Yeast is the most important ingredient in beer brewing. It is a single-celled organism; a fungus (phylum Ascomycetes; class Hemiascomycetes; including 10 different families). Yeast is a living creature, metabolizing, reproducing, and living off the ingredients in the beer. It is responsible for the converting of sugar to alcohol and carbon dioxide in the fermentation stage. Yeast is also the final component that determines the flavor of the beer.
There are thousands of varieties and strains of yeast. Even in the air, wild yeast is floating around ready to contaminate. Only cultivated strains of yeast should be utilized in the brewing of beer. If other yeast contaminate the beer, the results can be over carbonation, strange flavors, and all kinds of fermentation peculiarities. Therefore, picking the right yeast for the desired beer is an absolutely critical.
The two main varieties of yeast used for beer brewing are top-fermenting yeast (Saccharomyces cerevisiae) and bottom-fermenting yeast (Saccharomyces uvarum). The names of both are descriptive of where the fermentation takes place. The top-fermenting yeast is similar to the yeast for baking bread. It is applied for making ales and stouts. The bottom-fermenting yeast is utilized for production of lagers and steam beer.
Top fermenting yeast is named as such because most strains exhibit the tendancy to floculate (gather) at the surface of the beer during the first few days of fermentation. After which the yeast settles to the bottom of the fermenter while a large percentage stay in dispersion. Top Fermenting yeast, 'Ale yeast', finds optimum performance in the temperature range of 55-75 deg F. Lower temperatures tend to inhibit fermentation, causing the yeast to become dormant.
Bottom fermenting yeast, 'Lager yeast', is best suited for the temperature range 55-32 deg F. The process of fermentation take substantially longer when using Lager yeast, this time is often refered to as 'Lagering'. Bottom fermenting yeast, as is expected, flocculates at the bottom of the vessel and spends most of its life-cycle in the sedimentary state.
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