The surface activity of beta -casein, caseinate, and a whey protein isolate
in aqueous solutions has been investigated over a range of protein concent
rations (1 (.)10(-5) to 5% w/w) at pH 5 and 7. The surface pressure data we
re determined by the Wilhelmy plate method. Surface pressure data at low pr
otein concentration indicate a low surface activity that rises to a plateau
as the monolayer is saturated at higher protein concentrations. The protei
n concentration and the surface pressure at the plateau depend on the pH an
d the type of protein in the aqueous phase. Protein-monoglyceride interacti
ons were investigated by spreading an insoluble monoglyceride (monopalmitin
, monoolein, or monolaurin) on a film of protein previously adsorbed on the
interface at equilibrium. The existence of protein-monoglyceride interacti
ons depends on the interfacial composition and on the protein/monoglyceride
ratio. The surface activity of mixed protein-monopalmitin and protein-mono
olein films is determined by the lipid as the surface pressure of the mixed
film is the same as the monoglyceride equilibrium spreading pressure, and
the monolayer is not saturated by the protein. However, the protein determi
nes the surface activity of mixed protein-monopalmitin and protein-monoolei
n films as the protein saturates the monolayer. For protein and monolaurin
mixed films, protein determines the surface activity over the range of prot
ein-monolaurin compositions due to monolaurin dissolution in the bulk aqueo
us phase.