After some thought, and after trying to remember what I did actually think all those years ago, I decided it was probably Biology. You see, Physics is certain. At least it is certain, assuming standard temperature and pressure. Chemistry is kind of fun; we got to distil alcohol when I was at school, under strict supervision I might add. We even made things explode, occasionally, although that might have been due to a lack of supervision.
I did remember getting interested in things that happened in petri dishes. I also developed a deep fascination of the nervous system, neurones, synapses and other things that made us "wired" As it happens, my "O"1 level results showed me to be slightly better at Biology than Chemistry, which was a shock to me. Or perhaps that was the Van de Graaff generator, not sure.
The longer I spend in the beer brewing industry the more there seems to be to learn. There is knowledge needed in micro-biology, chemistry, physics and a good handful of engineering to go along with it. There also seems to be a lot of mis-information, half truths, old wives tales, urban myths and damn annoying lies from some, just to make the beer drinker think something other than the facts.
Lager is different to "ale" we are told, because it's made with "bottom fermenting yeast" where as "ale" is made with "top fermenting yeast" Other people, who are more knowledgeable, talk about bottom or top cropping yeast. However, I think even this is a flawed explanation.
As I said in my previous post, I'm no yeast expert, but I'm fascinated by the subject. I have read about the fascinating organism, I've used them in various forms and have enough experience to be sure that most beer experts and some brewers still have a lot to learn, me included.
What I know is that when yeast is at the bottom of the tank, or floating at the top, it does a very poor job of fermenting. It only ferments when in suspension. So we could all do with cutting out this top and bottom fermenting false-jargon.
I also know that it is perfectly possible, and indeed probably preferable, to bottom crop nearly every yeast. Most big breweries and some of the more progressive micro-breweries crop their yeast at the bottom, even when making "ale". There may, or may not be a crust of yeast forms on the top of the beer after primary fermentation, but some of the more modern "ale" strains, especially if supplied dried, completely fall to the bottom of the primary fermentation tank.
The use of he words top and bottom are completely misleading, in my view.
Wort, the sugary liquid on which the yeast works to make beer, has a number of various complexities of carbohydrates dissolved within it. The precise types of carbohydrates depend on a number of factors including the grain used, other carbohydrates used like corn starch and whether or not any additional enzymes are added. Broadly, in brewing terms, we talk about fermentable sugars and un-fermentable sugars. However, the boundaries are perhaps not clear.
Various yeast strains are able to digest different carbohydrates, in different environments, with various degrees of success. It is mainly to do with the enzymes that the yeast cell generates. And, just to complicate things, there are enzymes available that will allow a brewer to ferment just about anything, irrespective of the yeast used.
It is true that a traditional lager yeast will drop to the bottom of the tank and will crop at the bottom.
It is true that in traditional ale making the types of yeast used would readily crop from the top and quite a lot of the yeast remained on the top of the beer after fermentation.
Generally ale yeast is known as Saccharomyces cerevisiae and lager yeast as Saccharomyces pastorianus or Saccharomyces carlsbergensis. However, there is some reading I have done that suggests that they really are all the same thing and just different strains of the same yeast. All strains have different behaviours.
However, the key real practical differences seem to be to do with temperature of fermentation and ability to attenuate the wort.
"Ale" yeast, or what I would prefer to call "warm" fermenting yeast is normally fermented at perhaps 20 degrees centigrade and fermentation might be done in 3-7 days.
"Lager" yeast will work at perhaps 5-12 degrees and might take a little longer to get around to turning all those sugars into alcohol, perhaps a couple of weeks with very long maturation times.
However, I can assure you, "ale" yeast does not stop working at 12 degrees and there are brewers who will deliberately ferment "ale" at lower temperatures to achieve required flavour profiles. Equally, modern lagers are often made much more quickly than the traditional lagers and this can only be done with at least some of the fermentation being done at a higher temperature. The line is not as clear as the beer geek would like it to be.
But, what we can say about lager over other beers, is that it tends to be much more attenuated. That is to say the yeast has got to more of the sugars and turned them into alcohol. The beer is drier and has less calories. Perhaps that's the major difference with lager yeast; the fact that it makes a drier beer.
Perhaps, but remember those enzymes you can buy? They'll help just about any yeast turn wort into a dry beer. For that matter, miss out a whole load of barley and add in easily fermentable sugar made from corn starch, and away you go. This is probably the process used by most bigger brands of beers and, interestingly, even small brewers are encouraged to add enzymes to help out with difficult fermentations. Certainly measured amounts of enzymes would tighten up ABV variations. No doubt if a brewers bonus2 depended on satisfying over a hundred different QA checks, he would be tempted to use it.
Of course, there are other ways of fermenting down lower in gravity, as in the case of Orval and lambics, other yeast strains or even bacteria might consume more sugars and dry the beer out.
I may have some of my facts wrong in this piece. I apologise and I am very happy for the more technically adept reader to chip in and correct me. What I am trying to do here is show the reader that to pigeon hole beers by some notion of exactly which yeast strain is used is perhaps less certain than it might be.
Lager is dryer than ale and can use more delicate hops as they are not overpowered by excess sweetness. Ale is sweeter than lager, generally and needs more hops to counter the sweetness. That, perhaps, is all you really need to know.
I hope I've broadened the readers mind a little.
1For you young people, "O" levels and CSE qualifications were later replaced by GCSEs. I know most of you won't remember that far back.
2Perhaps another indicator of craft?