Adsorption and Exchange of ß-Lactoglobulin onto Spread Monoglyceride Monolayers at the Air-Water Interface

Spread monolayer isotherms of β-lactoglobulin, monopalmitin, monolaurin, and their mixtures were measured at the air-water interface. The isotherms exhibited hysteresis behavior. A phase transition from a liquid expanded (LE) to liquid condensed (LC) was observed for monolayers containing monopalmitin. At highest surface pressures, isotherms of mixed β-lactoglobulin-monoglycerides coincided with those of pure monoglycerides, which indicated that the protein was squeezed out from the interface. Monopalmitin monolayer exhibited higher values of surface pressure as well as dilatational elasticity as compared to monolaurin and β-lactoglobulin. The dynamics of surface pressure (π) and of surface concentration (Γ) of ¹⁴C radiolabeled β-lactoglobulin adsorbed onto spread monolayers of monoglyceride (monopalmitin and monolaurin) at the air-water interface was measured. The adsorption of β-lactoglobulin onto loosely packed spread monoglyceride monolayers was enhanced at short times with a corresponding increase in the surface pressure, indicating synergism due to possible dissolution of the protein molecules into loosely or moderately packed monoglyceride layers. The rate of adsorption of [¹⁴C]-β-lactoglobulin was enhanced onto a spread monolayer of monopalmitin of 19 and 27 Ų/molecule at short times. However, the amounts of protein adsorbed after 10 h were lower with the values being 0.8 and 1.2 mg/m², respectively. Spreading of a monopalmitin monolayer of close-packed area after 2.5 h onto the air-water interface with adsorbed β-lactoglobulin led to complete displacement of the protein from the interface, possibly because of the surface pressure and steric exclusion effects.Onthe other hand, the rate of adsorption of [¹⁴C]-β-lactoglobulin was lowered by the presence of a monolaurin layer at the interface, but the steady-state surface concentration of the protein was not changed. Spreading of monolaurin monolayer of close-packed area after 2.5 h onto the air-water interface with adsorbed β-lactoglobulin resulted in an initial displacement of the protein from the interface. However, this resulted in an initial jump in the surface pressure immediately followed by its relaxation to much smaller values possibly due to the rearrangement of the monolayer. This resulted in slow readsorption of β-lactoglobulin at longer times.