[COHO] Yeast and how it affects taste

[Huntkng@aol.com]

I saw this and thought I would share.  It should dovetail into Brett's  class 
this Thursday:
 
 
Yeast Guide


Yeast are single-celled  microorganisms that reproduce by budding. They are 
biologically classified  as fungi and are responsible for converting 
fermentable sugars into  alcohol and other byproducts. There are literally hundreds of 
varieties  and strains of yeast. In the past, there were two types of beer 
yeast: ale  yeast (the "top-fermenting" type, Saccharomyces cerevisiae) and lager  
yeast (the "bottom-fermenting" type, Saccharomyces uvarum, formerly known  as 
Saccharomyces carlsbergensis). Today, as a result of recent  reclassification 
of Saccharomyces species, both ale and lager yeast  strains are considered to 
be members of S. cerevisiae.  

Top-Fermenting Yeast
Ale yeast strains are best used at  temperatures ranging from 10 to 25°C, 
though some strains will not  actively ferment below 12°C (33). Ale yeasts are 
generally regarded as  top-fermenting yeasts since they rise to the surface 
during fermentation,  creating a very thick, rich yeast head. That is why the term 
 "top-fermenting" is associated with ale yeasts. Fermentation by ale yeasts  
at these relatively warmer temperatures produces a beer high in esters,  which 
many regard as a distinctive character of ale beers.  

Top-fermenting yeasts are used for brewing ales, porters, stouts,  Altbier, 
Kölsch, and wheat beers. 

Bottom-Fermenting  Yeast
Lager yeast strains are best used at temperatures ranging  from 7 to 15°C. At 
these temperatures, lager yeasts grow less rapidly than  ale yeasts, and with 
less surface foam they tend to settle out to the  bottom of the fermenter as 
fermentation nears completion. This is why they  are often referred to as 
"bottom" yeasts. The final flavour of the beer  will depend a great deal on the 
strain of lager yeast and the temperatures  at which it was fermented. 

Some of the lager styles made from  bottom-fermenting yeasts are Pilsners, 
Dortmunders, Märzen, Bocks, and  American malt liquors. 

Spontaneous Fermentation
Beer  that is exposed to the surrounding open air to allow natural/wild yeast 
 and bacteria to literally infect the beer, are spontaneous fermented  beers. 
One of the typical yeasts is the Brettanomyces Lambicus strain.  Beers 
produced in this fashion are sour, non-filtered and inspired by the  traditional 
lambics of the Zenne-region. This brewing method has been  practised for decades 
in the West Flanders region of Belgium.  

Byproducts of Yeast
Yeast impact the flavour and aroma  of beer more than you might think. The 
flavour and aroma of beer is very  complex, being derived from a vast array of 
components that arise from a  number of sources. Not only do malt, hops, and 
water have an impact on  flavour, so does the synthesis of yeast, which forms 
byproducts during  fermentation and maturation. The most notable of these 
byproducts are, of  course, ethanol (alcohol) and carbon dioxide (CO2); but in 
addition, a  large number of other flavour compounds are produced such as:  
    *   acetaldehyde (green apple aroma)  
    *   diacetyl (taste or aroma of buttery, butterscotch)  
    *   dimethyl sulfide (DMS) (taste or aroma of sweet corn, cooked  
veggies)  
    *   clove (spicy character reminiscent of cloves)  
    *   fruity / estery (flavour and aroma of bananas, strawberries, apples,  
or other fruit)  
    *   medicinal (chemical or phenolic character)  
    *   phenolic (flavour and aroma of medicine, plastic, Band-Aids, smoke,  
or cloves)  
    *   solvent (reminiscent of acetone or lacquer thinner)  
    *   sulfur (reminiscent of rotten eggs or burnt matches) 
There are other yeast byproducts, and some of the listed can be both  desired 
byproducts and/or undesired depending on the beer style or what  the brewer 
was trying to achieve. 
 
Bill
 
William (Bill) Herrick
Central Oregon Shooting Sports  Association (COSSA)
PO Box 7499
Bend, OR 97708
(541)  318-8862
Fax: (541) 318-0083

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.bendtel.net/pipermail/coho/attachments/20060611/131baf4b/attachment.html