Soy globulin spread films at the air–water interface

Structural and topographical characteristics of two major fractions of soy globulin from a soy protein isolate, β-conglycinin (a 7S globulin) and glycinin (a 11S globulin)—including the effect of chemical reduction of glycinin with dithiothreitol (DTT)—spread at the air–water interface at pH 8 and at 20 °C were determined from π–A isotherms coupled with a microscopic, non-invasive technique—Brewster angle microscopy. The structural characteristics of 7S and 11S globulin spread monolayers depend on film ageing. We have observed a significant shift of the π–A isotherms towards higher molecular areas over time. The ageing effect was due to unfolding of the protein at the interface. A change in the monolayer structure was observed at surface pressure of 17–19 mN/m. At a microscopic level, the heterogeneous monolayer structures visualised near to the monolayer collapse and during the monolayer expansion proved the existence of large regions of protein aggregates. Relative reflectivity increases with surface pressure and was a maximum at the monolayer collapse. The monolayer structure was more expanded for 7S than for 11S, but the maximum of both surface pressure and reflectivity (at the minimum molecular area) was observed for 11S globulin. The chemical reduction of glycinin with DTT produced a significant expansion of the monolayer structure.