Ale has been brewed since at least ancient Egyptian times. Ale yeast goes by the latin name Saccharomyces cerevisiae and this species includes bread yeast, distillers yeast and many laboratory yeast strains. Ale yeast is distinguished by their unique flavor production. The use of bread yeast or other “wild yeasts” by brewers would result in phenolic tasting beer. Ale yeast, as well as lager yeast, do not produce phenolic tasting beer because they have a natural mutation that prevents it from production of phenolic off flavors. (Specifically, brewers yeasts — sometimes called POF (-) strains — lack ferulic acid decarboxylase, the enzyme that decarboxylates ferulic acid to produce 4-vinyl guaiacol.) Ale yeast do what a brewer wants: they ferment quickly, consume the correct profile of sugars, tolerate moderate alcohol levels and can survive the anaerobic conditions of fermentation.
There are a huge variety of ale yeast strains. In fact, all wheat and Belgian strains are classified as ale yeast. For the purposes of this article, however, they will be treated separately. Because of the large variety of ale yeast strains, there are many differences in performance among these yeasts. They flocculate differently, attenuate differently and produce different flavor profiles.
They do have some similarities, however. Almost all ale strains have an ideal fermentation temperature that hovers around 68 °F (20 °C). Most ale yeast will tolerate conditions to 95 °F (35 °C), but they produce the best flavors when they ferment at 68 °F (20 °C). Flavors that ale yeast produce are varied. If they produce a small quantity of these flavor compounds, they are known as “clean” fermenters. The more esters and fusel alcohols, the “fruitier” the yeast is considered. The late George Fix — in his book, An Analysis of Brewing Techniques — uniquely labeled these as group 1 and group 2 ale yeasts, respectively. Examples of clean, group 1 yeasts offered by White Labs include WLP001 (California Ale), WLP029 (German Ale/Kölsch) and WLP051 (California V Ale). The clean fermenting ale strains are very popular, because they can produce lager-like beers using ale techniques and fermentation times. They usually ferment a little slower than other ale yeast and exhibit medium flocculation properties, which ensure they will be in the beer long enough to condition it properly. They can also produce trace sulfur, but not as much as lager yeast strains.
Ale yeast are famous for their ability to top ferment. After the first 12 hours of fermentation, many ale yeast strains will rise to the surface and ferment from the top of the beer for 3–4 days. This allows brewers to collect the yeast from the top, a practice called top cropping. The advantage of top cropping is a great crop of yeast, healthy and with little protein mixed in. The disadvantage of this method is the exposure to the environment. If the fermentation room is not sanitary, the yeast can easily be contaminated. There are few homebrewers that top crop, but I suggest giving it a try. The strains WLP001 (California Ale), WLP004 (Irish Ale), WLP013 (London Ale) and WLP300 (Hefewiezen) are strains that are particularly well suited for this. Homebrewers that ferment in glass will have trouble cropping the yeast through the small opening, but if fermenting in plastic buckets, this can be done with good cleaning practices. It is worth doing as an experiment, and you the brewer can evaluate its effectiveness.
Ale yeast that produce fruitier beers are less versatile, but extremely interesting. The strains WLP002 (English Ale), WLP004 (Irish Ale) and WLP005 (British Ale) are examples of fruity, group 2 yeasts. These are yeasts of distinction and can add a lot of character to your beer. They ferment at the same temperature as clean fermenters, but in doing so create more compounds that are excreted from the yeast cell. They usually flocculate quickly, which aids in leaving acetaldehydes and diacetyl in solution. It’s fun to produce a beer with highly flocculent yeast because it looks different while fermenting. (Lots of chunks!) Then the yeast drops out right when fermentation is done. The beer can be bottled and consumed rapidly. These strains usually do not top crop as well because they flocculate too quickly.
For ale yeasts, a pitching rate of 5–10 million cells per milliliter promotes cell growth and good beer flavor. For a 5-gallon (19 L) batch of beer, this corresponds to the amount of yeast from a 1–2 qt. (~1–2 L) yeast starter.
Lager yeasts ferment best at colder temperatures than ale yeasts — in the 50–55 °F (10–13 °C) range. Lager yeast is classified as Saccharomyces cerevisiae, same as ale yeasts, but many brewer still use the old classification of S. carlsbergensis or S. uvarum. This special yeast was first isolated in the Carlsberg Laboratories under the direction of Emil Christian Hansen, in 1881. Hansen was the first to develop pure culture techniques, techniques that we still use today in microbiology laboratories. Not only was Hansen able to grow this new yeast, lager yeast, in pure form, he was able to store it for long periods of time on a combination of wort and agar, which creates a semi-hard surface. This long- term storage allowed lager yeast to be transported all over the world, and soon lager brewing overtook ale brewing worldwide.
Why did lager beer become so popular? At the time lager yeast was discovered, most ale fermentations contained some wild yeast and bacteria and the resulting beer had a very short shelf life. Lager beer could be fermented cool, which suppressed the growth of wild yeast and bacteria. (Modern lager brewers tend to have problems with Pediococcus because of the slower fermentation, but they likely have less problems than pre-modern ale brewers.) Lager beer therefore had a longer shelf life, which meant greater distribution area and increased sales. Breweries probably began to switch to lager brewing to increase their sales.
