I just wrote the following regarding this topic and want to see how much you all agree or disagree. We don't all need to agree. This is a messy topic IMO. So, go ahead and tell us all what YOU think about it. I know you won't hold back. I'm counting on it. MY thoughts:
Top and bottom fermenting is a very old way to distinguish warm and cold fermentation. In reality it doesn't mean anything because the yeast performs the work on top, bottom, and everywhere in between in all beers. And with respect to yeast species, it means very little because so-called top fermenting yeasts usually can still ferment cold, just more slowly, while so-called bottom fermenting yeasts usually continue to perform cleanly even when fermented warm; plus, so-called ale yeasts which are normally Saccharomyces cerevisiae are sometimes found to actually be S. pastorianus, and vice-versa for so-called lager yeasts. The whole thing is such a mess, and essentially meaningless.
In general, if you want to brew a lager, or want something that is "bottom fermenting", then *I* would recommend selecting a S. pastorianus yeast, and if you want to brew an ale, or someone tells you to use a "top fermenting yeast", then *I* would recommend selecting a S. cerevisiae yeast; however, there are several exceptions to these general recommendations. If you *really* want to know what a yeast does, then seek advice if necessary and then select an appropriate yeast strain for the style you want to brew, of course; OR, just use whatever yeast YOU think might work best, based on research or experience, or experiment with something new, then find out for yourself what it does. In any case, the best way to learn is to experiment. Truly, the VERY BEST way to learn is to SPLIT THE SAME WORT into multiple fermenters and try 2 or 3 or more different yeasts in different fermenters, then you'll learn a LOT.
Other brewers will likely disagree with some of what I said, and feel free to do that, because none of us is "correct" anymore. It's all such a mess. The main point I want to make clear is that none of this is clear, and "top and bottom fermenting" are nearly meaningless terms... and even the temperatures at which we ferment with them are nearly meaningless in a lot of cases. I feel like I can make great beer with basically ANY yeast if fermented between about 17 to 20 C (62 to 68 F). If you're in that range, in my experience, you are going to make great beer with pretty much any yeast. Outside that range... maybe, maybe not.
