Yeast 

Brewing yeast strains are normally divided into 3 types:

• Top-fermenting yeast (Saccharomyces Cerevisiae)
• Bottom-fermenting yeast (Saccharomyces Carlsbergensis also called S. Pastorianus)
• Spontaneous fermenting yeast (yeast from the surroundings)

When master brewers select a particular yeast strain they normally look for a handful of different parameters - both biochemical and physical behaviour - before selecting the one to use:

1. Strain
Before using a certain yeast strain it is important to agree upon which beer-style is desired to produce.

2. Aromatic qualities
What levels of e.g. fruity, sulphurous, and/or butterscotch notes are desired? The aroma from the particular yeast strains dominate the beer produced to a larger or smaller extent, see below.

3. Rate of attenuation
How long time does it take to ferment a batch of beer (days, weeks)?

4. Extent of attenuation
Which kind of sugar molecules are eaten by the yeast, leaving behind a sweeter or dryer beer?

5. Flocculation
Flocculating yeast form aggregates of cells into grapelike clusters, near the end of fermentation. Bottom fermenting yeast settles at the bottom while top-fermenting yeast rises to the surface of the fermenter.  

6. Yeast growth
How much yeast is generated during the fermentation?

7. Fermentation temperature
What is the optimum fermentation temperature for the particular yeast strain and what types of aromas are generated?

Top-fermenting Yeast

Top-fermenting yeast strains are best used at temperatures ranging from 18 to 30°C, but it depends very much on the actual strain and desired aroma. Top-fermenting yeast rises to the surface of the fermenter, creating a thick, rich yeast head. Fermentation by top-fermenting yeast produces a beer relatively high in fruity aromas which many regard as a distinctive character of top-fermenting beers.

Top-fermenting yeasts are used for brewing of e.g. ales, wheat beers, stout and some porters.

Bottom-fermenting Yeast

Bottom-fermenting yeast strains are best used at temperatures ranging from 7 to 18°C, but it depends very much on the actual strain.  As the temperature is lower, the yeast growth is slower than top-fermenting yeast. Bottom-fermenting yeast settles at the bottom of the fermenter, near the end of the fermentation.  

The final flavour of the beer is not so dominated by the yeast, resulting in “cleaner”, more neutral aromas.  
Bottom-fermenting yeasts are used for brewing of e.g. US-lagers, Euro-pilsners, bocks, black beers, and some porters.

Spontaneous Fermentation

Beer that is exposed to the surrounding open air to allow wild yeast and bacteria to infect the beer, are spontaneously fermented beers. Some call this a natural fermentation when no selection of yeast has taken place. The resulting flavour is dependent on the actual micro organisms but is normally quite sour and fresh.

Some breweries actually use wild yeast like Brettanomyces Bruxellensis, B. Lambicus or B. Clausenii to create more complex beers with additional aromatic compounds like earthy, horsey, leather, and pineapple.

Spontaneous fermented beers are e.g. gueuze, lambic and kriek.

How much the yeast is dominating the aroma of a beer is e.g. depending on the beer style, the malt-composition, the use of hops, and which yeast strain is used.

When comparing bottom-fermenting and top-fermenting beer styles and how much the ingredients and yeast strain is dominating the final aroma of the beer, I would give the following guidelines, with certain reservations:

Jens Eiken, Master Brewer