But what makes lager yeast so different than ale yeast? Unlike many ale yeasts, lager yeast does not usually collect on the top of the fermenting beer. Lager yeast is known as bottom fermenters because of their nature to ferment from the bottom of the tank. But as in everything else in science — there are always exceptions! For example, White Labs WLP800 (Pilsner Lager) forms a yeast cake on the top of the fermentation even though it is a true lager yeast. Even though most lager yeast ferment from the bottom, they are not known as high flocculators. In fact, most of the yeast stays in suspension and most lager strains are low to medium flocculators. Lager yeast needs to stay in suspension in order to ‘lager’ the beer, aging it with some yeast in order to reduce the sulfur and diacetyl levels produced during the cold fermentation.
The cold fermentation has many consequences for lager beer. Since the yeast ferment in a cool environment, usually 50–55 °F (10–13 °C), they produce fewer esters and fusel alcohols. But the cool temperature keeps more sulfur in solution and it makes it harder for the yeast to absorb diacetyl. A good diacetyl rest near the end of fermentation will greatly reduce this. Here is a good procedure for a diacetyl rest: Ferment at 50–55 °F (10–13 °C). When the beer reaches a specific gravity of 1.022–1.020, let the fermentation temperature rise to a maximum of 68 °F (20 °C). The fermentation will reach completion, usually in the specific gravity range of 1.010–1.014. Let it sit at this temperature for 3–5 days post terminal gravity, then cool over the next day to 50 °F (10 °C). Keep at 50 °F (10 °C) for one day, then lower to lagering temperature, usually 41–45 °F (5.0–7.2 °C).
The optimal pitching rate for lagers is roughly double the rate for ales, between 15–20 million cells per milliliter.
Traditional European wheat beer use special yeast strains, which produce lots of flavor. The flavors are what we usually classify with wild yeast, phenolic and clove. But these strains produce a pleasing amount of flavors that blends well the other wheat beer ingredients. There are not a lot of different wheat beer strains, perhaps half a dozen. They differ in small ways due to different flavor compounds. For example, White Labs has two regular wheat beer strain offerings. One, WLP300 (Hefeweizen), has a dominant banana ester character that can be dialed in and out with fermentation temperature. The other, WLP380 (Hefeweizen IV), produces very little banana ester regardless of fermentation temperature. For WLP300, temperatures in the 65–68 °F (18–20 °C) range yields little banana; fermenting above 70 °F (21 °C) greatly increases the level of banana esters.
Wheat beer strains produce little alpha-acetolactate, the pre-cursor to diacetyl, and they absorb diacetyl quickly. So butterscotch flavors are rarely a problem. Wheat strains will also produce sulfur. It is important to let the fermentation go to completion before capping it. Some brewers like to cap the fermentation near the end to trap the remaining CO2 as a way to carbonate the beer. If you do this, you will trap the sulfur in the beer and it will never go away. This applies to lager brewing as well. It takes approximately 24 hours post fermentation at fermentation temperature to scrub all the sulfur out of solution.
Most wheat beer strains do not flocculate well. This is a desired characteristic, which leaves the traditional cloudiness of wheat beers. Wheat malt, with higher protein content, also contributes to this. You still want the yeast to drop out some; otherwise the beer will be milky like a yeast culture. The pitching rate for wheat yeasts is the same as that for ale yeasts.
Many Belgian beers are brewed with unique yeast strains. There are many different strains used, so it is impossible to generalize about them. You could make a Belgian style wit beer with a normal lager yeast if you like, it will just not have the nose and taste of a Belgian wit fermented with an authentic Belgian wit yeast strain. These strains usually produce a lot of phenol and clove flavors, as with wheat beer yeast. Many Belgian style yeast go beyond just phenol and clove and produce a lot of esters, fusel alcohols and earthy flavors. The balance of these compounds helps determine the flavor profile of these strains. These strains would be ones that are used to make Belgian style Trappist beers, for example. White Labs WLP500 (Trappist Ale) has a good balance between esters and phenolics, while WLP530 (Abbey Ale) ferments faster and produces fewer esters. Many Belgian strains, as with wheat beer strains, do not flocculate well. I recommend a higher pitching rate — between 10–15 million cells per milliliter — for most Belgian strains.
Creativity is key when trying to create Belgian style beers. In fact, creativity is the key to brewing beer. Mix in a little science, good cleaning, and you have the recipe for great beers.
Like White Labs, Wyeast offers a wide variety of yeast strains for the homebrewer. Here are some Wyeast yeast strains that are good representatives of the categories mentioned in the article.
Clean Ale Yeasts
These yeasts produce fewer esters relative to the fruity yeast strains and reduce diacetyl well.
1056 (American Ale)
1272 (American Ale II)
1007 (German Ale)
Fruity Ale Yeasts
These ale yeasts produce distinctive estery fermentation characteristics especially suited to English-style ales. Strains 1084 and 1187 can leave residual diacetyl in beer.
1968 (London ESB)
1028 (London Ale)
1084 (Irish Ale)
1187 (Ringwood Ale)
These yeast rise to the top of the fermenter early in fermentation. They can be skimmed to yield a healthy pitch for another batch of beer.
1010 (American Wheat)
1318 (London Ale III)
3638 (Bavarian Wheat)
3787 (Trappist High Gravity)
These yeasts ferment at lower temepratures, producing far fewer esters than ale yeasts but with a hint of sulfur.
2007 (Pilsen Lager)
2124 (Bohemian Lager)
2278 (Czech Lager)
Wheat Beer Yeasts
For the distinctive taste of a German hefeweizen, you’ll need to use one of these strains of wheat yeast.
3068 (Weihenstephan Wheat)
3333 (German Wheat)
3638 (Bavarian Wheat)
As with wheat beers, Belgian Abbey ales require special yeast strains to produce the characteristic aromas and flavors of the beers.
3787 (Belgian Trappist Ale)
1214 (Belgian Abbey Ale)