Yeast

Yeast is a single-celled fungus that has been on Earth for millennia. It is responsible for breaking down the sugar in the wort (produced by the mashing and boiling stages) into carbon dioxide gas and alcohol, with added fizz. There are two major types of yeast: ale yeast and lager yeast.

Ale Yeast

The species Saccharomyces cerevisiae 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. Ale yeast is any one of a number of yeast strains belonging to this species. Ale yeast does what a brewer wants: it ferments quickly, consumes the correct profile of sugars, tolerates moderate alcohol levels and can survive the anaerobic conditions of fermentation.

Ale yeasts 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 to 4 days. Also, ale yeast tends to ferment best at relatively warmer temperatures (between 18 and 24°C).

Compared with lager yeasts, ale yeasts also tend to produce more of the esters that lead to fruity and/or complex flavours and aromas; the higher ale fermentation temperatures can also accentuate this tendency. Flavours that ale yeasts produce are varied. Ale yeasts generally fall into one of two groups:

Clean-fermenting group 1 yeasts are used for producing lager-like beers such as California style and German ales.
Fruity group 2 yeasts are used for producing English and Irish ales.

Lager Yeast

Lager yeast, Saccharomyces pastorianus (named in honour of Louis Pasteur), is a bottom-fermenting yeast used for brewing lager style beers. S.pastorianus is actually a hybrid of S.cerevisiae and another species S.eubayanus, and has the ability to ferment at cooler temperatures (between 5 and 14°C). Lager yeast likes to sit at the bottom of the beer as it ferments – this is referred to as ‘lagering’.

The presence of the two genomes in the hybrid S.pastorianus may have led to its observed ability to carry out fermentation better at cold temperatures than S. cerevisiae alone. This selection for a cooler fermenting yeast may have taken place during successive rounds of cold-temperature fermentations resulting from a 16th-century Bavarian law that prohibited brewing during summer months due to the inferior quality of summer-brewed beers. Many isolates of S.pastorianus have been collected from breweries since Hansen pioneered pure culturing of yeast in the late 1800s, and recent work has shown that all lager strains in use today likely descended from one of two hybridisation events between S.cerevisiae and S.eubayanus. In both cases, it is probable that the S.cerevisiae parent of these two original S.pastorianus strains was a strain already being used for brewing ale.

Flocculation

Flocculation refers to the ability of yeast to stick and clump together once all the nutrients have been used. As the yeast gathers, they become too heavy to remain in suspension in the beer, and fall to the bottom of the fermenter, mixing with the sediment and nutrients.

The strength of this ‘clumping’ is determined by a host of factors from oxygen levels in the beer, temperature, and nutrient levels, but the primary factor seems to be the strain of yeast itself (in truth, flocculation is still being researched, and has a few mysteries remaining). Flocculation occurs near the end of the yeast life cycle, so a yeast that flocculates early will not convert as much sugar, and result in a lower-than-expected alcohol content. Strong flocculation gives a clearer beer, while weaker flocculation can lead to a cloudy beer, sometimes with a yeasty taste. Hefeweizen yeast is a good example of a strain that has very low flocculation, meaning a lot of it is left behind in suspension. Traditional non-filtered hefeweizens should look cloudy from the yeast (not the wheat as some might think). The clove-like signature flavour in a hefeweizen comes from the yeast.

Flocculation for a particular yeast is listed as high, medium or low:

Highly flocculant strains do have advantages—namely, they can produce a brighter beer with less suspended yeast, making filtration easier. Producers of cask-conditioned beers will often desire this quality as well, as they want the yeast to drop quickly to the bottom of the cask when fermentation is complete.
Medium flocculation type strains tend to be some of the more commonly used strains and will start to flocculate out as sugars become less abundant. Often they need help in order to flocculate, and this can be accomplished by cooling the fermenter to lower temperatures.
Low flocculation strains are those that stay in suspension well after fermentation has ended. These strains tend to be wheat beer strains in which low flocculation is desirable, as the finished beers are meant to be hazy with yeast.

Attenuation

Attenuation describes the overall efficiency of a yeast strain in converting sugars into alcohol under a specific set of conditions. Higher gravity beers require the use of yeasts with higher attenuation rates. Attenuation is listed as a percentage, meaning the percentage of sugars present in the wort that get converted into alcohol. Most beer yeasts have a 65 to 80% attenuation rate. The exact attenuation rate can be found for a given beer by comparing the starting (pre-yeast) original gravity (OG) to the final gravity (FG) of the finished beer – refer to the Calculations page.

When selecting yeasts, the beer style should be considered. When brewing a Belgian Witbier, a yeast strain with a 65-70% attenuation rate would be appropriate. A “stronger” yeast (higher attenuation) could be used, resulting in a higher alcohol content, but the overall flavour and style of the beer would be “off” from the standard. The attenuation for yeast for an English ale would be lower than the yeast for a Belgian brown.

The attenuation of a particular batch is affected by anything that would affect the health of the yeast. Unexpected flocculation, changes in temperature and insufficient nutrients can all lower attenuation.

Dried Yeast or Liquid Yeast?

Dried yeast is sterile, strain-pure, and highly capable of producing great beer. It has a shelf life of up to 3 years and is more tolerant than liquid yeast of warm storage or shipping conditions. Dried yeast is also packaged with nutrient reserves and ready to pitch without a yeast starter, although the preparation of a starter will get the fermentation going sooner.

On the flip side, not all yeast strains can be dried, so there are fewer options to choose from. That’s where liquid yeast comes into play.

A few examples of popular dried yeast:

Safale dried yeast, produced by Fermentis (a visionary leader in all things fermented), produce a line of dried yeast pouches that are one of the most popular options for easy brewing. Think of Safale as your all-purpose flour.
Lallemand, another esteemed yeast producer, carries the Danstar yeast line. They carry a wide variety of yeast for your next beer style, but by far the most popular option is the Nottingham Ale yeast, a neutral ale strain with excellent attenuation.

The range of available strains is the greatest benefit of liquid yeast. Any strain can be collected and cultured for use by home-brewers. However, it is most viable immediately after culturing, and viability decreases as time goes on. Therefore, brewers need to be aware of temperature. However, because liquid yeast is a live culture, it is usually more expensive and is much more perishable. Yeast shipped by mail order usually has a practical shelf life of 3 months (sometimes longer) and can be adversely affected or destroyed by temperatures above 30°C.

When brewing with liquid yeast, cell count must be kept in mind. Each yeast supplier varies in packaging and cell count, so some research is advisable before brewing.

A few examples of popular liquid yeast:

Wyeast has been producing yeast for over 30 years, they’ve stood at the forefront of the craft beer revolution. They’re known for their “smack packs,” special pouches that deliver enhanced yeast viability with just a slap of the hand.
White Labs provided a large yeast innovation in 2017. White Labs continues to astound the brewing world with PurePitch® and FlexCell™ packaging which produce the purest yeast available. The packaging allows the yeast to be grown and pitched out of the same package. At around 100 billion cells per package, they guarantee the very highest purity possible for your beer.