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Revision as of 18:32, 12 February 2007
Yeast is a type of plant fungus which is used in the fermentation process to create beer, wine, mead, cider, and any other similar alcoholic beverages. It converts the sugar content into alcohol and co2 along with some other by products.
There are thousands of known strains of yeast, and scientists believe only a small percentage of strains have been discovered and categorized. For the purposes of this wiki, only a limited number of yeast strains will be considered.
Wild yeast is present all around us. Wort should be covered to protect from wild yeast starting the fermentation and becoming the dominant yeast strain. Wild yeast was how old Ale was made before the discovery of modern controlled brewing yeast strains.
Dry yeast is comprised of very tiny "grains", making it easy to use, 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. Since the packets contain dormant yeast, their storage shelf-life is typically longer than that of liquid. When these dormant cells are "woken up" via rehydration, they reactivate relatively quickly. Dry yeast is typically available in small packets with contain roughly a teaspoon of these "grains". Its main drawback is the lack of different yeast strains that are commercially available in this form.
While liquid yeast is generally more fussy and harder to store than dry yeast, it offers the brewer much more flexibility. Dry yeast comes in a few basic strains; 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 wasn't fermented with Hefeweizen yeast strain, and instead, a Belgian Abbey yeast strain were used. 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 other drawback of liquid yeast (aside from the 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 advanced homebrewers use liquid yeast in their brews.
There are two primary companies that provide yeast strains for the homebrewer: White Labs and Wyeast. 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. There are also many smaller companies which sell various types of dry and liquid yeasts, such as Danstar and Brewtek.
There are as many varieties of dry yeast and liquid yeast as there are styles of beer, and they can have a profound effect on flavor. 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. Liquid Yeast Varieties, Dry yeast varieties For examples of commercial uses of strains of yeast check out the Known Commercial Yeasts list.
Washing yeast is a procedure that is used to seperate the yeast from the trub left over after a beer has completed primary fermentation.
Yeast Starters are used to boost the starting population of yeast before pitching it into the 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.
Storing yeast is not as simple as throwing it in a box and forgetting about it. Both dry and liquid yeast must be refrigerated in order to keep it dormant until it is time to pitch.
Freezing yeast is also a delicate process that requires some skill and consideration.
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