Yeast have the most important job in brewing: they start with sugar and break it down, leaving alcohol, carbon dioxide, and a variety of flavors. The importance of yeast often gets forgotten when conversations about beer turn to grain and hops, but yeast actually have the potential to contribute more unique flavors to your beer—both good and bad—than any other ingredient.

Last week we talked about grain, and next week we'll look at hops, but today I'll be giving you what you need to know about yeast to make the best homebrew possible.

When people first started brewing beer, they didn't even know that yeast existed. But tiny microbes were floating around in the air and living in wood barrels, and when those yeast came into contact with unfermented beer (known as wort) they would settle down in all the sugar, rapidly reproduce, and begin fermentation. Different strains of yeast naturally occur in different areas, contributing to the regional styles of beer. It wasn't until 1857 that Louis Pasteur wrote the first paper about yeast as a living organism and its role in fermentation.

Our understanding of yeast has come a long way since then. Today's homebrewers can purchase yeast strains that come from commercial breweries from around the world. Yeast varieties are divided into two major categories: lager yeast and ale yeast. Lager yeast requires cold fermentation temperatures that can be difficult for homebrewers to maintain—generally around 55F. Since this temperature control requires some special (and expensive) equipment, many homebrewers don't use lager yeast. Instead, they opt for the ale yeast varieties that can ferment well at room temperature or just below.

Commercial Brands of Yeast

Most homebrew recipes will use one of the following three commercial brands of ale yeast:

White Labs and Wyeast are the two most common brands of liquid brewing yeast. White Labs comes in a hard plastic test-tube package, and Wyeast comes in large foil packages. Either brand is available in many varieties including different types of American, Belgian, English, German and Irish styles. A single package of these liquid yeasts usually contains enough cells to properly ferment beers up to 5% ABV. If you're going to make a stronger beer, you will need multiple packages to get the best results. It's also possible to take a single package and "grow" the yeast before you brew in order to increase the number of cells, which would avoid the need for multiple packages when making a stronger beer. This is called making a yeast starter. (Stay tuned for more on on that.)

Safale is a popular brand of dry yeast that comes in small, foil packages. Dry yeast packages have the benefit of containing a lot more cells than their liquid counterparts. This makes it a good choice for new brewers because you don't have to worry about purchasing multiple packages or making a yeast starter. A standard 11.5 gram package will properly ferment beers that can be as strong as 9% ABV. The downside of dry yeast is that it doesn't come in nearly as many varieties as the liquid. Some people also claim that dry yeast produces a lower quality beer, but I have used it several times in my own homebrew without complaint. Dry yeast also has the benefit of being about half the price of liquid.

There are three major factors that will affect the health of your yeast and the quality of fermentation:

Temperature control: Yeast are very sensitive to temperature changes. If there are fluctuations over 10 degrees, the yeast get stressed out and can produce undesirable flavors. Ale yeast ferment best in the 60 to 70° F range, and the cooler end of this range generally produces better flavors from the fermentation. My strategy to keep a constant temperature for my fermentation is to put the fermentation bucket in the coolest closet in the house, and just check up on it a couple times a day.

Yeast are very sensitive to temperature changes. If there are fluctuations over 10 degrees, the yeast get stressed out and can produce undesirable flavors. Ale yeast ferment best in the 60 to 70° F range, and the cooler end of this range generally produces better flavors from the fermentation. My strategy to keep a constant temperature for my fermentation is to put the fermentation bucket in the coolest closet in the house, and just check up on it a couple times a day. Oxygen: Like most living organisms, yeast need oxygen to be healthy. Before you add the yeast to your unfermented beer (the wort), you have to increase the amount of oxygen in the liquid. This can be done by vigorously shaking or stirring the wort for a couple of minutes after it has been boiled and cooled. If you stir the wort with a spoon, be sure that the spoon has been thoroughly cleaned and sanitized. It's important to note that after fermentation has begun, shaking or stirring isn't beneficial, and can actually cause your beer to go stale quicker.

Like most living organisms, yeast need oxygen to be healthy. Before you add the yeast to your unfermented beer (the wort), you have to increase the amount of oxygen in the liquid. This can be done by vigorously shaking or stirring the wort for a couple of minutes after it has been boiled and cooled. If you stir the wort with a spoon, be sure that the spoon has been thoroughly cleaned and sanitized. It's important to note that after fermentation has begun, shaking or stirring isn't beneficial, and can actually cause your beer to go stale quicker. Cell Count: If there are not enough yeast cells, they will spend more time and energy reproducing and not as much quality time fermenting the beer. Having a low cell count will cause off-flavors, increase the time of fermentation, and can even cause your fermentation to stall out prematurely. If you're going to make a stronger beer using liquid yeast, be sure to make a yeast starter. If your recipe calls for dry yeast, you don't really have to worry about cell counts.

Got questions about yeast's role in brewing? Leave them in the comments below! Ready to go? Me, too. We'll be making beer together before you know it.

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