Fermentation Fascination

Ignore yeast at your beer’s peril because if you don’t know yeast, you don’t know beer.

Dave Carpenter Dec 4, 2015 - 13 min read

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The next time you watch the evening news and remark to yourself or a loved one that society is on a downward spiral leading to humankind’s inevitable demise, here’s something positive to keep in mind: We’re the only species that makes beer.
This is, of course, a lie. We’re the only species that can make wort, but science has identified several species that can turn that wort into beer. Once they figure out the wort thing, we are doomed: Yeast will outlive us all, even the roaches.

The Least Glamorous Ingredient

Breweries love to go on and on about premium barley, mountain spring water, and fresh whole-cone hops. But when is the last time you heard one build a marketing campaign around yeast? I didn’t think so.

Before I started brewing at home, I thought yeast was just something you pick up at the grocery store when you want to bake bread. It comes in a little packet, and when you add it to a warm mixture of flour and water, magic ensues. Perhaps it was the same for you. Perhaps it still is.

But this rudimentary understanding is still far ahead of ancient brewers who believed that fermentation just sort of happened. The famous Bavarian beer purity law of 1516, the Reinheitsgebot, _didn’t even mention yeast because nobody knew what it was. It wasn’t until Louis Pasteur proved that yeasts are not only responsible for fermentation but are, in fact, living things, that brewers began to understand these tiny microbes. So important was Pasteur’s discovery that lager yeast, _Saccharomyces pastorianus, bears his name.


That was just 150 years ago. Anthropologists say ancient Egyptians and Mesopotamians started brewing about 5,000 years earlier, which means we’ve known about yeast for all of 3 percent of the history of beer. No wonder it still seems mysterious.

What Yeast Is

Yeast is a unicellular member of the fungus kingdom, a group of organisms that also makes possible such delicacies as Camembert and chanterelles. Yeast cells are eukaryotes, meaning they have nuclei, whereas bacteria lack such basic features. Imagine having no brain. That’s how much more advanced yeasts are than bacteria.

Countless species of yeast have been identified, but only a few are of interest to beer enthusiasts. When brewers discuss yeast without further qualification, they’re referring to strains of the species Saccharomyces cerevisiae _(ale yeast) or _Saccharomyces pastorianus _(lager yeast). You’ll also hear about so-called wild yeasts, which include several members of genus _Brettanomyces. Other microorganisms critical for certain sour styles, namely Lactobacillus and Pediococcus,_ _are bacteria, not yeast.

What Yeast Does

At its most fundamental level, a yeast cell does three things:

  1. It consumes sugar.
  2. It produces alcohol and carbon dioxide.
  3. It creates other yeast cells.

But that’s only a small part of the story. In doing all of the above, a yeast cell also creates flavor compounds that can range from fruity (esters) to spicy (phenols), and even buttery (diacetyl) or rotten egg-like (sulfur).

New homebrewers are frequently surprised to learn just how much influence yeast exerts as it transforms wort into beer. If you’ve ever sampled American and English pale ales side by side, you may have noticed that English pale ale has a more robust malt character and less of a hops focus than its American cousin. But maybe you’ve also noticed that English ales have a characteristic fruitiness that’s not present in most American ales.

This difference is largely one of yeast selection. British yeast strains tend to produce lots of fruity esters, while many American yeast strains are relatively clean. In fact, certain beer styles such as Bavarian Hefeweizen, monastic tripels, and farmhouse saisons are so heavily defined by yeast that it’s impossible to brew authentic versions without the correct strains. The considerable differences between American wheat beer and Hefeweizen mostly come down to yeast.

Yeast also determines how dry or sweet a beer will be, how clear or cloudy, even how malty or hoppy. It also happens to be the only ingredient in beer that makes more of itself.


How Yeast Works

If you haven’t yet had the pleasure of brewing, wort (unfermented beer) is prepared by mashing grains or dissolving concentrated malt extract in hot water and boiling with hops for an hour or more. After the boiled, hopped wort is cooled to room temperature or lower, yeast is added (or pitched in brewing parlance). And then you wait a week or two for fermentation to occur. But what is actually going on during that time?

Yeast experts divide the fermentation process into three distinct phases: lag, growth, and stationary.

Lag Phase

For several hours after yeast is added to fresh wort, a whole lot of nothing appears to happen. This lack of apparent activity prompts many first-time homebrewers to wonder whether they did something wrong and voice their concerns in Internet forums, where they’re almost immediately told to relax and not worry. Think of the lag phase as the microbiological equivalent of jet lag or altitude sickness: a sudden transition from one environment to another takes a bit of getting used to.

But yeast cells aren’t being lazy during the lag phase. In fact, quite a lot is taking place, and much of the beer’s character is determined at this time. Yeast cells begin to absorb nutrients from the wort and prepare for growth. Especially important to yeast health during this phase is sufficient oxygen. But having just been boiled for an hour or more, wort is usually deficient in oxygen when it comes right out of the kettle. That’s why introducing oxygen is so critical at the start of fermentation.


