What is the Mystery Behind Lager Yeast's Bottom Fermenting Characteristics?

Whether you're a homebrewer, beer enthusiast, or bug-eyed yeast-o-phile, you've probably heard about the differences between ale and lager yeasts and how lager yeast has this magical ability to ferment on the bottom. But what exactly makes lager yeast "bottom fermenting," and is it really that different from ale yeast? Grab a beer, because we're going to dive into this fascinating realm of yeast behavior and uncover the mystery behind lager yeast's bottom fermenting characteristics.

Ale Yeast Characteristics

Before we talk about lager yeasts, it's worth looking at ale yeasts. Ale yeasts, known scientifically as Saccharomyces cerevisiae, are the most commonly used strains in craft and home brewing. They thrive in warmer temperatures, usually between 60-72°F (15-22°C), though some Belgian and exotic strains can go higher. At these room-type temperatures, ale yeasts are quite active, and they produce a lot of carbon dioxide (CO2) as they ferment the sugars in the wort. This vigorous fermentation creates strong convection currents in the fermenting beer, which keeps the yeast cells suspended in the liquid. This activity is why ale yeasts are often referred to as "top fermenting" – they are carried to the surface from these rapidly rising CO2 bubbles and form a thick layer of foam, or krausen, on the surface of the wort during fermentation.

Lager Yeast Characteristics

On the flip side, we have lager yeasts, or Saccharomyces pastorianus. Contrary to popular lore, these yeasts do not prefer a cooler environment. They want to survive just like every other organism, and they will live just fine at classic ale temperatures. On the other hand, when these yeast are allowed to ferment cool, typically between 45-55°F (7-13°C), they will produce the flavors we associate with lagers, such as crisp, clean, dry bready and biscuity. At these lower temperatures, the fermentation process is much slower, and less CO2 is produced. This means fewer convection currents, and as a result, the yeast cells are not buoyed by the rising CO2. Via gravity they gradually settle to the bottom of the fermenter. This slower activity and settling behavior have earned lager yeasts the "bottom fermenting" label.

The Temperature Connection

Here's where things get interesting: if you ferment lager yeast at warmer temperatures, it starts to behave more like ale yeast. The increased activity at higher temperatures leads to more CO2 production, which in turn creates those familiar mixing currents. These currents keep the yeast in suspension, causing the yeast to "top ferment" just like ale yeast.

Conversely, if ale yeast is fermented at cooler temperatures, its activity slows down, and it might settle more like lager yeast. However, most ale yeasts aren't as effective at cooler temperatures and might not ferment completely or produce the desired flavors. When the ale yeasts get too cold, they slip into dormancy.

So What are Practical Implications for Homebrewers?

For homebrewers, understanding these behaviors is key to experimenting with different styles and achieving the desired results. Here are a few tips to keep in mind:

  • Temperature Control: Use a temperature-controlled environment to maintain the ideal fermentation temperatures for your yeast. This ensures consistent results and helps avoid unexpected flavors. You don't have to geek out on this. Temperature control doesn't have to be fancy. For people who live in the north, basement temperatures in the winter are often perfect for fermenting lagers.
  • Experimentation: Don't be afraid to experiment, because that's what home brewing is all about. Try fermenting lager yeast at slightly warmer temperatures to see how it affects the flavor and fermentation process. Just remember that too warm might produce off-flavors. Home brewers have used Saflager 34/70 from Weihenstephan in Germany in the upper 60s with excellent results.
  • Patience with Lager Yeast: If you're brewing a lager and you're fermenting it cool - no Type A personalities allowed. Be prepared for a longer fermentation time. The slow, steady fermentation at cooler temperatures is part of what gives lagers their clean, crisp character.

The Big Takeaways

Like you'll remember any of this after a few beers....

So, what's the mystery behind lager yeast's bottom fermenting characteristics? It all boils down to (pun intended, ha ha) temperature. Lager yeast fermenting at cooler temperatures results in slower activity and less CO2 production, which allows the yeast to fall with gravity to the bottom. But, give lager yeast a warmer environment, and it will "top ferment" just like ale yeast. Understanding these dynamics may very well help you take your homebrewing to the next level and craft beers with precision and creativity, or it might just make you sound cool. And if your beer-mooching friends on Thirsty Thursday think you sound like a smart ass, then tell them to quit drinking your precious home brew and fall back upon some lamer hard seltzer.


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