This question is a fun one. On the surface, the answer probably seems straightforward, but closer inspection reveals that it’s not necessarily as cut and dried as we initially suspect.
So let’s approach the question by way of four of the most commonly offered answers. Along the way, we’ll dispel a few myths and learn more about what defines this seemingly stark division of the world’s great beer styles.
Common Answer #1: Ale is made with top-fermenting yeast, but lager is made with bottom-fermenting yeast.
This answer is misleading at best and simply not true. Observe an active fermentation for any length of time and you immediately notice that yeast activity is distributed throughout the liquid. As billions of yeast cells multiply and feed upon the available sugars, initially clear wort becomes rather cloudy. Bubbles rise to the top from all points in the fermentor, and the whole mass appears to churn violently. When fermentation is over, those yeast cells drop out of solution, and the beer becomes clear once again. Most yeast cells, ale and lager alike, flocculate and end up on the bottom of the fermentation vessel, at least to some degree.
The top-fermenting versus bottom-fermenting yeast distinction likely originated in the observation that ale typically features a large, fluffy kräusen on top of the fermenting beer. Lagers have a kräusen as well, but it’s usually smaller and less vigorous. This has less to do with any particular positional preference in the vessel than it does with the simple fact that cool fermentations are more subtle than warm fermentations. Yeast works more slowly when it’s cold than when it’s warm. And that brings us to the second commonly offered answer.
Common Answer #2: Ale is fermented warm, but lager is fermented cold.
There is a great deal of truth to this answer. Most ales ferment in the 60–75°F (16–24°C) range, with some saison strains requiring temperatures as high as 95–100°F (35–38°C) to fully attenuate. Lagers, on the other hand, generally do their best in the 45–55°F (7–13°C) range, but they are perfectly capable of fermenting in the ale regime. After all, most of us propagate lager yeast starters at room temperature, not in the cold.
The reason that lagers are fermented cold is twofold. First, the 1553 decree by Duke Albrecht V of Bavaria that prohibited summer brewing caused brewers to unknowingly select for cold-tolerant yeasts that could survive Bavarian winters. Thus, lager yeasts evolved to prefer—and thrive in—the cold. Second, cold fermentation suppresses many of the by-products of yeast fermentation and allows the finished beer to be largely malt- and hops-focused. We’ve grown to appreciate the clean, crisp character of a good lager.
But there’s another reason that the whole warm-versus-cold distinction isn’t the entire story. Some lager yeasts perform well at relatively warm temperatures, and some ale yeasts are comfortable with conditions that would cause other strains to just give up and flocculate. California Common strains, such as Wyeast 2112 and White Labs WLP810, act pretty lager-like at up to 65°F (18°C), and certain German ale strains used for Kölsch and Altbier can comfortably work down to 55–60°F (13–16°C). So-called hybrid strains blur the temperature lines a little.
Common answer #3: Lagers undergo a special cold-conditioning phase, but ales do not.
The German word lagern means “to store” and refers to the long cold-conditioning phase that follows primary fermentation. It’s true that this practice is more commonly associated with brewing lager beer than with brewing ale, but by no means do lager brewers hold exclusive rights to cold conditioning.
For instance, altbier and Kölsch, both German ales, traditionally undergo a period of cold storage after fermentation, just like their cold-fermented Bavarian siblings. And in Brew Like a Monk, Stan Hieronymus notes that Westvleteren’s 8 and 12 spend anywhere from a month to 10 weeks in 50°F (10°C) lagering tanks to give yeast, hops, and proteins time to settle. Ales are indeed less commonly cold-aged than lagers, but the practice is far from unheard of.
And while few among us would ever intentionally omit the cold-conditioning phase from a lager beer, I can say from personal experience owing to equipment constraints that, provided primary fermentation is conducted cold, a period of a few weeks or months at room temperature can still turn out a satisfactory beer with smooth lager characteristics. It’s not ideal, and it’s not traditional, but a warm-conditioned lager can still be excellent.
In Yeast: The Practical Guide to Beer Fermentation, Chris White and Jamil Zainasheff discuss the benefits of the lagering phase and note that “every beer improves with some period of cold conditioning,” regardless of the strain with which it was fermented. They then offer Jean De Clerck’s reasons for lagering, as outlined in his seminal two-volume A Textbook of Brewing in 1957:
- To allow yeast and turbid matter to settle out
- To carbonate the beer with artificial carbonation or secondary fermentation
- To improve flavor
- To precipitate chill haze, to prevent haze formation when the beer is chilled after filtration
- To avoid oxygen pick-up in order to prevent oxidation
These aims are important to beers of all types, not just those that are fermented in the cold.
Common Answer #4: Ales are fermented using strains of Saccharomyces cerevisiae, and lagers are fermented using Saccharomyces pastorianus.
If one must point to a single differentiating factor between ales and lagers, this is it. Ales and lagers are fermented with two different species of yeasts. Ale yeast, Saccharomyces cerevisiae, is, in fact, the same species of yeast we use to bake bread, which ought not be too surprising given the close historical relationships between brewers and bakers.
Saccharomyces pastorianus is, however, a microbe all its own. Aside from its obvious penchant for the cold, S. pastorianus also has the ability to metabolize melibiose and raffinose, two complex sugars that S. cerevisiae can’t really deal with. The practical implications of this metabolic capacity are not great, for malt-based wort contains very little in the way of melibiose, and no raffinose whatsoever. Nonetheless, it’s important from a taxonomic standpoint in that it supports the distinction of ale and lager yeasts as unique organisms.
Older texts may refer to lager yeast as S. carlsbergensis or S. uvarum, but the most commonly accepted modern term is S. pastorianus. Interestingly, S. pastorianus appears to be an interspecies hybrid of S. cerevisiae and S. eubayanus, the latter of which has been traced as far from Bavaria as Patagonia, Tibet, and Mongolia. How it managed to find its way to Continental Europe remains the subject of much research and the sort of lively scientific debates that one only hopes feature lab-coat-clad scientists of the sort found in The Far Side.
Yes, when we really get down to it, the only meaningful distinction between ales and lagers concerns the microbes used to ferment them. Everything else has more to do with tradition.
A Question of Intent
Ultimately, I don’t think it matters what you call your beer, as long as you understand the technical requirements of your selected yeast strain. Weihenstephan W34/70 simply won’t give satisfactory results at ale temperatures, and the Brasserie Dupont strain (blend? mutant? alien microbe?) may not even give you the time of day unless it has been offered an opportunity to spend time in a sauna.
Great beers can be made as both ales and lagers. And we need only look to such boundary-blurring and tasty styles as Kölsch and India pale lager to see that the two have more in common than we might ever have thought.
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