Yeast and Fermentation
Yeast and Fermentation
Yeast are the main drivers of quality beer making. Yeast ferment malt sugars, creating carbon dioxide and alcohol as by-products.
During fermentation, yeast produce a whole range of flavoring compounds, including esters, phenols, and a large variety of other chemicals. These compounds will dramatically change the character of the final beer. Comparing a traditional German Hefeweizen to an American Blonde Ale, the main flavors of each are directly related to the specific strains of yeast chosen.
Yeast are a fungus, and there are millions of different types. Brewers are only concerned with two species, saccharomyces cerevisiae and saccharomyces pastorianus, ale yeasts and lager yeasts, respectively. The two strains are mainly differentiated based on their general flavor profiles and ideal fermentation temperatures (ales typically being fruitier, warmer-fermented and lagers being cleaner and cooler-fermented), and give the two main divisions in beer styles: ales and lagers.
After cleanliness and sanitization, controlling fermentation is the most important aspect making good beers. This entails choosing the correct strain of yeast, treating it well, and giving it all the conditions it needs to go through fermentation in a healthy manner.
Fermentation occurs in three stages. First, the yeast is pitched into the wort. Here, yeast absorb dissolved oxygen, and use various minerals that exist within the wort to build up cell walls. Yeast then begin splitting, increasing cell count for the conditions they are in. This is the phase where most flavor compounds are created, and will last anywhere from 6 hours to two days. This delay timeframe is referred to as lag time.
The second stage is active fermentation, wherein yeast actively digest sugars, creating alcohol and carbon dioxide. This is the critical stage where temperature will play a big role. Keep the temperature within the range specified by the yeast, otherwise the beer may develop off-flavors (if fermentation gets too hot) or stall out (if fermentation becomes too cool). This stage is visible; a krausen will form, and the airlock should bubble consistently. Krausen is the term for the thick frothy layer that builds up. It consists of proteins, by-products, and yeast cells being pushed up by produced carbon dioxide. This stage typically lasts 5-10 days.
The third and final phase is conditioning. Yeast will drastically slow down and chew through the final sugars. Visibly, the krausen will begin to fall and sink back into the beer, and a layer of trub will develop on the bottom. After, yeast will clean up harsh by-products produced during the first two phases. This stage is important, as it will help the beer taste “cleaner”, with less off-flavors, and generally help the beer taste better. This stage will typically last 4-7 days, though this is highly dependent on alcohol percentage. For this reason, best practice is to keep the beer in the primary fermenter for at least 2-3 weeks before bottling or kegging.
There are several terms that are important to know when making a yeast selection. These include temperature, pitching rate, attenuation, alcohol tolerance, and lag time.
Fermentation temperature refers to the ideal temperature require to ferment at to produce expected results. Keep in mind that fermentation is exothermic, it will create extra heat. If fermentation is planned in a 62°F ambient room, during active fermentation, the temperature may reach as high as 72°F. Typically a 2-4°F increase can be expected. Fermenting too hot will yield higher-then-expected ester levels, and possibly other harsh fusel-related off-flavors. Fermenting too cool will yield a cleaner profile, but may stall a fermentation. When checking temperature of the beer during fermentation, it is best to measure the beer temperature itself, rather than relying on ambient air temperature. Refer to the ideal temperature of the yeast you are using on the package, and try to stay between the range for best results.
Pitching rate refers to the amount of yeast cells being added to the beer. Under-pitching will cause the yeast to split-more, causing excess ester production and possibly straining the yeast, causing harsh off-flavors. Over-pitching is much less of a worry, the downside being a cleaner profile from less cell-splitting. The common adage is 1 million cells per milliliter per degree plato (plato is a measure of sugar content). Generally, 1 dry-yeast pack will ferment 5 gallons of ale up to an original gravity of 1.060. More cells should be pitched for higher gravity batches, larger batch sizes, and lagers.
Attenuation is the percentage of sugars the yeast strain is likely to consume. This will alter the final mouth-feel of the beer and total alcohol. Yeasts with low attenuation will eat less sugars, resulting in a thicker, lower-alcohol beer. High attenuative yeasts will create drier, crisper beers with more alcohol.
Alcohol tolerance is the ability of the yeast to withstand alcohol. Alcohol is a poison to yeasts, but some yeasts are better at handling larger amounts of alcohol then others. Generally, alcohol tolerance will range between 5-15% ABV. Similar to attenuation, choosing the right yeast with the correct alcohol tolerance is mostly a matter of beer style.
Lag time refers to the amount of time between pitching the yeast to seeing visible signs of fermentation. This is essentially the first stage of fermentation. The amount of time is dependent on several factors, including pitching rate, temperature (both of the wort and yeast at pitching time, and ambient temperature the beer is sitting at), yeast strain, gravity, and available nutrients. Typical lag times range between 6 hours to 2 days. If your lag time persists past 2 days, you may want to re-pitch your yeast.
When making a yeast selection, knowing the above terms is helpful, as well as know the general type of yeast, and associate flavors, one can expect. For example, some ale strains can be very clean and are good for letting hop or malt flavors shine. Others, however, will be very flavorful (such as Belgian strains), and should not be used in certain styles. Generally, keep yeast strain choice close to style choice (e.g. use an American ale strain for an American ale).
