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Fermenting High Gravity Beers

Fermenting a batch of high-gravity beer can be a challenge for homebrewers because it can also be a challenge for yeast. Yeast in a high-gravity brew not only has a lot of work to do, but the work is not easy. To successfully brew a high gravity beer, brewers must set the stage for the yeast to have the best chance of getting the job done. This installment of “Techniques” examines what it takes to brew big beers that begin between 15 and 20 °Plato (specific gravity of 1.061—1.083). We’ll also look at some special considerations for brewing beers of more than 20 °Plato (specific gravity > 1.083), big beers with high percentages of adjuncts and what to do if fermentation slows down or stops before a big beer is finished.

Big beer basics

To increase your chance of success when fermenting a big beer it is important to choose the right yeast for the job, pitch an ample amount of yeast, provide the yeast with adequate aeration and nutrition, control fermentation temperatures and don’t let the yeast quit until the job is done! Not just any yeast is suitable to ferment a high gravity beer. It is important to select a strain of yeast that not only produces the desired flavor profile and degree of attenuation, but can also tolerate the level of alcohol expected in the finished beer. Information on specific yeast strains is available on the Web at (www.byo.com/resources/yeast), at White Labs at (www.whitelabs.com) and Wyeast at (www.wyeastlab.com).

Once you choose the strain (or combination of strains) of yeast to ferment your high gravity wort, it is critical to send in a large enough workforce of cells to tackle the task at hand. According to the suggested pitching rates shown in; “Pitching Rates for Fresh Yeast” Table 2 “Pitching Rates from a Fresh Starter” at http://www.byo.com/resources/pitching, an example wort with an original specific gravity of 1.080 calls for 440 billion yeast cells. A large pack or vial of yeast may contain around 100 billion cells. Thus to pitch an adequate amount of yeast would require three times that amount at a minimum. In this case, building a big starter culture is the best way to go.

Preparing yeast starters is not a difficult task. However a big beer needs a big starter . . . of about a gallon (3.7 L) in size rather than the typical a one-quart (0.9 L) measure of yeast used for most brews. Wyeast and White Labs liquid cultures typically contain approximately 100 billion cells, which is adequate to pitch into a full gallon (3.7 L) of at least 10 °Plato (specific gravity 1.040) starter. Be sure the wort used for your starter is well aerated. A good way to assure good aeration is to continuously aerate the starter (using filtered air supplied by an aquarium pump) as the yeast grow. A typical starter culture usually ferments in a day or two. After fermentation is nearly complete, turn off the aeration (and magnetic stirrer if you’re using one) and the yeast should settle to the bottom of the vessel. This way, most of the liquid can be decanted off and the remaining yeast slurry pitched into the high gravity main batch of wort.

The next critical element to a successful high gravity fermentation is supplying oxygen and nutrients in the wort that the yeast cells need for growth early in life while making sterols and unsaturated fatty acids crucial for building cell walls. Wort aeration can be accomplished by bubbling either air or pure oxygen through the wort cooled to fermentation temperature. For wort of 15 °Plato (specific gravity of 1.061) fifteen minutes of air or two minutes of oxygen should be sufficient. The nutrients contained in all-malt wort along with the nutrients included in a smack pack of liquid yeast should be sufficient for beers up to 20 °Plato (specific gravity of 1.083).

Once fermentation has begun, temperature control is the next important consideration. If your batch of brew gets too warm, you may end up with more estery (fruit) or phenolic (clove) flavors than desired . . . too cool and your fermentation may slow or even stop. Typical temperatures recommended by Wyeast for their high-gravity yeasts are 65­–80 °F (18–27 °C) and those from White Labs are in the 65-75 °F (18-24 °C) range. So, make note of the fermentation temperature range recommended for your chosen yeast and take measures to assure your big brew stays in that range.

With all the work yeast is required to perform in a high-gravity beer, sometimes the yeast may fall dormant before the job is done despite our best efforts as brewers. Should this occur, you can use the time-honored trick of, “taking the yeast for a walk” where barrels of fermenting beer were rolled about the brewery to rouse the yeast back into suspension and wake them up for a final phase of activity to finish the job. While many of us do not ferment our beer in barrels that can be rolled around, a sanitized spoon or racking cane can be used to stir up the beer and get the yeast back in suspension and in contact with any unfermented sugars that might remain in the beer.

