Four main ingredients make up beer: barley, hops, water and yeast. Yeast, which we didn’t even know existed until 1680 and didn’t understand the purpose of until 1857. Yeast! It’s pretty great.

Let’s talk about yeast.

Yeast: the basics

Yeast is a fungus. It’s best to try to forget that as you drink any fermented beverage, because while I like bleu cheese and mushrooms and so on I wouldn’t exactly call them “thirst quenching.” So: your beer is alive.

Yeast is also responsible for the alcohol in your beer, wine and spirits. That’s right, liquor too: there’s a reason the Scottish call beer unfinished whisky.

The nonscientific way I describe fermentation: yeast eat sugar and poop carbon dioxide and alcohol. I suppose that might be more gross than the fungus bit.

But it’s true! We owe beer to yeast, and so it’s important to treat them well. Homebrewers know this well: pitch yeast into wort (unfermented beer) that’s too hot? Congrats, you just boiled your fermentation army alive. Don’t pitch enough yeast? They get stressed trying to ferment everything and things taste off. Ferment too hot? Don’t give them enough oxygen in the wort beforehand? So on and so forth: be kind to your yeast.

Yeast: the classifications

Broadly speaking, all yeast is the same. That is: they all ferment sugar into alcohol and co2, whether you’re using it to make beer or bread (adding the sugar to let the dough rise makes more sense now, huh?). Bu all yeast is the same in the same sense that all dogs are the same, since they have four legs at at least a bit of a tail and look adorable when you dress them in costumes.

You can use beer yeast to make bread, and bread yeast to make beer, but you probably won’t like the results very much. There are two main species of yeast used to make beer:

A yeast particle. Photo from NIAID, under Creative Commons license

• Saccharomyces cerevisiae ferments at the top of the liquid, likes higher fermentation temperatures, produces more esters (fruit, spice, etc) and is generally used to make what we call ales. Every beer we’ve made has been an ale.

• Saccharomyces pastorianus ferments at the bottom of the liquid, likes lower fermentation temperatures, ferments “cleaner” in taste and is generally used to make what we call lagers.

Then there’s the Brettanomyces genus but I’m trying to make this a good introduction, and if you like brett then you probably know all this already. These two species also have strains, like dogs have breeds, and so a Belgian variety will prefer higher temperatures and provide more esters and phenols whereas an American strain will ferment cleaner. Some attenuate more, meaning they ferment more of the sugars and produce a drier beer.

Lagers are named for the German word meaning “stock” or “to store”, because they need to be aged longer than ales do. This is the real reason why many breweries, ourselves included, don’t make them: it isn’t a philosophical aversion to “fizzy yellow beer”, it’s that we can’t make enough beer as it is and something with a 28 day production cycle would only exacerbate the situation.

An interesting bit of evolutionary history

I first heard this theory on Basic Brewing Radio somewhere around 2008, but there’s also a post about it on Zythophile and hey, if Martyn has researched it it must be true.

Yeast eats sugars to survive. It is not the only creature to do this, and so in the microbial world the yeast had some competition.

Then, in the not-actually-a-sentient-choice nature of evolution, yeast realized something: if it invested some of its evolutionary skill points into the “ferments alcohol” perk it could gain an advantage. Creating alcohol from some of the sugars meant that they would eat slower, yes, but fairly soon on in the process the alcohol would make the surfaces of, say, the overripe fruits that were their battleground covered in alcohol and therefore inhospitable to their competition.

They poisoned the well. Slow and steady won the race.

That’s something I can lift a pint to.