Yeast come in two forms: dry and liquid. Dry yeast looks like small pellets, and come in 11.5 gram sachets which will ferment 5 gallons of beer up to a gravity of 1.060. Dry yeast has a very long shelf life, and can be pitched directly into wort. However, there exist only a handful of different strains of dry yeast.
Liquid yeasts come in a variety of packages, and usually will ferment 5 gallons of beer up to a gravity of 1.048. Despite the lower cell-count of liquid yeasts, there is a much larger variety of strains available. Additionally, pitching rates can be customized more easily by creating a starter wort in order to increase cell count.
Ensuring healthy fermentation
As previously stated, the best way to ensure beer will turn out well is to ensure a healthy fermentation. This comes down to three key points: aerating adequately, pitching enough cells, and controlling temperature.
Aerating adequately is a simple process, and an easy way to ensure happy yeast. Common off-flavors that develop from a failure here usually revolve around a harsh, strong fruity flavor, and a slight astringency. These flavors are commonly referred to as “stressed yeast flavors”, as they are directly related to the stress yeast have when oxygen is not readily available to them. If you encounter this flavor, and are aerating by stirring your wort, you may be better off tossing the wort between a couple sanitized vessels. This method allows much more splashing to occur, allowing quicker and more efficient aeration. If using an aeration system, make sure you leave the system on for a long enough time. With the two above methods, you cannot overdo aeration.
Pitching enough cells can be a bit complicated, as the answer to the question “how much is enough?” is dependent on a lot of factors. Gravity, batch size, ale or lager, specific strain, and even beer style all play a role here. For most beers, however, there are a couple rules of thumb. For dry yeast, if your gravity of a 5-gallon batch is above 1.060, pitch 2 packets. If the gravity is above 1.090, pitch 3 packs. Generally, over-pitching is much more difficult than under-pitching yeast, thus if you are unsure, pitch more, rather than less, cells.
For liquid strains, the best practice is to make a starter tailored to the batch size and original gravity of your batch. A starter is a small wort that is used to increase cell count. After yeast is pitched in the starter, it is allowed to ferment for 2-3 days, then pitched into the main batch of beer.
The process for making a starter is simple. Dissolve dry malt extract (DME) in water at a rate of ½ cup DME per pint of water (if you have a scale, use 100 grams DME per liter of water). This is then boiled for a couple minutes to sanitize, then cooled and inoculated with yeast culture. Typically, flasks are used for the starter, as they have convenient volume markings, and (if made from a borosilicate glass), can be boiled inside of. During the starter fermentation, the flask should be swished consistently, allowing for plenty of oxygen for the yeast. Because of this, some homebrewers will employ a stir plate for their starters. Stir plates spin a small magnet inside the flask, allowing yeast access to a large amount of oxygen. The more oxygen contact yeast have in a starter, the more yeast cells will be produced from the starter, and the smaller the starter is needed.
The easiest way to determine the specific size of starter needed is to run your numbers in a yeast pitching rate calculator. Input the batch size (volume into the fermenter), target original gravity, and mode of aeration (simple starters will not be stirred, intermittent shaking will be stirred every time the brewer passes by it), and the calculator will output size of starter, and number of yeast packages needed.
Finally, temperature control during active fermentation will help ensure the yeast don’t stall out or cause heat-related off-flavors. A simple method is to find an area in your house that is dark, has a consistent temperature (watch out for day-to-night temperature swings, yeast do not like this), and is in the range the yeast tolerate. For ales, usually a closet is a safe bet, or a garage (provided the garage is insulated). Brewing for the season is also a good bet; American ales can typically be fermented cleaner during spring and fall time, Belgians (especially Saisons) can be kept warmer summer months, and lagers can be stored cool during the winter.
There is a large array of various pieces of equipment to help control fermentation temperatures, from heating wraps and pads, insulated jackets, and others. One common, very precise method involves having a dedicated refrigerator or chest freezer hooked up to a temperature control unit . A probe is place in a thermowell in the beer, and the control unit turns on and off the refrigerator or freezer to maintain a specific, constant temperature. While not the only way to have full control over fermentation, it is one of the more effective means.
For more in-depth information on yeast and fermentation, Yeast by Chris White and Jamil Zainasheff is a very comprehensive resource.
The Wild Side
Recently, a popular trend in craft beer and homebrewing is the creation of sour beers. These beers use “wild” yeasts and bacteria to create a beer with a sour bite and a variety of flavors and aromas not commonly seen in “clean” (non-soured) beers. Common flavor descriptors include fruity, dank, sweaty, musty, or smokey.
Sour beers are commonly fermented with a saccharomyces strain in conjunction with a blend of brettanomyces (the classic “wild” yeast variety), lactobacillus, and pediococcus strains. The “bugs” can be added along with the clean yeast, or added during secondary. Either way, the bugs take time to make their by-products, thus sour beers are typically aged a minimum of 3 months before bottling.
Those looking to brew sour beers or seek further knowledge on the subject should consult American Sour Beers by Michael Tonsmeire.