Plant vs Dairy protein stabilised cappuccino foams: how protein and hydrocolloid conformational changes affect foam stability

Plant based dairy alternative milks are seen as to have low foam appeal due to the dry, stiff texture and sometime rapid collapse. The current study sought to understand the factors affecting the formation and stability of foams made with plant and dairy proteins. Two different aeration processes were studied, steam injection and whisking, to be representative of coffee shop and in-home foam applications. These two aeration processes were found to have a significant impact on total air volume fraction and final foam bubble size. Whisking produced foams with higher air volume fraction and coarser bubbles compared to steam injection. Further, the effect of bulk viscosity on foam drainage and coalescence was investigated by adding high acetyl gellan as viscosifier. Bulk solution viscosity played a dominant role in foam stability, with higher viscosity leading to slower liquid drainage and reduced bubble coarsening. Conformational changes to high acetyl gellan viscosifier upon heating explained why steam injected foams underwent faster drainage compared to whisked foams at equivalent gellan content. Importantly only minor change in protein secondary structure and aggregation state was observed after foaming. This work shows that the main driver of the difference in dryness between plant and dairy cappuccino foams arises from the aeration process and the speed of liquid drainage. The inability to resist foam drainage, a key weakness of plant-based cappuccinos, can readily be overcome using a shear thinning hydrocolloid such as gellan to boost bulk viscosity. These findings help to understand the factors affecting the stability of cappuccino foams and contributes to the development of plant-based alternatives with improved foam quality.