Milk and soy protein films at the air–water interface

In this contribution we present a systematic comparison of the two major fractions of globulin from a soy protein isolate (β-conglycinin and glycinin, including the chemical reduction of glycinin with dithiothreitol (DTT)) and typical milk proteins (β-casein, caseinate, and WPI). The comparison is centred on the most important functional properties of spread and adsorbed protein films at the air–water interface (such as adsorption, structural, topographical, and dynamic characteristics), as a function of the aqueous phase pH and the protein concentration in the bulk phase and at interface. A combination of surface techniques (tensiometry, surface film balance, Brewster angle microscopy and surface dilatational rheology) was used in this study. A notable feature of soy proteins is the strong pH dependence of the molecular conformation and the associated functional properties, such as surface activity, film structure, surface dilatational viscoelasticity, and especially, the rate of adsorption at a fluid interface. Optimum functionality occurs at pH<5, which limits the application of soy globulins as food ingredients. In this respect the behaviour of milk and soy proteins is quite different.