Converts sweet wort into beer. Yeast consume the sugar in wort, and turn it into CO2, alcohol, and flavour compounds.
Yeast have the potential to contribute more unique flavors to your beer—both good and bad—than any other ingredient.
We didn't know it existed until 1857 when Pasteur discovered it's role as a living organism.
First we pitch the yeast. Then it attenuates a percentage of the beer, eating the sugars and making alcohol. Then it flocculates to the bottom to leave a clear beer.
White Labs (test tube liquid) and Wyeast (smack pack liquid) have the largest variety, but only go to 5% ABC unless using a yeast starter.
Safale make dry yeast, which is cheaper, goes to 9% from the packet, but dry yeast has less variety available.
Summarized from White Labs
Taken from Serious Eats
Temperature: Yeast is very sensitive, if it goes into high temperatures it gets stressed out and produces off flavours. Most yeast likes it aruond 60-70F, with lower temperatures giving a better taste.
Oxygen: Before pitching the yeast oxygen must be added to feed the yeast. Vigorously shake or stir the wort for a couple of minutes. After pitching do not shake any more, this can cause the beer to go stale.
Cell count: If there are not enough yeast cells, they will spend more time and energy reproducing and not as much quality time fermenting the beer. Having a low cell count will cause off-flavors, increase the time of fermentation, and can even cause your fermentation to stall out prematurely
How does the brewer really know when fermentation is done? The answer: by testing the degree of attenuation.
Attenuation percentage is the percentage of sugars that yeast consume. Attenuation varies between different strains. The range for brewers yeast is typically between 65-85%.
To measure you need to know the specific gravity (taken just before yeast is pitched, also called Original Gravity: OG yo!). This is the density of the water. If water is 1.000 then the sugars make it more dense. It's this reading that says how much sugar the yeast has consumed. To be accurate the alcohol must also be removed as alcohol is lighter than water. That's why home brewers measure "Apparent Attenuation".
To calculate attenuation percentage, the following equation can be used: [(OG-FG)/(OG-1)] x 100
The yeast you choose depends on the style. IPAs are dry to let the hops shine, so you'd use an attenuation of 70-80%. An English mild would use 65-70%. It's taste good regardless, but it wouldn't be to style.
Flocculation-the state of being clumped together.
In the case of yeast, it is the clumping and settling of the yeast out of solution.
-John J. Palmer HOW TO BREW
When yeast finish the fermentation process, they shut down, clump together, and fall to the bottom of the fermentor, or "flocculate". The magical art of yeast coming together, dropping to the bottom of a fermentor, is called flocculation.
Flocculation is unique to Brewers Yeast. It gives clear beer. Floc too early and the beer will be too sweet, floc too late and it'll be cloudy and yeasty.
Wild Yeast is yeast that does not flocculate well. Historically brewers have only reused the Yeast at the top or bottom, thus naturally selecting the Yeast that flocculates as they desire.
Yeast is described as low, medium or high flocculation. Ale yeast fits in all categories, lager are mainly medium. English beer tends to be high, US medium and Wheat beer low. Commercial beers are filtered before bottling, so it's hard to know what they use.
The ideal yeast strain would be highly flocculent, with good attenuation, and provide the desired flavor contribution.
High flocculation (3-5 days to floc, sometimes needs roused. High diacetyl. Low attenuation. Good for malty ales) creates a yeast cake fast, which can get into the taps.
Medium flocculation (6-15 days to floc. Ideal for ales. Clean balanced flavour. "Powdery") gives a clean taste, useful with hops.
Low flocculation (Won't floc even at day 15. Wild yeasts are like this. Hard to filter) is rarely used as it remains in suspension. Really just Wit and Hefe beers.
All very strain dependant. Taken from Wyeast labs
- Oxygenation (Poor or low wort aeration can result in early and incomplete flocculation. Adequate aeration can result in delayed and more intense flocculation)
- Temperature
- pH (Optimum range is 3.5 to 4.8)
- Ethanol Concentration
- Pitch Rate (how many cells at what point)
- Trub (can increase or decrease)
A list of Yeast strain numbers can be fount at byo.com
The main strains of yeast used in beer and wine production are from the genus Saccharomyces, primarily Saccharomyces cerevisiae (top-fermenting ale yeast), Saccharomyces carlsbergensis (lager yeast, also called Saccharomyces pastorianus), and Saccharomyces bayanus. Within these species, there are as many varieties of yeast as there are beer and wine.
Some yeasts are very neutral in profile and clean-fermenting, adding little flavor, while others, such as Belgian or wheat beer yeasts, give distinctive characteristics to beers with which they are used.
Some beers are fermented with other yeast species, such as Brettanomyces. Because these beers were often traditionally fermented in open fermenters rather than inoculated with pure yeast strains, these are often known as wild yeast.