This is the Tech Article I wrote on Yeast and Fermentation for our BJCP Exam study group earlier this week. It is long, but not overly technical:
Pitch Rate & Health
- Most of the off flavors/flaws in a beer are directly connected to yeast health and pitching rates (as well as fermentation temperatures), making yeast care and fermentation the most important factors in brewing aside from sanitation. In order to properly ferment a beer wort and avoid all the flaws associated with poor fermentation you need to ensure that your yeast are healthy and that you have the proper amount of them, as well as the ability to control the temperature of the fermenting beer. Old yeast will not be sufficient to fully ferment a beer and clean up after itself; neither will pitching too low of a cell count. Yeast should be pitched at a rate of 0.75 million cells of viable yeast, for every milliliter of wort, for every degree plato, 1.5 million for a lager (Jamil Z, Mr Malty.com) As the gravity of the beer increases so does the amount of healty cells required, as with volume as well. In order to achieve the correct amount of healthy cells you have a number of options. You can brew a low gravity beer such as a Mild, Bitter, or APA, then use some or all of the yeast cake for the larger beer. You can also make a starter by fermenting a mini-batch (1 pint to 1 gallon based on the beer to be fermented) then use that yeast to ferment the main beer (a stirred starter of roughly 1.040 OG works best). Or you can go to a local brewery that has a yeast which matches your needs and ask for a pitch of their slurry. Consult an online pitch rate calculator for the proper amount of yeast to pitch as well as the best way to achieve this.
- You should have enough fresh yeast to not only finish the job properly but to start it quickly to avoid contamination from bacteria, wild yeast, or molds which all want at your sugary goodness as well.
- In order for yeast to have the optimal health and ability to reproduce it requires nutrients which are typically found in standard beer wort but can be added through using commercially available nutrient blends. It also requires oxygen for reproduction which the beer wort is highly deprived of as it has been driven off in the boil. Oxygen should be added to the beer prior to fermentation for proper yeast growth.
- Each yeast has a temperature range that it is best suited to ferment in. If yeast gets too cold it will go dormant leaving unfermented sugars as well as failing to clean up off flavors like aceteldehyde or Diacetyl. It could also finish the job but not be very vigorous which could leave compounds like DMS in the beer. If the temperature gets too high it could cause excessive phenols, esters, Diacetyl (Palmer), and/or fusel alcohols, as well as causing the krasuen to blow through the top or your fermenter leaving a huge mess. Excessive temperature fluctuations can cause both of these to happen: if the temp goes too high it could cause phenols, esters, fusels, or Diacetyl, then dropping too low, would leave the beer an unfermented mess with lots of off flavors. Keeping temperatures at the low end of the range for the strain should keep the esters lower, while fermenting near the high end of the range would lead to higher ester production.
FLAVORS AND OFF FLAVORS
- Ale yeasts are known for the fruity aromas and flavors they contribute to a beers profile. These yeast-created fruity flavors can range from subtle to fully expressive. The range of fruits is broad: red apples, pears, peach (Conan, West Yorkshire), strawberry, grape, citrus, and banana (the typical ester in German Hefeweisse and Belgian Beers). Lager beers do not typically create these esters when used at their proper temperature range. If a lager is fermented too warm it will have esters and this is considered a flaw in Lagers.
- Like Esters, Phenols are created by yeast during fermentation and can range from subtle to fully expressive. Phenols can be tasted and smelled as well as felt in the mouth. Phenols are the spicy character in the beer like cloves (German Weisse and many Belgians), as well as cinnamon, black pepper, ginger, vanilla, and more. In most beers this character is undesirable, but in German Hefe the clove is a requirement of the style, and in Belgians, especially Saisons, phenols are desired. Phenols should never be present in Lagers, and in most Ales, and can be created by too high of a fermentation temperature. There is another set of phenols which present as smoke, bandaid, burnt rubber, or plastic which are a flaw in any beer.
- As yeast consumes the sugars present in wort it produces ethanol, an alcohol. This alcohol can have different flavor ranges depending on the temperature of fermentation, the malt bill, the additions of sugars, and the starting gravity of the beer. Sometimes alcohol can have a lingering spicy character in the finish, but should never be HOT. A HOT beer is a flawed beer (unless it contains peppers). Alcohol can also take on a rose water flavor/aroma.
- If fermentation gets too hot the yeast can create fusel alcohols which are higher alcohols that for the most part are flavorless, but are thought to be linked to hangovers. Along these lines are the hot alcohol flavors found in beers that were too warm during fermentation. As mentioned above, a "HOT" beer is a flawed beer. Controlling fermentation temps will keep this from happening.
- This is the “Green Beer” flavor which is created as a byproduct of fermentation; it tastes like green apple jolly ranchers and has a distinct mouthfeel. Ramp up temp a few degrees near the end of fermentation or allow beer to remain on yeast a little longer so they can clean it up. Once the beer is removed from the yeast, or they go dormant (cold temps), this will remain in the beer.
