mandag 3. mars 2014

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Yeast: The Practical Guide to Beer Fermentation av bl.a Jamil Zainasheff.

  BeerSmith Home Brewing

Understanding Brewing Yeast

I started brewing in the 1980′s, which was the modern equivalent of the wild west days of home brewing. We simply did not have either the technical knowledge or access to the high quality ingredients that brewers have today.
Yeast, in particular, was one ingredient that improved dramatically in the last 20 years. When I started, the only yeast available was dry packet “bread-style” yeast. It came in two flavors – ale and lager. The quality of these two yeast strains was questionable, resulting in significant variation in flavor and character.
In the early 1990′s Wyeast and White Labs introduced high quality liquid yeasts to the US homebrewing market. Companies like SafeAle also brought high quality dry yeasts into the picture. This revolutionized homebrewing in a way that is difficult to explain today.
Brewing Yeast
Brewing yeast is a single cell microorganism (technically a fungi), and both ale and lager yeasts are members of the family Saccharymyces Cerevisiae. Lager yeast was earlier classified as S. Uvarum, but a recent reclassification put it in the S. Cerevisiae family. Ales are traditionally called “top fermenting” for the yeast layer that forms at the top of the fermenter, while lagers are called “bottom fermenting”
Ale yeasts ferment in the range 10-25C and produce beers high in esters and often lower in attenuation, both distinctive characteristic of ales. Ale yeast strains are often mixed together to aid in attenuation and flocculation. Lagers ferment in the range from 7-15C and produce a cleaner beer with lower esters.
The Fermentation Life Cycle
Yeast goes through four overlapping phases when fermenting beer into wort. After pitching, yeast starts in the “Lag Phase”. During the lag phase, the yeast strives to reproduce as fast as possible. The key ingredient during this phase is Glycogen, an sugar stored internally in the yeast, which is broken into glucose to fuel yeast reproduction. If an insufficient amount of yeast is pitched (or the yeast itself is low in glycogen), you will get an excess of dicytl (buttery or butterscotch flavor) in your finished beer. A properly sized Yeast Starter is critical.
The lag phase is followed quickly by the respiration (also called the growth) phase. In this phase the cells grow logarithmically (usually one to three doublings) by cellular division. In this phase the critical ingredients are oxygen and a variety of yeast nutrients. The cells will continue to grow until the oxygen or nutrients have been depleted.
Once all of the oxygen has been scrubbed from the wort, the fermentation phase begins. Yeast cells convert simple sugars into carbon dioxide, alcohol and beer flavors. As the sugars are consumed, the gravity of the beer will rapidly drop. Fermentation normally takes 3-7 days.
Sedimentation is the final phase of fermentation. Yeast will begin to form into clumps and settle to the bottom of the fermenter in a process called flocculation. In this phase, yeast will also store the glycogen needed for future reproduction as it prepares to enter a dormant state. Some yeasts will settle (flocculate) much more rapidly than others, so settling can take from days to weeks depending on the yeast variety.
Understanding Yeast Characteristics
Selecting a yeast appropriate for a given beer involves some knowledge of key yeast characteristics:
  • Attenuation: Refers to the percentage of sugars converted to alcohol and CO2. A high attenuation yeast will result in a clean, dry finish. Low attenuation yeasts typically leave ester, malt and other flavors behind leaving a more full bodied complex beer flavor.
  • Flocculation: Flocculation refers to the ability of yeast to form clumps (flocs) at the end of fermentation and rapidly sediment (or be skimmed) from the finished beer. Low flocculation yeast is often called “powdery” yeast. High flocculation yeasts tend to fall out of the beer before completing fermentation, leaving a more complex, higher gravity beer. Low flocculation yeasts such as lagers will completely ferment, leaving a cleaner finish but it can then be difficult to separate the yeast from the beer.
  • Temperature Range: Different yeast strains have differing temperature tolerances. Ales ferment at a higher temperature. Higher temperature fermentation is associated with higher ester production, and lower temperature with clean dry flavors.
 
  • Alcohol Tolerance : Many yeasts are sensitive to alcohol content, and will have trouble fermenting very high original gravity wort. Champagne, Wine, or other alcohol tolerant yeasts are frequently used either for primary fermentation or as a second yeast addition to assure full fermentation of very high gravity beers such as barley wine.
  • Beer Flavor: Individual yeast strains can add widely varying flavor profiles to your beer. A good example is Hefe-Weizen, where a significant majority of the clove and banana beer flavor comes directly from the yeast. Matching the yeast strain to the style of beer is a the best way to provide proper flavoring.
An Examples of Yeast in Beer Design
In most cases, brewers tend to select the Wyeast or White Labs yeast that matches their beer style. Lets look at a few examples where varying from the script might pay off: One example is brewing an Irish Stout. Irish stout has a very dry roasted character derived from roasted barley.
A traditional yeast choice might be an Irish Ale yeast such as White labs WLP004. However, looking at WLP004, it has a modest attenuation of 71.5%, which will produce a more fruity finish. One could experiment by picking WLP007, “Dry English Ale” yeast. This yeast has a much higher attenuation (75%) and will leave a drier finish on the beer, while still retaining some of the English esters needed for the stout.
At the other extreme, I know several brewers that use a single strain, White Labs California Ale WLP001 for just about every beer they brew. Their justification is that this yeast is relatively flavor neutral for an ale yeast, has a very high attenuation and leaves a clean finish on any beer. It ferments rapidly and flocculates quickly from the beer minimizing storage time needed. Personally I don’t subscribe to this “one size fits all” approach, but a number of brewers have demonstrated great success with it.
The above are just examples, but the key to great beer design is to understand the ingredients you are working with. Knowledge of the characteristics of yeast, and its significant effect on your beer will help you to become a better brewer.

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