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Yeast is a single-celled organism used to create beer, wine, mead, cider, and most other fermented beverages. Yeast converts sugar into (primarily) alcohol and carbon dioxide.

Types of yeast

There are thousands of known strains of yeast, and scientists believe only a small percentage of strains have been discovered and categorized. For beer and wine brewing, only a few simple varieties are commonly used.

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.

Technically, bacteria are a separate type of organism from yeast, which is a fungus. However, many strains of bacteria are used to contribute flavors, especially sour flavors, to some types of beer and other fermented beverages.

Forms of Yeast

Prepackaged yeast is available to home brewers and wine makers in either dry or liquid form.

In addition, yeast may be available from noncommercial sources. Wine may be fermented with the natural yeast found on the grapes; beer yeast may be collected and cultured from some bottles of commercial beer, or collected from a previous batch of homebrew.

Dry yeast

Dry yeast is comprised of very tiny "grains" and usually does not require a starter because of its high initial cell count and its own supply of nutrient. It can be pitched directly into the wort or "jump-started" by rehydrating shortly before pitching. It should be noted that some yeast companies recommend rehydration of their dry yeast, while others do not. It is therefore important to consult the specification sheet for a particular yeast to find out if it should be rehydrated before use. Since the packets contain dormant yeast, their shelf life is fairly long and is typically good if used within two years from the date of manufacture if kept under proper storage conditions. The recommended storage temperature for dry yeast is usually below 50°F (10°C) and it is possible to store dry yeast in a freezer because the cell walls have hardened and will not rupture when exposed to the extreme cold. It is important to allow cool yeast packets to come to room temperature before using. This usually takes about an hour and will help to prevent the yeast from being thermally shocked. When these dormant cells are "woken up" via rehydration, they reactivate relatively quickly. Dry yeast is typically available in small packets which contain roughly a teaspoon of these "grains". The typical amount of dried yeast needed for a successful fermentation is approximately 1 to 2 packets to 5 US gallons of wort, but the amount of packets needed can vary with typical cell count and weight of the amount of yeast per packet. Again, it is advisable to consult the manufacturer specification sheet for a given strain. In terms of variety, the dry yeast strains are somewhat limited because not all strains can be successfully freeze dried and packaged.

Liquid Yeast

While generally more fussy and more difficult to store than dry yeast, it offers the brewer much more flexibility. While there are only a few basic strains of dry yeast, liquid yeast strains number in the hundreds. Many of these strains are very specialized, and lend particular characteristics to beer. Most traditional beer styles of the world have special characteristics that are only achievable by using the specialized yeast strain meant for that style. For example, a German Hefeweizen would taste completely different if it were fermented with a Belgian Abbey yeast strain instead of a Hefeweizen yeast strain. Since many yeasts lend subtle flavors and aromas to beer, liquid yeast allows the brewer to control yet another variable in his/her beer. The main drawback of liquid yeast (aside from shorter shelf-life) is that the cell count in most commercially available yeast packs is inadequate for inoculating 5 gallons of wort, thus creating the necessity for a starter. However, even given these drawbacks, most homebrewers prefer to use liquid yeast in their brews.

Using and Culturing Yeast

Yeast Pitching

Yeast pitching means more than simply opening the packet or vial and dumping the contents into the fermenter. Taking the time to calculate proper pitching rates and techniques can have a marked improvement on your beer.

Yeast Starters

Yeast Starters are used to boost the starting population of yeast before pitching it into wort. They are usually prepared a few days before brewing, but they can be stored for longer if refrigerated. Using yeast starters generally results in a shorter lag time and a quicker fermentation.

Yeast Washing

Washing yeast is a procedure that is used to separate the yeast from the trub left over after a beer has completed primary fermentation.

Storing Yeast

Storing yeast is not as simple as throwing it in a box and forgetting about it. Both dry and liquid yeast must be kept cold in order to keep it dormant until it is time to pitch. The recommended storage temperature for most freeze dried yeast is below 46°F (8°C) and it may be stored in the freezer. Average shelf life is approximately two years from the date of manufacture under proper conditions. Liquid strains should be kept in the range of 33-38°F (0.55-3.33°C) and should be used relatively soon after the date of manufacture, although many unofficial reports claim viability can easily exceed one year. In order to freeze large quantities of liquid yeast the consensus is that it be added to spent beer before freezing. Cultures are commonly stored in an equal amount of yeast slurry to glycerol. The reason for either of these procedures is to prevent the cell walls from rupturing when frozen.

Freezing yeast

Freezing yeast is also a delicate process that requires some skill and consideration. Some people have reported success in freezing dry yeasts.

Making a Yeast Bank

See this forum thread for detailed instructions on making your own yeast bank courtesy of FlyGuy from the forums.

Reusing yeast

Used yeast still in the primary fermenter, or a "yeast cake", may be used again to achieve fermentation of a subsequent batch. This is easy to do, but there is some discussion as to whether fresh yeast would achieve better results. The number of yeast cells in a yeast cake is more than a typical starter, and without washing, will retain some flavors from the previous batch.

Sources of commercial yeast

Liquid Yeast

There are two primary companies that provide liquid yeast strains for the homebrewer: White Labs (offering 46 different styles of beer yeast) and Wyeast (offering 48 types of beer yeast). White Labs yeasts come in vials, while Wyeast come in what are called "smack packs," or small pouches which are crushed to activate them before pitching them into the liquid to be fermented.

Dry Yeast

There are also many other companies which sell various types of dry, such as Fermentis, Danstar and Brewtek.