Oh the Difference a Yeast Makes

by Matt DeMarco

One amazing thing about beer is that despite the multitude of brands and styles there are only 2 classifications of beer--lagers and ales.  For example, a pilsner is part of the lager family and IPAs are part of the ale family. These classifications are based on the type of yeast used in fermentation. Prior to fermentation, malt is cracked to release the starch within the grain and then mixed with water.  The mixture or wort is then heated to convert the starch into sugars.  Fermentation begins when yeast is added to the wort.  During fermentation, the yeast feeds on the sugar excreting alcohol and C02.  Top fermenting or ale yeast stays on top of the wort while it is active and then sinks to the bottom when all of the fermentable sugar is consumed. Bottom fermenting yeast or lager yeast is the opposite and stays on the bottom of the wort the whole time.

Lagers use bottom fermenting yeast and fermentation must be at a controlled temperature

of 45° to 55°F or it will produce harsh flavors. If you have ever had a California Common beer or Anchor Steam Beer, it uses a special type of lager yeast that works at higher temperatures but produces a beer that tastes more like an ale. Lager yeast also takes longer to turn the sugar into alcohol because of the low temperature. The combination of bottom fermenting yeast and low temperature produce a mild and crisp taste with minimal flavor provided from the yeast. So when you taste a lager and it tastes clean and crisp, it was fermented at the right temperature. Lagers are more than just Pilsners or American Pale Lagers. Bocks, Doppelbocks, Maibocks, Ocktoberfests/Marzens, Rauchbiers (smoked beers), and Schwarzbiers (black pilsners) are all types of lagers.

Ales use top fermenting yeast that can be fermented at 68° to 72°F. Ales tend to have a more complex taste and aroma than lagers. The warmer fermentation temperature allows the yeast to work faster and produce a type of chemical called esters. Depending on the strain of yeast, the esters produced create  different flavors. For example, Hefeweizen yeast produces banana and clove flavors and some Belgian style strains can produce plum, raisin, or even pepper flavors. Belgian ales are the best example of yeast flavors since the ales tend to showcase the fruity and spicy notes of the yeast. There are so many different strains of yeast that a brewery can dial in exactly what flavors they want from the yeast.

Some flavors and aromas are desired and some are very undesirable. Here is a list of the most common yeast byproducts and what flavors and aromas they produce:

• 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 - flavor and aroma of bananas, strawberries, apples, or other fruit
• Medicinal - chemical or phenolic character
• Phenolic - flavor 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

Unless a beer is highly filtered and pasteurized, the yeast lingers in some capacity and can change the flavor of the liquid overtime. This change can be good or bad. If you have ever had an old beer and it tasted like a band-aid or cough medicine, you can thank the yeast for that. Some yeasts though help the beer get better with age, two examples that come immediately to mind are aged barleywines and stouts. With some yeasts the flavor can be completely unpredictable. (see my post about brettanomyces.)

Finally, just to clarify, beer styles are just types of ales or lagers. An individual style of beer is based on color, bitterness, and alcohol. So no matter what beer you are drinking, it is either an ale or a lager. There are tons of different styles as you already know. Besides the ones you are used to drinking, there are many obscure styles that you may have never heard of but that is a post for another day.

Cheers!

Here is a link to a nice reference of yeast strains by beer style:
https://byo.com/resources/yeast