary structures. Dilatational rheology measurements confirmed that alkaline-treated YPI formed highly viscoelastic interfacial films, characterized by a high dilatational modulus (34.5 mN/m) and a lower degree of nonlinearity, resulting in a significantly enhanced foam capacity (2.4 times higher than YPI) and foam stability (with a half-life (t₁/₂) exceeding 2 h). The ultrasound-assisted pH treated YPI yielded smaller sizes with rapid initial adsorption kinetics and favorable foaming ability, but the resulting interfacial films lacked sufficient viscoelasticity. Thermally treated YPI exhibited pronounced particle aggregation reaching sizes up to about 500 nm, which led to weakened interfacial film integrity and poor foaming performance. Enzymatically hydrolyzed YPI produced small particles, sized at around 70-80 nm. Yet, their low surface hydrophobicity and weak intermolecular interactions prevented the formation of cohesive interfacial films, resulting in the highest interface strain softening and nonlinearity degree and the weakest foam stability.