As brewers we take great consideration in selecting the ingredients we use for our recipes, because it is the recipe that defines the beer we make. Developing the recipe and deciding which ingredients would work best in a beer is the most creative and rewarding aspect of brewing, and the result is the brewer’s signature.
Each brewery’s product has a unique character partly from the brewer’s signature and recipes, and partly from the house strain of yeast. Choosing a strain of yeast for a particular beer style is an integral component of recipe formulation, whether you’re making the choice for a production brewery or a homebrewery.
The National Collection of Yeast Cultures in the United Kingdom catalogs more than 500 strains of Saccharomyces cerevisiae, and this list is not all-inclusive. Choices abound, and the brewer’s selection of strain has as great an influence on beer flavor and aroma as any other ingredient.
There are several features to consider in choosing a strain. These include flocculation, flavor and aroma characteristics, production of fermentation byproducts, attenuation, rate of fermentation, style appropriateness and, above all, the brewer’s experience with the strain.
Flocculation, a yeast’s ability to clump and settle at the end of fermentation, can help determine the clarity of the beer. Flocculation ability is generally classified as low, medium, or high. Many English and American ale strains have medium to high flocculation characteristics. Most lager strains are low to medium flocculators, and hefeweizen yeast and many Belgian strains are low flocculators. If you are brewing a hefeweizen, the obvious choice is a low-flocculating weizen yeast, because some of the yeast should remain in the beer.
Flavor and Aroma
The next factor to consider is the flavor profile. Are you brewing a wit, a German weizen, or an American wheat? Each yeast is unique. You wouldn’t expect the clove and banana character in an American wheat, and it would be just as inappropriate not to have that character in a German wheat. Most American ale yeast strains work well in American hefeweizen, but a low flocculator would be the best choice, and there is an American hefeweizen yeast available on the market. The defining character of the style is its turbid appearance and mild fruity esters. The defining character of a German weizen is the clove and banana aroma, and this is what differentiates it from American hefeweizen. The Belgian wit yeast is similar to the German yeast but leaves a crisper, tart character.
These yeast strains are unique in that they are not desirable in a wide range of beer styles — an American hefeweizen yeast usually is used only for American-style hefeweizens. Many other strains are versatile, and perhaps the most common and versatile are the American ale strains. There are numerous American varieties, a few of which are available for purchase by homebrewers. It has become quite common for homebrewers to collect a pitch from the local brewery, and many breweries have proprietary strains.
American ale yeast strains generally are medium to high flocculators and leave a clean, slightly fruity finish. Ester profiles vary from strain to strain. The finish tends to be somewhat malty but not quite as malty as most English ale strains. If your pitch comes from a local brewery, ask the brewer about his yeast strains’ characteristics and the origin of the yeast. Of course, the best way to determine whether you wish to brew with a yeast strain is to try beers brewed with it.
Other varieties, such as English ale yeast strains, leave a sweeter, maltier profile than the American, but the strains are somewhat interchangeable. An English-style ale can be brewed with an American yeast and vice versa. A stout, for instance, can be brewed with an Irish stout yeast or almost any of the American or English choices. Keep in mind, however, that while each of these may be appropriate for the style, each will produce a different beer. One will be sweeter than the next and accentuate malt more or less, and levels of byproducts such as esters (fruity) and diacetyl (buttery) will differ.
Fermentation byproducts are another aspect of flavors and aromas. Byproducts include diacetyl, esters, fusel alcohols (spicy, wine-like), and other products. The advantage of using an English ale yeast when brewing an English style is the maltier profile it produces, but some English strains have the tendency to produce detectable levels of diacetyl. While diacetyl is not necessarily a desirable character, very low but detectable levels are acceptable in many English ales. Some American ale strains are also very prone to diacetyl production. If you wish to brew a beer without this flavor profile, it would be best to select a different yeast.
Degree of Attenuation
Degree of attenuation directly affects the flavor of the final product and is one of the most important factors to consider. Attenuation is the process of sugars in the wort ransforming into alcohol and carbon dioxide through fermentation. Knowing a strain’s degree of attenuation helps the brewer predict the final gravity of a given beer. One yeast might complete fermentation at 1.010, while another stops at 1.014 given the same wort.
Compare degrees of attenuation among strains. The less attenuative a yeast strain is the higher the final gravity will be and, thus, the more unfermented sugars will remain in the beer. The amount of this residual sweetness in a beer dramatically affects the final flavor.
A brewer needs to know before the brew begins whether the yeast is a poor attenuator or a high attenuator. To produce beers of similar terminal gravities, a brew fermented with a yeast with a low degree of attenuation will have to be mashed cooler than one fermented with a high degree of attenuation.
Attenuation may be listed as low, medium, or high, or it may be represented as a percentage range. For instance a California ale yeast is listed with an attenuation of
73 percent to 80 percent, which is fairly high. An English ale yeast from the same lab is listed as having an attenuation of 63 percent to 70 percent, which is quite a bit lower. Expect a much sweeter, maltier product from the English yeast than the Californian. To brew a beer with the same malty character that the English yeast would produce, the mash temperature would have to be adjusted upward by a few degrees to compensate for the higher fermentation characteristics of the California yeast.
Rate of Fermentation
Rate of fermentation generally does not concern the homebrewer to a great degree but is related to the degree of attenuation. Slower fermenters are generally lower attenuators and produce maltier beers. Fermentation rates of various strains are hard to come by in any literature available to homebrewers.
It would be nice to know if your beer is going to be ready for that bachelor party next month, but you used a Vienna lager yeast and your batch is chugging along very slowly at a rate of one or two points of gravity a day. Had you known that it was a slow fermenter and low attenuator, you might have chosen another yeast. Now you have two worries. Will the beer be ready on time? And should you have dropped the mash temperature three or four degrees to compensate for a low attenuator so the finished beer won’t turn out overly sweet?
Style and Experience
Two remaining factors to consider are the appropriateness for style of a particular strain and the experience of the brewer with that strain. All of the factors described above lead the brewer to determine whether a strain is appropriate or not. If you wish to brew that German hefeweizen, then you must choose a strain with the characteristics expected of the style. The best way to assure that whatever style you brew is as close to the authentic style as possible is to use the yeast suggested for that style, such as a Czech yeast for a Czech pils and an English yeast for an ESB. If you prefer not to use the authentic yeast or if it is unavailable, look for yeast strains with similar characteristics and adjust your brewing processes to compensate for any expected differences.
While reading the literature about a strain is helpful, nothing is more important than the brewer’s experience with that yeast. The brewer needs to know the yeast well to be able to predict with confidence what the terminal gravity will be and how the final beer will taste and smell. There is nothing wrong with experimentation for the fun of it and for the learning experience. But when you want to brew a certain style of beer, arm yourself with the information to achieve your goal. Nothing is more rewarding than achieving what you expected because you knew what you were doing.
Larry Lesterud is head brewer at Humboldt Brewing Co., Arcata, Calif.