Above: What a difference a yeast makes!

I’ve sometimes wondered what the technical difference is between a lager and an ale. Apparently the difference comes mainly from the type of yeast used. From the Kitchn:

There are two main strains of yeast that have been cultivated specifically for beer brewing: ale yeast and lager yeast. It’s possible to use the wild yeast living naturally in our environments to brew beer (as for some Belgian styles), but wild yeast is somewhat unpredictable and can result in some very funky brews.

What is it then that the yeast contributes to the flavour of the beer?

Both ale and lager yeasts feed on the sugars in the malt to produce alcohol, carbon dioxide, and other flavors. This fermentation process goes on for between 5 and 14 days, after which the yeast has more or less finished converting all the existing sugars. Additional sugar can be added during bottling to encourage more carbonation in the final brew.

Realbeer has an illuminating mini-history of the use of yeast in brewing. Apprently “lager” comes from the German word for storage: Brewing of one kind of yeast was seasonal and so these beers were stored.

Ales came first, when brewers weren’t exactly sure what role yeast played. Because ales were unstable, brewing ceased in warm weather and brewers would store reserves in as cool or cold an environment as they could find. Brewers storing their beer in very cold Alpine caves found that their beer was more stable because the yeast had sunk to the bottom.

How do all the beer sub-types fall into these two yeast categories? Realbeer says:

Ales include everything with ale in the name (pale ale, amber ale, etc.), porters, stouts, Belgian specialty beers, wheat beers and many German specialty beers. They generally have a more robust taste, are more complex and are best consumed cool (50F or a bit warmer) rather than cold.

Lagers include pilseners, bocks and dopplebocks, Maerzens/Oktoberfests, Dortmunders and a few other styles found mostly in Germany. They are best consumed at a cooler temperature than lagers, although anything served at less than 38F will lose most of its flavor.