- Tastes like movie theater popcorn butter and has a distinct mouthfeel. It is created as a byproduct of fermentation; ramp up temp a few degrees near the end of fermentation or allow to remain on yeast a little longer to have them clean it up. Once the beer is removed from the yeast, or they go dormant (cold temps), this will remain in the beer. Can be caused by a fermentation temp that exceeds the standard temp range for the yeast, or infection.
- During fermentation yeast is active and moving about fluidly in the wort converting sugars. As the process proceeds the yeast will either stay suspended in the beer or drop out of solution. Most beer styles have clarity as a much desired aspect of the beer (Weisse and Wit being the largest exceptions). Each yeast has a flocculation rating based upon its natural propensity to drop out of solution in a timely manner without assistance. A yeast such as 1968 London ESB is rated as highly flocculant which means that the yeast cells drop out of solution quickly and easily leaving behind a very clear beer. Sometimes it is too quick to flocculate leaving the beer under-attenuated and with off flavors (rouse the yeast cake back up and raise the temperature of the beer). Other yeasts like Weissen strains do not want to come out of solution without assistance from fining agents (additives of chemical or organic make-up) which grab onto the yeast cells and take them out of suspension. High flocculance means that the yeast will come out of solution easliy and readily, Medium takes longer, Low can take an extended aging at cold temperatures or finings to remove.
|Hefeweisse Yeast vs English Fuller's Yeast|
- Attenuation is the degree to which the yeast is able to convert the sugars present in the wort into alcohol. This is registered as apparent attenuation, which measures the residual sugars after fermentation (FG) in relation to the original sugar content (OG), and is displaced by the lower density of alcohol which impacts the readings given. This is usually expressed as Low Attenuation (65-70%), Medium Attenuation (71-75%), and High Attenuation (76-80%) (Palmer). Yeast companies are typically reserved in all of their numbers for their yeast, many of which can exceed the manufacture’s specs. 1968 is rated at 67-71% but can get into the mid to upper 70% range. 3724 is rated at 76-80% but easily achieves 88% without sugars. Attenuation plays a key role in the final outcome of a beer. If the attenuation is too low it can leave the beer too heavy, full bodied, sweet, and/or low on alcohol content for specific style requirements. If the strain is highly attenuative it could leave the beer too dry, watery, and with a much higher ABV than desired.
- Each strain has a character in the beer to which it focuses. Some strains, like American/Cal Ale is known for subduing malts and pushing hops to the forefront making it many brewer’s yeast of choice for IPAs. Other strains restrain hops and downplay bitterness while bringing the malts to the center. Some can create a dry beer with a fuller mouthfeel such as West Yorkshire. Some yeasts have a mineral character like 1028 London Ale. This is not just typical of Ale strains, but Lager strains as well. Even with the same malt bill and FG a beer can taste fuller, or lighter, crisper and dry based on the yeast selected.
CLEAN YEAST STRAIN FAMILIES
- American Strains: Typically a clean character with very low ester production, and no phenols. These strains can push some temperature boundaries and still not produce off flavors, though going too high with any strain will make a bad beer.
- English Strains: These yeasts have a more characterful expression than American Ale strains lending to more soft esters in the apple, peach, and pear range, as well as nuttiness, and some mineral contributions.
- Belgian Strains: These are the most expressive yeast in the Ale family, they are known for their higher ester and phenol production, and ability to create complex alcohol notes without creating fusels while at a higher fermentation temp than other Ales strains. These should not be allowed to run wild though as they can still create too much expression or high alcohols if used to warm.
- Lager strains are fermented in lower temperature ranges, should be very clean (no esters or phenols), and due to the lower temperatures can require a Diacetyl Rest and require a larger yeast pitch than Ale yeasts. Some of the yeasts can produce sulfur compounds that will remain in the beer if the fermentation is not vigorous enough.
- Hybrid strains such as Cal Lager, Kolsch, German Ale, and others have the ability to ferment very clean like a lager strain at Ale temperatures, as well as at higher Lager temps. These strains can be used to produce Lager style beers without the temperature control needed for Lager strains, though an extended cold storage time after primary is usually done with these strains as with Lager yeasts.
WILD YEAST AND BACTERIA
- These yeasts are in a class of their own. They can ferment beer wort on their own just like an Ale or Lager strain lending many fruity contributions like pineapple, mango, cherry, and peach. When used in conjunction with other yeasts, ie, as a secondary strain (post fermentation), these yeasts can take existing alcohols, acids, and esters and change their profiles to create other compounds. Some of these can be very nice such as pineapple, cherry pie, and spicy phenols. They can also be odd, but still pleasant in the right beer, like mild mousiness, goaty, horse blanket, faint sweatiness, and hay. There are also other compounds that can render a beer undrinkable like high mouse taint, band-aids, plastic, burnt rubber, heavy sweat, and smoke. This yeast is the main flavor/aroma contributor in Flander’s Red, Lambic, Gueuze, as well as Orval, and adds character to some English Old Ales and Berlinerwiesse.