Homebrewing guides usually instruct brewers to vigorously shake the carboy, rock the plastic bucket back and forth, or otherwise agitate chilled wort to introduce oxygen. Don’t skip this step! Experienced homebrewers will even go so far as to dissolve pure oxygen into wort using sintered diffusion stones, but a good shake is almost as effective: it just takes a little more time and muscle.

The quality of the lag phase is also affected by pitch rate, which refers to how many yeast cells are used to inoculate the wort. It is a topic unto itself, remaining the focus of much debate and experimentation among professionals and hobbyists alike. An oft-quoted rule of thumb is to pitch 0.75 million cells per milliliter of wort per degree Plato of wort strength for ales, and twice as much for lagers. Pitching too many or too few cells can lead to off-flavors or beer with little character.

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Also important is temperature. Every yeast strain has a range of optimal temperatures, typically 60–70°F (15–21°C) for ales and 45–55°F (7–12°C) for lagers. Aiming for a temperature below the optimal range can render yeast cells sluggish and even drop them out of suspension altogether. Allowing the temperature to rise too much will likely result in unpleasant flavors. Generally speaking, starting fermentation at the low end of the range and letting the temperature gradually increase to the high end gives good results.


Growth Phase

The growth phase is what most of us think of as “active fermentation.” This is when yeast cells are enthusiastically consuming wort sugars and converting them to carbon dioxide and alcohol. You’ll sometimes hear this phase referred to as logarithmic or exponential because of the rapidity with which cells reproduce.

This is also the most visually arresting portion of the fermentation cycle. A thick, fluffy white foam called _Kräusen _(pronounced “croy-zen”) appears on top of the beer, flecked with brown bits of hops and other wort sediment. If fermentation is taking place in a transparent vessel such as a carboy, you can see streams of carbon dioxide bubbles rising up the sides, and the airlock or blowoff will vigorously release gas.

This is also the phase during which homebrewers, be it their first batch or their five hundredth, can be observed crouched on the floor as they stare at their churning beer and sniff the bubbling airlock.

Stationary Phase

Alas, all great journeys must come to an end, and the stationary phase represents the end of the road for our friendly fermenters. But even as they sense things slowing down, yeast cells remain active to the bitter end.


Unlike children, yeast cells enthusiastically pick up after themselves. During fermentation, they produce all kinds of unpleasant compounds that you definitely don’t want in your beer. But as the process winds down, yeast cells will actually reabsorb these compounds in preparation for going dormant.

One example of this janitorial role is the so-called _diacetyl rest, _which some lager brewers employ during this phase. Diacetyl is a compound that tastes like movie theater popcorn—in fact, food manufacturers add copious amounts of the stuff to artificial butter flavoring to make it taste more like real butter. Raising the temperature of a lager by just a few degrees toward the end of fermentation can encourage yeast to reabsorb diacetyl, among other by-products of fermentation.

Another process of vital importance to homebrewers (and most craft brewers as well) is flocculation, which is the tendency for yeast cells to clump together. When the clumps reach critical mass, they drop to the bottom of the fermentor, clarifying the beer. Some strains do this more than others. British ale strains are famously flocculent, while German Hefeweizen yeast are famously resistant.

After flocculation, fermentation can be considered complete, though flavors may continue to evolve for some time.


Yeast in Practice

Any of the topics I’ve covered could easily fill its own article. But what does it all mean for the practical homebrewer? We can break it all down into five easy guidelines:

>> Use the right yeast strain for the job. Fermenting a German Hefeweizen with an American ale strain may very well create good beer, but it won’t be Hefeweizen.

>> Use enough yeast cells for the job. Calculate an appropriate pitch rate based upon the volume and strength of your wort (brewing software makes this a breeze). Five gallons of average-strength wort may be fine with just one yeast packet, but higher gravities and higher volumes require more yeast.

>> Give yeast the right tools for the job. Most of the nutrients that yeast cells need are readily available in wort. As long as you provide plenty of oxygen up front, they’ll treat you well. Add a pinch of yeast nutrient if you’re nervous. Healthy yeast is happy yeast.

>> Ferment at the right temperature for the job. Know the optimal temperature range for your yeast strain of choice and stay within it. This might mean setting up some form of temperature control or even brewing certain styles only at certain times of the year. But your efforts will be repaid in better beer.

>> Give yeast enough time to do the job. Waiting may well be the hardest part of all. We’re all eager to taste our first or latest batch of beer, but remember that yeast cells operate on their own schedule, not ours. An English bitter might be ready to bottle in five days; a persnickety saison may need five weeks. With time and practice, you’ll be able to predict how your yeast will behave.

Treat your yeast well. Give it food, oxygen, and time. But above all, give it respect. And you will be rewarded.