Solubility and sorption kinetics of gases in complex media

Coffee crema is one of the main drivers of coffee beverage quality and it is highly dependent on the used technique to brew coffee, since the final composition depends on many process parameters. Due to this, coffee systems were simplified by model solutions that reasonably represented key foaming properties.

Once the effect of individual coffee components was analyzed, a novel foaming technique was investigated to improve cold brew coffee foams. The process consists of 1) gas dissolution into a coffee solution under pressure; 2) gradual pressure release, provoking bubble nucleation, bubble growth due to gas diffusion and bubble rise; 3) bubble accumulation at the surface, forming the foam.

The studied gases were nitrogen and carbon dioxide. Nitrogen is not well soluble in aqueous systems, and it cannot penetrate in the liquid phase via diffusion, which limited nucleation and foaming studies. Carbon dioxide offers higher solubility, good diffusion properties and it can reduce the interfacial tension considerably, parameters that are enhanced above its critical pressure. More nucleated bubbles and lower bubble growth during their ascent were observed at higher pressures. However, foams created under these conditions were very unstable and they collapsed during generation. Furthermore, nucleation and bubble growth were theoretically evaluated in a 3D model to overcome the experimental limitations, which suggests that interfacial tension and liquid supersaturation are key drivers’ parameters.