What's the difference between top fermenting and bottom fermenting?

Question

Some friends and I are attempting our first home brew. When the question came up I couldn't answer except to say that lagers are one and ales are another. I understand the some of the basic principles of fermentation from my college days in biochemistry but what are the differences between the two fermentation methods (right word?)? Be as detailed as you can/care to be please!

Answer 1

They are two different species: Saccharomyces cerevisiae and Saccharomyces pastorianus. They both do the same thing--convert sugar into alcohol--but they thrive in different conditions.

The main difference is that lager yeasts continue to function at almost-freezing temperatures, while ale yeasts go dormant.

The terms 'top-fermenting' and 'bottom-fermenting' are merely descriptive, as they describe the flocculation tendency of the yeast. An ale yeast at optimal temperature will tend to clump together and rise to the surface, while a lager yeast at optimal temperature will act more slowly and stay in suspension.

Answer 2

These yeast started out with a similar roots s.cerevisiae and diverged some time around the 15th Century, when it is thought to have hybridized with a new world yeast(Saccharomyces eubayanus). It is then likely that the yeast harvesting methods of different brewing techniques progressively selected for more specific varieties.

In British and similar brewing practices the yeast to be pitched into the next batch was skimmed off the top of one and deposited into the others. This culminated in the complex but highly effective Burton Union System for harvesting yeast.

Where as in other brewing cultures using this new lager yeast they would have harvested from the bottom of the fermenter, further selecting for the bottom fermenting properties.

The two fermenting properties are very similar taking sugar and spitting out ethanol in the absence of oxygen. Lager yeast can operate at mush lower temperatures, and at these temperatures tend to clean up and metabolize away a number of the esters it makes during early stages of fermentation. This leads to a far cleaner flavour profile if you brew the same wort with lager yeast and keep it in its preferred temp range vs an ale yeast in its temp range.

Lager yeast will ferment and stay active down to ~38°F/3°C, but large sudden temp drops will cause it to become dormant, so you need to cool things slowly if you want to do a slow secondary fermentation or the yeast will drop dormant. It prefers 45°-50° F max 60°F about 7°-12°C max 15.5°C, above this it will produce many off flavour esters. Sometimes you may want these and specifically stress the yeast to achieve them, but not on a first brew.

Ale yeast on the other hand tends to like 14-20C or 55-68°F and I would suggest keeping it in the middle portion of the band.

If you want to brew at higher temps then look at some specialty yeasts some Belgian strains are happy fermenting away up to 27°C/80°F at these temps they will spit out lots of clove and banana smelling esters giving your beers Belgian fruity notes, the exact profile depends on temp and strain.

NB: For a first brew I would go Ale yeast, as less need for temp control in most climates and less likely to give you an unexpected result.