Yeast, Fermentation and Flavor

In this post:
Top Fermenting and Bottom Fermenting Yeast
Ale and Lager Fermentation Temperatures
Conditioning Beer: Aging and Lagering

Lagers aren’t So Bad
Many of those “born again” into the flavorful world of beer become bitter (no pun intended) to the ale counterpart, lager. I experienced this first hand when a close friend introduced me to home brewing, and the rich flavors in a variety of ales. I decided that lagers were not worthy of my time. Naively, I began to boycott lagers and even badmouth them as lousy, tasteless beer.

As my appreciation of craft beers grew, I began to realize that good lagers do exist and I wasn’t giving them a fair shot. A little exploration is a good thing, and I learned that there are better lagers out there than what I was used to.

Some mass produced commercial brands tend to be rather pale and less flavorful, but let’s not forget classic styles like Bocks, Dunkels and Pilsners, they are lagers too and worthy of any beer connoisseurs respect. A Helles Lager is a pale straw color, but full of flavor, so lagers can be just as bold and flavorful as ales. But what really determines the difference between them?

Lagers are made using the same basic ingredients as ale, with certain malts and hops used for certain styles, but the two main differences between ales and lagers comes down to yeast and fermentation temperature. “Lagering” also has a lot to do with it, and I’ll touch more on that in a bit.

Top Fermenting and Bottom Fermenting Yeast
Let’s take a look at the yeast. Yeast is available in a variety of strains, each developed for a certain style of alcoholic beverage. There are champaign yeasts, wine yeasts, yeats specifically engineered for ciders, and several varieties of lager and ale yeast. Each yeast strain has unique qualities that impact the final flavor and aroma of the beverage, and different tolerances to alcohol which determine the highest alcohol level possible during traditional fermentation.

Lager yeast is considered “bottom fermenting,” while ale yeast is considered “top fermenting.” Contrary to popular opinion this does not mean that lager yeast sits on the bottom to ferment, or that ale yeast floats on top to ferment. During the fermenting process for either style the yeast completely permeates the liquid and is actively consuming sugars in the wort. active yeast

The reason lager yeast strains are referred to as bottom fermenting is because they do not produce substantial krausen during the height of fermentation. Krausen is that thick head of foam that forms on the top of the beer as it ferments.

During “High Krausen,” or the most active period of ale fermentation, yeast cells collect in a thick foam at the top of the vessel, while active fermentation is ongoing in the liquid. This is also sometimes called “Top Cropping” although that term really refers to the harvesting of yeast from the fermenter.

“Top Cropping” means to skim yeast from the top of the vessel to be saved and used for other batches of beer. This is common with open fermentation vessels. True top cropping yeasts are better for top harvesting.

“Bottom Cropping” refers to harvesting yeast from the bottom, a process which is specific to conical fermenters, but not necessarily specific to lager yeast. Bottom Cropping yeasts are not always lager yeasts, but Top Cropping yeast is always an ale yeast.

All strains of yeast, whether wine or beer, flocculate and settle out, forming a sediment at the bottom of the fermenter. This trub also contains hops and other particulates that may have been part of the brewing process. Make wine, ferment lager or ale, you’ll see the same result.

Ale and Lager Fermentation Temperatures
The other main difference in producing a lager or ale is in the temperature during fermentation.

Both ales and lagers can begin the lag phase of fermentation at a higher temperatures, typically between 72°F and 75°F, but most ale should enter their active fermentation phase between 65°F and 70°F, while lagers need to be cooled lower, between 50°F and 55°F.

During the active fermentation (or the logarithmic phase) yeast begin actively consuming sugars and the cell count exponentially increases, and aside from producing alcohol and CO2, yeast create a variety of flavor and aroma compounds as well.

At this stage lagers and ales are relatively neck and neck as far as alcohol, flavors and aromas. The inherent differences between lager and ale yeast strains will create distinct style-specific flavors and aromas, as will the general ingredients, but basically, though different, both fermented drinks are rather well rounded and are somewhat robust.

Conditioning Beer: Aging and Lagering
While lager yeast and ale yeast behave differently during fermentation the main difference, and the reason many lagers taste “cleaner” than ales is due to lagering.

Lagering is the term generally used to describe the cold-aging, or conditioning, stage of lager beers. After fermenting at temperatures between 50°F and 55°F, lagers will be chilled further, to about 40°F for secondary fermentation, or about 35°F for lagering.

These colder secondary and lagering temperatures clean up the beer significantly. During this stage yeast will reabsorb compounds, flocculate and settle out, tannins and proteins will precipitate out as well.

Colder temperatures will have a more profound effect on the flavor than will warmer temperatures associated with aging, or conditioning, ales. Commercial breweries will slowly reduce the temperature of their ales to force flocculation, but aging ales in their bottles at temperatures between 50°F and 60°F for a few weeks will help beer clear and settle. Because of the higher aging temps, ales tend to have more complex flavor and aroma.

The goodness of a beer’s flavor is relative to the person who enjoys it. Ultimately it is the craft of brewing that matters, the recipes and traditions that fill the world of beer with such a variety of types and styles. There should be no battle over what is better. They are simply different.

Reference: Yeast Life Cycle (PDF) – Lager Brewing
Designing Great Beers: The Ultimate Guide to Brewing Classic Beer Styles