Yeast is one of the most overlooked ingredients in homebrewing. However, the selection of yeast can make a vast difference to the characteristics of the finished beer.

Yeast is available in two forms: liquid and dried. Liquid yeast is transported in a solution, while dried yeast has been dehydrated into a powder. Don't be fooled, though, just a few grams of yeast contains billions of yeast cells!

Most brewers start their brewing exploits using dried yeast, usually the one found under the lid of a tin of concentrate. This yeast is often, although unfortunately not always, sufficient to produce a quality beer. Such yeasts have been chosen to, among other things, survive long periods without being refrigerated and produce a reasonable beer even if used outside ideal fermentation temperatures. Of course, that means these yeasts are not always the best choice to make the best beer. One of the first changes kit brewers may make to their beers is to use a different dried yeast.

There is a reasonable variety of dried yeasts available, from suppliers such as Fermentis, Danstar and Lallemand. Many homebrewers frown on the use of dried yeast, but the fact is that plenty of small breweries use dried yeast for all or most of their beers. For example, Fermentis US-05 is used widely by craft brewers and homebrewers because it ferments cleanly (i.e. does not produce strong flavours) and is tolerant of a wide range of temperatures.

Other sources of yeast are a culture from commercial bottle-conditioned beer, liquid yeast and yeast saved from a previous brew.

There are many more types of liquid yeasts available than there are dried yeasts, partly because some types of yeast just don't survive the drying process well. Therefore, liquid yeasts are essential to accurately reproduce certain styles of beer.

Find out about preparing dried and liquid yeast for use, culturing yeast from a bottle of commercial beer and yeast farming on the section about preparing the yeast.

What is yeast?

Yeast is a fungus that loves turning sugar into alcohol and carbon dioxide, which is a good thing because those are two of the things that are essential to a decent beer.

The yeasts used in brewing fall into two broad categories: top-fermenting (Saccharomyces cerevisiae) and bottom-fermenting (Saccharomyces pastorianus, formerly known as Saccharomyces carlsbergensis, after the Carlsberg brewery). Top-fermenting yeasts are also known as top-cropping or ale yeasts and tend to rise to the top of the brew while they work their magic, forming a thick "krausen" of foam on top of the beer. As fermentation slows and finishes, this krausen will collapse into the beer and sink through the beer to the bottom of the fermenter. Top-fermenting yeasts typically produce a beer which has a more fruity nose and more body (due to the yeast being able to ferment fewer sugars) than a beer made with a lager yeast. Lager, or bottom-cropping, yeast will result in a beer that is more “crisp” and have a less-malty body than if the beer had been made with an ale yeast.

Individual yeast strains within these categories will produce hundreds of chemical compounds in different quantities and ferment more or fewer sugars, each producing a beer with a specific character.

In general — and there are exceptions — ale yeasts produce the best results at 18°C to 20°C, although most will not become dormant until below 15°C and some will ferment to about 13°C, but fermentation will be very slow. Yeast will work very quickly into the 30s before being killed by the heat, but at these temperatures it will produce undesirable flavours in the beer. For this reason it is important to keep the brew as close as possible to the ideal 18°C to 20°C range when brewing with ale yeast.

Lager yeasts on the other hand will work down to much lower temperatures, in the range of 7°C to 13°C, and produce the cleanest, crispest beer at the lower end of these temperatures. Of course, there are exceptions to the temperatures that lager yeasts are used at, one example being Californian steam beers, which are brewed with lager yeasts at ale temperatures.

Something very important for kit brewers to know is that a can of "lager" hopped concentrate isn't necessarily — and arguably usually isn't — supplied with a lager yeast, rather with an ale yeast. If you want to brew at the lower, lager temperature and are unsure whether the supplied yeast is a really a lager strain, ditch it and buy a sachet of dried lager yeast instead. Trying to ferment at lager temperatures with the yeast under the lid of the tin if it's an ale yeast won't work because the yeast will become dormant.

Some beer, almost exclusively lambic beer in Belgium, is made by spontaneous fermentation, in which the wort is left to cool in large open vats and attract wild yeast. We recommend that you don't try this at home because you're more likely to end up with an undrinkable beer than a Belgian-style masterpiece. After all, the baceria flying around your house is unlikely to be the same that in the centuries-old brewhouses of the Belgian countryside!

There are two phases the yeast goes through when turning the sweet wort into alcoholic beer: an aerobic phase (requiring oxygen) and an anaerobic phase (best done with little or no oxygen). The aerobic phase at the beginning of fermentation is when the yeast multiplies. For this it needs oxygen, which is why when filling the fermenter there's no problem doing as much splashing of water as you like, and why it's necessary for all-grain brewers to re-oxygenate the wort that has been de-oxygenated by boiling. Even kit beers will benefit from oxygenation, which can be done simply and easily by stirring the wort vigorously. Other methods of oxygenation include using a paint stirrer on an electric drill, and an air stone (which diffuses air) attached to an aquarium air pump or oxygen bottle.

As the yeast finishes multiplying, the anaerobic phase begins, in which the yeast consumes sugar and expels oxygen and carbon dioxide. When there is no food left for the yeast, it falls out of suspension to the bottom of the fermentation vessel. Some yeasts "clear" better than others, and the process can be helped by "crash chilling" the beer, which involves chilling it as quickly as possible to as close to 0°C as possible. Because beer contains alcohol and sugar, it will not freeze until below freezing point.


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