When most people discuss beer, they speak of its flavor, color, hop aroma, alcohol content, and style in which it was brewed but rarely do they ever discuss one
of the most critical ingredients of beer. This ingredient is arguably the most important of all the ingredients that make up our beloved beverage, and can be directly linked to our survival throughout the ages. This special ingredient shall be forever named yeast!
Yeast is the workhorse of all beer, but is often overlooked or ignored when one talks about and rates beers. There is an old saying, “Brewers make wort, yeast makes beer,” and without yeast, beer as we know it wouldn’t exist. Instead, we would be drinking on an overly-sweet and unpleasantly bitter beverage that would not give us the euphoric buzzed feeling we’re accustomed to. In this article, we attempt to simplify the complexity of yeast: how they work, the differences yeast can create in beers and how various regions produce different yeast samples, thus making the styles of beer that we all know and love.
The standard dictionary definition of yeast is “any of various small, single-celled fungi of the phylum Ascomycota that reproduce by fission or budding, the daughter
cells often remaining attached, and they are capable of fermenting carbohydrates into alcohol and carbon dioxide.” To put it more simply, yeast is a single-celled fungus that eats simple sugars to produce alcohol and carbon dioxide. The scientific name for brewer’s yeast is Saccharomyces, which comes from the Greek word “Saccharo,” meaning sugar and myces (mushroom or fungus). Yeast plays the beginning role of decomposition in nature and is literally found everywhere, living on the skins of fruits and vegetables, hibernating inside the bellies of bees, and even living on our skin.
Why does yeast aid in the production of tasty beverages such as beer? Before we can discuss yeast’s role in beer, we must first understand what beer is prior to adding yeast, or what brewers call “wort.” Let’s briefly go over how beer is made all around the world. The main ingredient is usually malted barley, which are barley seeds that have been allowed to sprout. Once sprouted, the barley is then heated to stop the germination process, which preserves the complex carbohydrates, or
starches, that would have been used if the seeds were to grow into mature plants. These carbohydrates are the beginning of what will become food for the yeast; because they’re too complex for standard brewer’s yeast at this stage, they are converted into simpler sugars through a process called mashing. This is when the barley is soaked in water at a specific temperature for a set amount of time. During the mashing process, natural enzymes in the grain start to break down the
carbohydrates into smaller chain carbohydrates, or what we would refer to as simple sugars. We measure the density of these sugars in the water by specific gravity, or in degrees Plato, for use in calculating the ABV (Alcohol by Volume) in the final product. The wort is now boiled and the recipe is followed as planned with various hop and/or spice additions. After boiling, the wort is cooled to a precise temperature–usually around 70° Fahrenheit–and placed into fermentation vessels where it’s inoculated with yeast cells. Within 12-24 hours there should be considerable evidence that the yeast are doing their job, or fermenting: an excessive amount of CO2
gas being released from the fermenter and a krausen (thick yellow/beige foam) on top.
Alcohol and CO2 are the main by-products of yeast fermentation. Most of us know what the alcohol does, and is one of the main reasons we enjoy drinking beer, but
CO2 is usually an afterthought. The majority of the breweries today allow fermentation to release the CO2 into the environment, and when it’s time to carbonate the beer, they purchase commercial CO2 and force it into the beer. Without CO2, beer would be a “still” beverage much like wine, and not have the bubbles and the frothy head that we enjoy. Before we were able to capture and pressurize CO2 into large steel tanks, there were only “cask conditioned” and “bottle conditioned” beers available, which are naturally carbonated by the yeast. This style of beer is becoming more and more popular today and is created a number of ways. The main way breweries today are naturally carbonating beers is by allowing fermentation to finish and then adding a precise amount of some type of sugar before it goes into its bottle or cask. This slowly starts fermentation up again, and the yeast start producing a small amount of CO2 and alcohol, just enough to carbonate a beer.
There are currently about 1,500 different yeast strains, but most brewers usually select from a small number of good, known strains that have been passed down for hundreds of years. The three easiest ways to categorize yeast are by ale yeast (Saccharomyces cerevisiae), lager yeast (Saccharomyces pastorianus) and wild yeast (Brettanomyces). Top fermenting ale yeast work best in our average comfort temperatures, 65°-75° Fahrenheit, create more fruity aroma and flavors (phenols and esters) and can take an average of ten days to complete fermentation. A lot of the new craft beers around today use ale yeast–IPA’s, hefeweizens, American pale ales, porters, stouts, and even kölsch. Bottom fermenting lager yeast prefers cooler fermenting temperatures, between 48°-58° Fahrenheit, and produces a more “clean” flavored beer, but can take up to forty-five days to complete fermentation. This is the yeast strain that has been favored over the years and is used to make most of
the larger commercial beers, such as lagers and pilsners. Wild yeast isn’t new to the brewing industry. It has always been a part of lambic production, as well as some of the funkier saison or farmhouse style beers. It wasn’t until the early 1900s, however, when Brettanomyces was identified as cause of spoilage in English ales, and thusly named “British fungus.” Brettanomyces, called Brett for short, is a form of yeast that is acidogenic and, when allowed to grow over time, creates acetic acid; this is the same acid that makes vinegar so tart. Recently, American breweries like Russian River, Lost Abbey, Jolly Pumpkin, Allagash, Cascade and many others have been welcoming this yeast into their brews, producing a complex character that can only be obtained by the use of Brett, other bacterial strains and usual long aging. Little is still known about the various strains of wild yeast and their actual reactions during fermentation, but the recent interest in sour beers is striking up significant scientific research.
