Which factor contributes to the formation of bubbles in beer?

The presence of nucleation sites is a critical factor in the formation of bubbles in beer. Nucleation sites are tiny imperfections or particles in the beer that provide a surface for gas bubbles to form. When carbon dioxide is dissolved in beer under pressure, it remains in solution. Upon opening a beer or when the pressure is lowered, the carbon dioxide begins to escape from the liquid. This escape is facilitated by these nucleation sites, where the gas is able to gather and form visible bubbles.

While temperature can influence the solubility of carbon dioxide and the behavior of bubbles, it does not directly cause bubbles to form. The type of yeast used primarily affects fermentation and flavor, rather than bubble formation itself. Alcohol content can influence the overall mouthfeel and stability of the beer but does not play a direct role in bubble formation like nucleation sites do. Therefore, the presence of nucleation sites is the most direct factor contributing to the formation of bubbles in beer.