h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

One of the basic ingredients for any beer, including real ale, is yeast. There exist many different strains of brewing yeast - the style of beer can be heavily influenced by which yeast is used. For example, the dominant characteristic of a Belgian Trappist ale is created by the unique strain of yeast used for that beer.

Yeast is the microorganism which converts sugars (from the malted barley, and anything else that's been thrown in) into two products: alcohol and carbon dioxide. Once the yeast runs of of sugar to consume, it becomes dormant and falls to the bottom of the fermenting vessel (though some remains suspended in the fluid). When homebrewing ale, you put extra sugar into the bottle before capping it - the dormant yeast is reactivated, creating a little extra alcohol and CO2 - however, with nowhere to escape to, the CO2 is dissolved in the beer. When the bottle is opened, the CO2 forms bubbles, and if you've done it properly, the beer gets a "head" of foam - your beer is "carbonated".

Yeast is crucial to the brewing process, and really ought to be listed amongst the "basic" ingredients!!

Yeast is in fact officially one of the four basic ingredients of all beer.

In fact, according to German law, beer consists of yeast, barley, hops, and water. ANY variation, and you can't call it beer. Thus, different types of German beers are made by using different strains of yeast, kilning the barley at different temperatures, etc.

Oh Dear,

I've not been around for a while...

First off, yes, Yeast should have been included. I can't imagine why I missed it except for possibly too many beers or maybe just following the trailblazing german purity laws which did not originally include yeast. When the Reinheitsgebot were introduced they didn't know about yeast. They just brewed the beer and it worked. They were unaware of yeast and the role it played. I can only claim a mistake rather than ignorance.
Mind you, even though the yeast really does play a huge part in the flavour, not all beers have yeast added, relying on the naturally present yeast. Some also have it added in uncontrolled measures, notibly the Lambic styles where they just throw open the windows and let the wild airborne yeast in. In the past breweries have closely guarded which yeast they use, nuturing it from batch to batch to preserve its flavour.
Mind you, I also know of breweries where they just chuck in some home brew yeast into the tank.

So to summise, strictly speaking yeast isn't necessarily a basic ingrediant as it can exist naturally in the other ingrediants or come in from the wild. Even the germans were able to brew beer without yeast. But, I agree it should have been listed and I apologise that it wasn't.