Tactics for really big beers

For a really big beer or barley wine with a starting gravity of 20 °Plato (specific gravity > 1.083) or more, consider pitching multiple strains of yeast together at the start to assure complete fermentation and achieve a more complex flavor profile. While it is essential to have a strain of yeast in the mix that can tolerate higher levels of alcohol, a less alcohol-tolerant strain can contribute its own flavors up until the alcohol content of the beer exceeds its limit. At that point, one or more alcohol-tolerant strains can carry on until the desired level of attenuation and alcohol production is achieved. Do not be tempted to use a wine or champagne yeast to finish out fermentation, such yeast could finish too dry, leaving your beer without the desired flavors available from beer yeast. If you need a strain to serve as a “closer,” consider one of the Trappist high gravity, Belgian strong ale or Abbey ale strains that can tolerate 12–15% alcohol by volume while not straying from the desired flavor profile.

As the concentration of the wort increases, so should oxygenation. For wort of 20 °Plato (specific gravity 1.083) or more, aerate up to an hour with air, or ten minutes with oxygen to be sure oxygen is not lacking for the yeast to grow. While additional aeration after the start of fermentation is often discouraged in brewing as it may result in off flavors, it may be useful to supply additional oxygen to your wort up to the point of high kräusen (peak of fermentation activity). This can be accomplished by simply inserting a sanitized aeration apparatus into the fermenting wort and briefly bubbling in oxygen or filtered air. By briefly, I mean less than two minutes using oxygen or less than ten minutes with air. This can give the yeast a boost without running the risk of developing off-flavors associated with late aeration (cardboard or sherry-like flavors). Additional aeration during fermentation should typically not be necessary if a sufficient quantity of yeast was pitched at the start of fermentation.

Tactics for big beers with adjuncts

In addition to the information mentioned earlier for really big beers, there are some additional considerations when brewing a big beer that includes a significant amount of starchy adjuncts in the mash or additional sugars added to the boil. Though oxygen is essential to yeast reproduction, proper nutrition is also important to yeast health. Yeast requires a certain amount of nitrogen in the form of amino acids and fatty acids to multiply and grow. While yeast get most of these compounds from the barley malt used to make wort, other sugar added to take the place of barley malt often do not include nitrogen and other essential yeast nutrients. Therefore, the addition of yeast-specific nutrients becomes necessary to assure that nutrition will not be lacking.

There are four types of yeast nutrient typically sold for use in home brewing; Di-ammonium Phosphate, yeast hulls, yeast nutrient and Servomyces. Yeast nutrient and Servomyces include vitamins, minerals and other compounds that provide the most complete yeast nutrition of the products listed. To borrow an approach used by vintners, some of the yeast nutrients may be added during fermentation to help the yeast along. A portion (half) of the yeast nutrients might be added to the boil and the remainder added after the commencement of fermentation. Yeast nutrients should not be added once the beer has achieved about two-thirds of the total expected attenuation. Use the amount of yeast nutrient recommended by the supplier (this is usually shown on the package). Another potential yeast nutrient is yeast itself. I sometimes have dry yeast that has past its expiration date. Instead of throwing it out, I will add the old yeast to the boil as a yeast nutrient because it contains the nutrients living yeast need to grow and ferment wort.

Tactics to help get big beers to the finish line

There are times when high gravity fermentation may slow down or stop before reaching the desired terminal gravity. In such circumstances the brewer may have to provide a boost to get things going again. One approach to kick starting a sluggish fermentation is to make a small batch of similar strength wort, aerate it, pitch a “closer” strain of yeast into it and get it to a high state of fermentation (kräusen) before adding it to the main batch. This technique can recharge a slowly fermenting beer and get it back into gear to help it reach the final desired level of attenuation. If you plan to add this type of boost, or supplement other fermentables during the primary or secondary stages of fermentation, be sure to allow space in your vessel for the added volume, plus headspace for any foam produced during the renewed activity.

There is no doubt that fermenting a high-gravity beer requires more work for both the brewer and the yeast. The challenge for the brewer is to create an environment for the yeast to have the best chance of success. Careful planning is necessary to choose the right strain(s) of yeast, and then provide proper aeration, nutrition and temperature for the yeast to reproduce and grow. The brewer must be committed to monitoring and managing fermentation through the possible addition of oxygen, nutrients or fermentables . . . and rousing the yeast if necessary to reach the goal of superb beer. Despite the extra effort it takes to ferment a high-gravity beer, your efforts will be well rewarded with a product that is in a class by itself.