Yeasts the difference between ale and lager

Beer snobs will insist that there are truly only two types of beer: ales & lagers. While it’s amusing to hear them explain why a German Dopplebock is a lager while a Wietzenbock is an ale, the truth is that they’re not actually talking about flavors or styles, but yeasts.

Yeasts are living organisms. The distinction between ale yeasts and lager yeasts is based on the temperature range at which they are most active, and where in the wort-suspension the yeast concentrates. Ale yeasts are top-fermenting. During the most active stages of fermentation, usually within the first 5 days of pitching (or adding) the yeast to the wort, ale yeast is visible on the surface of the beer, amidst the foam and bubbles. Lager yeast is active at the bottom of the beer, and is easily missed after the first week as it mixes in with the sediment.

The top- versus bottom-fermentation is an aesthetic difference though; the real distinction is the temperature at which the beer should ferment. Ale yeasts ferment best between 60 -75 degrees F; (12-24C) going below this range slows fermentation, causing the yeast to be “sluggish”, while going above the range increases the risk of bacterial growth and other contamination, as well as causing off-flavors from the yeast. A friend’s batch of hefeweizen suffered because the home thermostat was set to cycle to lower temperatures at night.

Lager yeasts are a slightly different animal. While they ferment best between 35 & 55 degrees F (2-13C), for the first day, the temperature should be in the ‘ale’ range. This is because of the life cycle of yeast: for the first 12-24 hours, the yeast is converting starches in preparation of reproduction and releasing CO2. After this initial “blow off” is done, the beer should be placed in a temperature controlled environment (like a refrigerator) and gradually chilled down to lagering temperatures. Note ‘gradually’; when cooling the wort, the goal is to chill quickly to limit exposure to wild yeasts and bacteria. When chilling the beer to lager it, no dramatic steps are necessary, as the beer should already be in a carboy with an airlock. Sudden, rapid changes in temperature can ‘shock’, or even kill, yeast. Smaller batches are inherently more susceptible to temperature shock due to the smaller volume of liquid.

Some people prefer to make a lager yeast starter at room temperature, then pitch into cooled wort from the kettle, then move the fermenter into the fridge. Lager yeast may also need a dialectal rest, which involves raising the temperature into the 60-65F range for a couple days to let the yeast clean it self up. Diacetyl is a butterscotch like flavor and can be seen as a defect in certain types of lagers, especially light lagers.