So at the most simple scientific level, brewing is when sugars from malted grains, mainly barley, are used by yeast to grow giving alcohol as a tasty byproduct. Yeast also makes lots of other stuff in this process which gives beer many of its distinctive flavours. When we brew beer, the healthy yeast sticks together in Carbon dioxide bubbles on the top to make a thick foamy layer.
Fermentation has to be monitored all the time - we want to measure how much of the sugar is turned to alcohol to give your ABV (alcohol by volume, i.e percent!) and this must be consistent. When enough alcohol has been produced, the brewers have to take off as much of the healthy yeast as possible to stop it fermenting too much further. This process is call skimming and it makes my arms hurt an awful lot.
Skimming the yeast usually takes me anywhere between 5 and 30 minutes depending on how much there is - sometimes it's a layer only 3 inches deep, sometimes its over a foot! No matter how long it takes I'm always covered in the stuff, I think yeast must love me a bit. I think its because I sort of lean in to the fermenting vessel to reach it all, although I'm getting better at it now.
This leads me, in rather a tenuous but I think timely manner, to talk about one of the differences between lager and ale styles of beer. The ingredients and processes of making the two styles are quite different, and whist it's a minor factor this does includes our tiny friend - the yeast.
In a very basic explanation, lager is brewed at cold temperatures, a practise originating from cave and underground fermentation processes back in the day. The strain of yeast which thrives at these temperatues, about 10°C, lives at the bottom of the fermenting vessel (ale is at the top, remember?) and it can break down more sugars than ale yeast strains, turning these to alcohol and leaving the lager with less natural sweetness. Ales are fermented at about 18 - 24°C which is optimum for the yeast which floats on the top and ferments fewer of the sugars extracted from the barley.
For all you uber science geeks out there, ale brewers yeast is Saccharomyces cerevisiae while lager yeast is Saccharomyces carlsbergenesis. It is a diploid eukaryote that reproduces by budding which you can see on this lovely artifically coloured electron micograph. Every time yeast is skimmed from the beer we save it in sterile buckets in the fridge (yes, just like the Bosch one at home) and re-pitch this into the next brew. This makes sure that we're always using the same strain to give a consistent product. Breweries sometimes swap their yeast strains to experiment or brew similar recipes.
And here endeth the lesson for today. Any questions, please raise your hand/comment on here.