Once the type of beer to be brewed is determined and the yeast strain has been selected, a specific amount of yeast needs to be “pitched,” which is referred to as the “cell count.” Wyeast, a large manufacturer of brewer’s yeast, claim that, “A good rule to follow is a million cells per milliliter per degree Plato.” This means that you need 20 million cells per ml for a 20 degree Plato (1.080 specific gravity) beer, or about 9.5 billion cells per pint for a 7% beer. That’s more than the total population of the earth for a single pint of beer. Once the yeast has been pitched, the lag phase begins. This phase occurs during the first twelve hours and mainly consists of the cells absorbing the oxygen, minerals and various amino acids which will help them process the sugars and bud (replicate) daughter cells during the main fermentation. After the lag phase, the yeast starts budding exponentially and begins processing the various sugars in the wort. There are a number of different carbohydrates in the wort, consisting of glucose, fructose, sucrose, but mainly maltose. The yeast cannot fully consume some of the more complex sugars such as maltotriose, which gives the beer a sweet and malty flavor.
As we near the end of fermentation, the yeast shuts down, clumps together and falls to the bottom of the fermentation vessel. This process, called flocculation, is unique to brewer’s yeast. Chris White, from White Labs explains, “The ability to flocculate is a product of natural selection. Brewers have continually collected yeast either from the bottom or top of a fermenter and in doing so, selected for increasingly flocculent strains.” The reason for this flocculation is to help clarify the beer. If the yeast is less flocculent, like in a hefeweizen, they stay in suspension and yield a cloudy and hazy beer. Wild yeast, such as Brettanomyces, do not flocculate well; this is why some of the newer style “wild ales” are recommended to age like a bottle of wine.
The rate at which the yeast eats the malt sugars is called “apparent attenuation,” which is measured by the percentage of sugars the yeast consumes. White Labs, one of the nation’s main yeast providers, says their California ale yeast strain is said to have a 73-80% attenuation, which means the yeast should consume 73-80% of the sugars in the wort. The higher the attenuation percentage, the less residual sweetness and body the finished beer will have. After fermentation is complete, the specific gravity, or Plato, is taken again, calculated with the original gravity, and the attenuation is measured along with the ABV. For professional breweries, if the attenuation of the yeast is not precise, then the final product will never be consistent and will fail at the quality control department. Yeast is very particular to the region in which they come from. If you have ever visited San Francisco and had the sourdough bread, you will know that it is unlike anywhere else in the US. This is due to the types of wild yeast and bacteria that are native within that region and the by-products they produce during fermentation. The exact same concept goes for beer.
Up until recent laboratory technology, breweries would use the same mature yeast strain time and time again, and it was considered their “house” yeast strain. This is what gives some of the older brew houses in Europe such a unique smell and flavor. Now with modern transportation, we can safely ship yeast samples from all around the world without worry of excess heat, time, or contamination of some other yeast or bacterial strain. Today, Belgian style beers that are made in America are mostly using yeast strains that have been brought over from Belgian many years ago.
These regional differences inhibit many different styles of flavors and aromas in the beer. These flavors and aromas are referred to as phenols and esters. Consider the difference between American pale ale and a German hefeweizen: these are found more in the esters produced by the yeast than in the wheat added in the hefeweizen. Hefeweizen, literally meaning “yeast wheat,” has a slight banana or clove essence that’s caused by the ester, Isoamyl acetate. Hefeweizens are fermented at the warmer end of ale temperatures and use low flocculating yeast from Germany to produce these robustly-flavored esters. Most American pale ales are fermented at a slightly lower temperature and use cleaner-flavored and more flocculent yeast from California. Another common flavor found in some English and Belgian Style ales that are strictly due to yeast are green apple/pear ester, or Isoamyl butyrate. The human tongue is very sensitive to these flavors, which means they can be very unpleasant at higher concentrations. Wild yeast, or Brettanomyces, is sometimes blamed for ‘solvent’-like flavors in beer, but these can come from commercial yeast strains as well.
Besides the production of Ethel alcohols, yeast is the microscopic fungus responsible for providing the bubbles, the alcohol and many of the aromas and flavors in beer. In this light, yeast can easily be argued as being the most critical component of making beer. Many breweries keep their yeast strains under lock and key with high levels of security, while others simply use the wild yeast that just happens to be floating in the air around their region. Without these single-celled organisms, there would be no beer, wine, cider, mead, hard spirits, bread, doughnuts, or even pizza to enjoy. It is time to give the hard working yeast cells the attention and respect that they deserve. The next time you sit down to enjoy your favorite craft beer and criticize its flavors and aromas, don’t forget it’s not just about the hops and malt. Trillions upon trillions of microorganisms created that oh so tasty beverage.