Ale has been brewed since at least ancient Egyptian times. Ale yeast goes by the Latin name Saccharomyces cerevisiae. This species includes bread yeast, distillers yeast, and many laboratory yeast strains. Ale yeast is distinguished by its 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 the production of phenolic off-flavors.
Ale yeast does 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, but, 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. 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 is varied. If they produce a small quantity of these flavor compounds, they are known as “clean” fermenters. The more esters and fuseloil alcohols, the “fruitier” the yeast is considered. 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. They 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 is famous for its 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.
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 test its effectiveness.
Ale yeast that produces fruitier beers is less versatile but interesting. 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, the same as ale yeasts. But many brewers 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, but he was also 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 fewer problems than pre-modern ale brewers.) Lager beer had a longer shelf life, which meant a greater distribution area and increased sales. Breweries 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 its 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 to ‘lager’ the beer, aging it with some yeast to reduce the sulfur and diacetyl levels produced during the cold fermentation.
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 fuseloil alcohols. But the cool temperature keeps more sulfur in the solution and it makes it harder for the yeast to absorb diacetyl. A good diacetyl rest near the end of fermentation will 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.
Attenuation refers to the percentage of available wort sugars that a yeast strain actually ferments. Brewers talk about apparent attenuation, which is the attenuation calculated from hydrometer readings. Typical values are
The desired degree of attenuation is partly a matter of style and partly one of personal preference. Using a low-attenuation yeast for a Saison or a high-attenuation yeast for a mild ale is likely to disappoint.
Flocculation is the readiness with which yeast cells clump together and, having reached a critical mass, drop to the bottom of the fermentor. British strains are flocculent: After fermentation is complete, yeast cells form a compact cake on the floor of the fermentor that comes off in chunks. Strains that show low flocculation, such as Weizen yeasts, tend to laze about and remain in suspension well past the end of the party. In extreme cases, the beer has to be refrigerated (or, in a commercial setting, centrifuged) to separate the yeast.
Alcohol tolerance describes how much alcohol a yeast strain can tolerate before it stops working. Brewers have pressured yeast strains over the years to adapt to different conditions. Breweries that have favored high-alcohol beer will have yeast strains that have risen to the challenge. A cold-fermenting Pilsner strain works well for lagers with less than 10 percent alcohol, while Rogue’s famous Pacman ale yeast can carry you into barleywine territory.
Temperature range is the—wait for it—range of temperatures in which yeast works best. Note that I said “best.” All yeasts will continue to ferment at temperatures well above the indicated limit. But you won’t like the results: Think overpowering esters and off-flavors. Below the recommended range, you might experience sluggish fermentation. Knowing a yeast’s optimal temperature range is important for a couple of reasons:
The sensory profile is a major driver of the flavor and aroma profile of the finished beer and also the hardest to describe. Attenuation, flocculation, alcohol tolerance, and temperature range are all quantifiable. But sensory descriptors are imperfect and subjective. A yeast strain’s sensory characteristics change depending on temperature, pitch rate, oxygen levels, and other variables. In short, the only way to know is to brew with it. Read the descriptors to get a general idea, and then take notes on how you perceive the results.
The pitching rate is a fundamental property of fermentation that many homebrewers get wrong. You should always know what the pitching rate should be for the beer style you want to brew. All characteristics of a yeast strain that you will find published assume you will be pitching the proper amount of healthy yeast. Under or over-pitching will yield results outside those printed specs that may or may not be obvious to most homebrewers. Use the pitching-rate calculator on MrMalty.com to determine the proper amount of yeast to pitch.
You can’t expect your yeast to produce consistent results unless they are healthy and viable. There are a lot of factors that affect yeast viability such as age, storage conditions, how they were transported from the manufacturer and arrived in your home brewery, how many generations you have pitched and the wort density of those previous generations, etc. The specs published about the particular yeast strain you are using assume you are using clean, healthy, viable yeast. When making your yeast selection decision, if possible, always use the freshest yeast available.
Store your yeast cold and keep the temperature stable. Don’t repitch on a yeast slurry from a high-gravity fermentation. Don’t re-pitch too many times. Wash your yeast from a prior fermentation before storing. Make a suitable starter with an OG of around 1.040 and step it up for higher gravity fermentations (literature states that you shouldn’t step it up to a volume of more than 10 times the original). Provide nutrients and plenty of oxygen for your starters. Pitch at high krauesen if possible or allow the yeast to settle and re-pitch into a starter the day you plan to pitch to kick start the yeast. Don’t store in a lot of alcohol (back to washing your yeast).
Using olive oilk in beer instead of oxygenation aside, you must provide the yeast with plenty of oxygen and nutrients. So they can get through the lag phase of their reproduction and continue with the fermentation as healthy and viable as you can make them. This means you must provide some form of nutrients for high-adjunct worts (it doesn’t hurt to provide a little nutrient in every beer you ferment). Plenty of oxygen before the yeast take on the daunting task of converting all that sugar into alcohol.
Every fermentation produces compounds that you don’t want in your finished beer. Allowing the yeast to reabsorb these compounds before bottling or kegging is generally called “conditioning”. It can be done in a secondary fermentation where the beer is racked off the dregs of primary fermentation. It can be done by manipulating the temperature at the end of primary, or it can be done through lagering. Be patient and allow plenty of time for proper fermentation and conditioning or lagering phase. Pulling your beer off the yeast too soon will leave the beer under attenuated with a lot of undesirable compounds. Allow the yeast to do their job and clean up these byproducts before transferring or bottling.
You can make the right yeast selection decisions and get your yeast in perfect health for fermentation. But without good sanitization, it’s all for nil. Get in the practice of scrubbing with the right cleaners, rinsing well, using the best sanitizer, sanitizing everything that will come in contact with the yeast before and after using the items, sanitizing your hands, sanitizing the mouth of your starter vessel and the scissors and yeast package before pouring them into the starter or fermenter. And never expose the yeast to the open air for any longer than necessary. It may seem like overkill, but it is an important part of brewing and will pay dividends for the rest of your homebrewing hobby/obsession.
Source: beerandbrewing.com, winning-homebrew.com, byo.com