Which term describes the microorganisms' response to oxygen in brewing?

The term that describes the microorganisms' response to oxygen in brewing is "pellicle." A pellicle is a thin film that forms on the surface of liquid, particularly in the presence of certain bacteria and yeast when exposed to oxygen. This phenomenon occurs during the fermentation process, especially in some styles of beers and wild ales, where the presence of oxygen facilitates the growth of surface microorganisms that create this protective layer.

In contrast, biofilms generally refer to aggregates of microorganisms (bacteria, yeast) that adhere to surfaces in a matrix of extracellular polymeric substances, which may not be specifically related to an oxygen response in brewing contexts. Scum typically refers to an undesirable surface layer that may form during fermentation or spoilage but does not denote a specific microbiological response to oxygen. A yeast raft, on the other hand, describes a cluster of yeast that can form during fermentation, but this term is not directly related to the response of microorganisms to oxygen.