Structural characteristics of adsorbed protein and monoglyceride mixed monolayers at the air–water interface

Monolayer technique has been used successfully to study the properties of mixed emulsifiers spread at the air–water interface. In this contribution we are concerned with the analysis of structural characteristics of mixed monolayers formed by milk proteins (β-casein and β-lactoglobulin) and monoglycerides (monopalmitin and monoolein), and how these differ depending on the method used for the formation of the mixed film at the air–water interface (spreading, penetration or by spreading of a lipid into a previously adsorbed protein film). Measurements of the π-A isotherm were obtained in Langmuir- and Wilhelmy-type film balances coupled with Brewster angle microscopy (BAM). The π-A isotherm deduced for adsorbed β-lactoglobulin monolayer in this work is practically the same as that obtained directly by spreading. However, for β-casein the adsorbed monolayer is more condensed than the spread one. For protein–monoglyceride mixed films, the π-A isotherms for adsorbed and spread monolayers at surface pressure (π) higher than the equilibrium spreading pressure of protein (πeprotein) are practically coincident, a phenomenon which may be attributed to protein displacement by the monoglyceride from the interface. At π<πe protein and monoglyceride coexist at the interface and the adsorbed and spread π-A isotherms (monolayer structure) are different. Monopalmitin has a higher capacity than monoolein for the displacement of protein from the air–water interface. However, some degree of interaction exists between proteins and monoglycerides and these interactions are higher for adsorbed than for spread films. The topography of the monolayer corroborates these conclusions.