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. BSA's 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 protamine's lack of demonstrated surface activity, but obvious enhancement of BSA's foaming behavior. (c) 2006 Elsevier Ltd. All rights reserved.