- There are two types of acid that can find their way into beer and depending on style and concentration can be desirous. One acid is acetic acid (vinegar) and it can be desired in Flander's Red beers in lower concentrations, but higher concentrations of acetic acid in any beer (including Flander's Red) is a severe flaw. The other acid is Lactic acid which is created by Lactobacillus and Pediococcus. This acid is required for all sour beers: Gose, Berlinerweisse, Lambic, Gueuze, Flander's Red, Oud Bruin, and Fruit Lambic. It is also acceptable as part of Wit and Saison styles in low to medium levels. Outside of these styles lactic acid is a flaw. Pedio also creates high levels of Diacetyl which must be cleaned up by a yeast which is why it is typically used in conjunction with Brettanomyces which will reabsorb the Diacetyl as it works on the beer slowly over time.
STYLES & STRAIN SELECTIONS
Hefeweisse requires a German Wheat Strain that exhibits banana esters and clove phenols, with low flocculation.
English beers would of course be best suited for use of English yeasts, for Bitters and Mild the BJCP ingredients state it needs to be made with a characterful English strain.
Scottish beers can at times (not required) have a smoky phenol, from yeast, if this is desired then use the Scottish Ale strain would work best, otherwise any very clean strains can be used including Lager and Hybrid strains.
Saison is a style that requires a very expressive yeast that finishes very dry, there are specific strains to achieve this.
American Ales and IPAs do well with a yeast that exhibits clean characteristics although some English strains with softer esters have been used with great success.
Porter and Stout yeasts will be dependent on the profile you desire. If a clean character that lets the hops shine like in American Stout and some Robust Porters is your goal then a clean American strain is best. If a more expressive character is desired then English Yeast should be used. Irish Stout has its own yeast though it is not required as long as the profile fits to the style. Baltic Porter typically uses a Lager Strain, hybrid, or very clean Ale strain.
Lagers should be made with a Lager strain, though Hybrid strains can be used with an extended cold storage time following the primary fermentation. When brewing lagers you will need to take into account the style parameters as well since not all Lager strains are alike. When brewing a Pilsner you will want to use an attenuative yeast that finishes dry and crisp, and highlights the hops. When making a Bock type beer you will want something that attenuates well but accentuates the malts and has a fuller mouthfeel.
Lambic and Flanders Red would require a mix of Ale yeast (the more character the better), Lacto, Pedio, and multiple Brett strains, as well as other wild strains of both bacteria and yeast that are not covered in this overview. Flanders Brown (Oud Bruin) would be made with a characterful Ale strain and aged with Lacto. Berlinerweisse would be made with a clean strain such as a Hybrid yeast and Lacto with a mild Brett contribution.
Hybrid beers would be best suited to use Hybrid strains; many have their own strains as well: Altbier (1007 German Ale), Kolsch (2565 Kolsch), Cal Commons (2112 Cal Lager).
Belgian beer styles will always use a Belgian yeast. Each style has parameters that should be taken into account when choosing a yeast strain as some styles have higher esters and lower phenols, while others have higher phenols and lower esters, and each strain of yeast will produce these characters differently. When trying to clone a specific Trappist beer one may want to consult this list http://www.mrmalty.com/yeast.htm as it states the supposed brewery from which many of the commercial strains have been acquired.
Strong Ales should use an appropriate strain for which their flavor profile calls and ensure that the strain is able to tolerate the level of alcohol you are intending to achieve without going dormant.
FORMS OF YEAST
- The most diverse selection of yeast strains come in liquid form from Wyeast (OR) and White Labs (SoCal) as well as new comers such as East Coast Yeast (PA) and Yeast Bay (SF). These producers have isolated many of the most prized strains of yeast, Brett, and Pedio, but not Lacto. They have ensured the strains are clean, healthy, and ready for use by brewers. If purchased fresh, these packages of yeast should be able to ferment a low gravity batch of beer without need for a starter.
- These yeasts are freeze dried by producers before packaging allowing for a longer shelf life and viability than liquid strains, and usually have enough viable yeast in a packet to fully ferment most beer styles if properly rehydrated. The diversity of strains is nowhere near the range of liquid strains, but with a few new producers (Mangrove Jack) on the market they are attempting to close this gap. Dry yeast should be properly hydrated before use by adding it to a cup of previously boiled water that has cooled to 95*F.
- There are many strains of yeast out there that are not available from commercial distributors that you may want to use in your beers. Although Wyeast and White Labs both offer the dominant yeast strain in Saison Dupont, the actual Dupont bottles contain 4 different strains of yeast, and Orval’s bottle culture is more complex than pitching WLP510 Belgian Bastone Ale Yeast followed by WLP650 Brett Brux. Culturing dregs from the bottle of a beer is beyond the scope of this article though.