Foaming behavior of mixed bovine serum albumin–protamine systems

The foaming properties of mixed bovine serum albumin (BSA):protamine systems were investigated as a function of pH (3, 5 and 7) and volume ratio (20:1, 10:1, 1:1 and BSA alone), and correlated with physicochemical properties and surface behavior. BSAʹs foaming behavior was significantly improved in the presence of protamine (p⩽0.05), though protamine itself showed no surface nor foaming activity in the pH range studied. The bubble size distributions of BSA and BSA-protamine mixed foams studied as a function of time substantiated the conductivity results whereby the mixed system foams were more stable than the BSA foam. Zeta potential, turbidity, and viscosity were also studied as a function of pH and molar ratio in order to elucidate the mechanism by which protamine enhances the foaming properties of BSA. The formation of light scattering complexes at pH 5 and 7 was attributed to electrostatic interactions between the BSA and protamine molecules. It was postulated that protamine neutralized negative charges on the BSA leading to a less negatively charged complex. The enhanced foaming properties were not accompanied by significant differences (p>0.05) in apparent viscosity between the bulk film forming solutions, indicating that viscosity played no role in reducing drainage. Examination of the surface rheological behavior using a Langmuir trough equipped with a Wilhelmy wire probe indicated that the compression profiles amongst all systems were similar. It is proposed that BSA and protamine interacted electrostatically, such that a molecular double layer of BSA and protamine molecules that reduced drainage was formed at the gas–liquid interface. The novelty in the current study stems from protamineʹs lack of demonstrated surface activity, but obvious enhancement of BSAʹs foaming behavior.