When it comes to beer, a lot of ingredients hog the spotlight. Hops? A-list ingredient. Coffee? As important an adjunct in beer as it is in daily life. Heck, even water, hops, and malted barley get their day in the sun.
But what most casual drinkers forget is that without yeast — the funny little living (technically) fungus that's responsible for all alcohol — we wouldn't even have beer.
See, beer needs to be fermented to, well, exist. During the fermentation process, the yeast transforms sugar into alcohol and expels carbon dioxide, giving beer the buzz and bubbles we all love. If brewers didn't have yeast, all we'd have would be sad little pots of damp cereal and sugar water.
But despite its necessity, yeast's role in fermentation wasn't even completely understood until 1857 when Louis Pasteur proved that yeast was responsible for fermentation. Before then, beer came about more or less by accident. Many in ancient history even thought the drink was miraculous since it was safer to consume than water and was created without human interference.
So what does yeast actually do?
Well, to the average beer drinker, the flavors and colors may be all that discerns one beer from another, but yeast is what distinguishes ales from lagers (See Understanding Beer: Lagers, Ales, and Acronyms). There are two types of yeast: top-fermenting and bottom-fermenting. Top-fermenting yeasts require temperatures between the mid 60s to low 70s and produce ales. By contrast, bottom-fermenting yeasts must ferment in temperatures in the low to mid 50s and yield lagers.
Most breweries purchase their ale and lager yeasts, but some will take a gamble on a batch of beer and allow it to spontaneously ferment. Instead of containing the hoppy sugar water known as "wort" in a closed fermentation tank, the brewers will actually expose the vulnerable liquid to wild yeast by leaving barrels of unfinished beer just kind of hanging out in the open air. There, wild yeasts and bacteria that are floating in the air can begin fermentation spontaneously. This process, while unpredictable, can produce a more tart and sour flavor (or it can produce something so bad that it's better used to clear your drains).
Like other ingredients, yeast can — and will — affect aroma and taste. Wild yeasts can create a tart, almost yogurt-y tartness in beer. And then take the average hefeweizen.
The second most important thing you can do with a beer is smell it (behind, of course, drinking it), and when you smell a hefeweizen, most people will detect scents of banana, clove or bubblegum. Yes, brewers do sometimes add various fruits, nuts, or other ingredients to their beer to alter aroma or taste, but that is not the case for most hefeweizens.
That bubblegum smell? It's from the yeast. During the fermentation of a hef, compounds known as "esters" form after the yeast reacts with the wort. These esters are what cause hefeweizens to have notes of banana, clove or bubblegum without any of those ingredients having been added to the beer itself.
Man, ancient brewers were right to believe in the divinity of beer.