Whey protein isolate-monoglyceride mixed monolayers at the air-water interface. Structure, morphology, and interactions

The surface pressure (pi)-area (A) isotherms and Brewster angle microscopy (BAM) of monoglyceride and whey protein isolate (WPI) mixed films spread on buffered water at pH 5 and 7 and at 20 degreesC were determined as a funct ion of the mass fraction (X) of monoglyceride (monopalmitin or monoolein) i n the mixture. From the X and pi -dependence on excess area, free energy, c ollapse pressure, BAM images, and the evolution with pi of the relative ref lectivity (I) of BAM images, it was deduced that the structural characteris tics, miscibility, and morphology of monoglyceride-WPI mixed films are very dependent on surface pressure and monolayer composition. The monolayer was more expanded as the monoglyceride concentration in the mixture was increa sed. Over the overall range of existence of the mixed film, the monolayer p resents some heterogeneities. At higher T, after the WPI collapse, characte ristic squeezing-out phenomena were observed. At the monoglyceride monolaye r collapse, the mixed film was practically dominated by the presence of mon oglyceride. However, some degree of interactions exists between monoglyceri de and WPI in the mixed film, and these interactions are more pronounced as the monolayer is compressed at the highest surface pressures.