In the early days of American microbrewing and homebrewing, when our modern beer culture was a shiny new thing, experts often divided the world into lagers on one side and ales on the other. Some would even compare lagers to white wines and ales to reds. The idea was that ales were bolder, stronger, and more bitter than lagers. Assuming that “lager” meant only pale, pilsner-like beers, maybe there was some logic to it.
But in terms of chemical reality, the difference is that lager’s cooler fermentations produce fewer chemical by-products. These can be smallish molecules called esters; heavier, even oily alcohols called fusels; and others.
I like to think of yeast cells as leaky bags of goo. Inside, multiple chemical processes are occurring—absorbed sugar is being snipped apart, and carbon energy units are popping off to fuel cellular activity. Excess energy is stored as fat. Waste is sequestered or expelled. Amino acids assemble into proteins. Sometimes, the cell is making baby yeast. Each process is a sequence of molecular transformations with multiple chemical actors. Some have potent aromas, and those can leak out into the beer.