Surface rheology and morphology of beer protein and iso-humulone at air-liquid surface

Foam is one of the unique properties of beer, but how the amphiphilic components stabilize the foam is still not fully understood. This study concerns the main surface active components. For this purpose, protein from barley and iso-humulone from hop, were extracted from raw materials separately to be able to consider their effect separately and in the controlled mixture. The layers were formed in three ways, i.e. single component, pre-mixed solution and sequential addition of the individual components. Drop tensiometry and Brewster angle microscopy were employed to reveal the surface tension effects, rheological properties and morphology of the adsorbed layers. Both protein and iso-humulone showed surface activity and the combination gave synergistic effects in terms increasing the surface pressure. Surface rheology showed the combination of pre-mixed solutions of protein and iso-humulone formed the stiffest surface film. The pre-mixed solution gave higher dilatational moduli than if the two components were added sequentially. Furthermore, protein had a major influence on resisting deformation of the surface layer. The attractive interaction was confirmed by comparing the surface rheology and morphology of adsorbed layers formed by pre-mixed solutions and sequential addition. Iso-humulone also had a significant influence on interfacial properties. But this depends on whether it was pre-mixed with protein before added in to a buffer sub-phase or added to a pre-adsorbed protein layer. BAM reveals different surface morphology between the sequential addition and the pre-mixed sample. Thus, the property of the adsorbed film depends not only on the chemical composition, but also on the addition order of components.