Yeast Fermentation Terminology

A beaker filled with brown liguid

Some yeast fermentation terminology will be given here so that you will understand what you are reading in the homebrewing texts, what’s going on in your carboy or bucket, and so you can communicate these terms concisely.

  • Attenuation: Beer Attenuation is usually represented as a percentage of the total wort that the yeast convert into alcohol and CO2. This is actually called the “apparent” attenuation. It is derived by comparing the original gravity (OG) of the beer with the final gravity (FG). The formula is (OG-FG)/(OG) and is represented as percentage. 

    As an example, if you have a beer with an OG=1.050, and a FG=1.010, the apparent attenuation would equal (50-10)/50=0.80 or 80%. Because alcohol weighs less than water, the apparent attenuation of a beer can be more than 100%. 

    Since this isn’t possible, the “real” attenuation will always be less. For our purposes though, apparent attenuation will suffice as a way of measuring the degree of fermentation. 

    The apparent attenuation of yeast strains differ and are usually quoted. Since each fermentation will depend on temperature and the amount and types of fermentable sugars in your wort, the actual apparent attenuation you get will vary from that that is quoted for each strain. The quoted amounts are usually averages obtained under laboratory conditions, but are a good guide for the homebrewer to use when formulating recipes and deciding which yeast is appropriate. 

    Apparent attenuation for most yeast ranges from 67-77%, but can be much lower or as high as 85-90% (although I don’t think I’d enter these beers in any competitions). Because it’s not totally up to the yeast how much sugar they have to eat, the fermentability of your beer, (which you determine with your recipe and mash schedule,etc.) generally determines how much a beer will attenuate.
  • Lag Time: This refers to the amount of time it takes the yeast to go from when you pitch the yeast into your wort, through the reproductive phase, until you begin to see signs of fermentation. These would include a change in color of the beer to an opaque creamy color, foam beginning to form on the top of your beer, and bubbles rising up from the bottom of your fermenter and out the bubbling airlock.

    Long lag times may or may not indicate problems. Long lag times are normal for lager yeasts and ale yeasts pitched cold. But in general, lag times for ales longer than 24 hours may indicate problems with oxygen levels, yeast health, or yeast pitching rates.
  • Pitching Rate: Pitching rate refers to the yeast cell count per volume of wort. Various pitching rates are encountered in brewing texts. As discussed on Jamil’s website Mr (Click Here to go to MrMalty’s Pitching Rate Calculator), the consensus pitching rates are about 1 million cells of healthy yeast per milliliter of wort per degree Plato, less for ales and more for lagers. He recommends .75 million yeast cells for ales and 1.5 million for lagers and those are the numbers his pitching rate calculator uses.

    I recommend you use the calculator and always know how much yeast you need to pitch for your beers. Pitching the correct amount of healthy yeast is just one less thing to worry about in your beers. 

    Both the White Labs vials of yeast and the Wyeast Activator packs of yeast have about 100 billion healthy yeast cells per container. As an example of proper pitching rates, for 5 1/2 gallons of wort with an OG of 1.048, you would need two vials or two smack packs of yeast to get the proper amount of healthy yeast ion your beer.
  • Flocculation: Flocculation refers to how yeast cells will congregate and form clumps of yeast that settle to the bottom of the fementer after fermentation is complete. Each yeast variety flocculates to a different degree. 

    Some form nice compact lees or sediment on the bottom while others form a fluffy mass of yeast which disturbs easily. Some of the highly flocculant yeast may even settle out before fermentation is complete. By gathering the bottom-most yeast from your primary for repitching, you are favoring those yeast that flocculate the most and drop out first.

    This sometimes leads to undue sweetness in your beer from the extra fermentable sugars that are left. And since the yeast that flocculate last are the ones that consume the byproducts such as diacetyl, this is sometimes the cause of high levels of that substance in your finished beers. 

    All you as a homebrewer can do is to pitch the proper amount of healthy yeast into a well aerated wort, and when choosing yeast to re-pitch from a prior fermentation, use the yeast from the middle of the yeast cake. Click here to go to the Yeast Selection Guide.

Understanding yeast fermentation terminology will help you understand the processes of fermentation and allow you to communicate these processes to other homebrewers or winemakers. Although yeast fermentation terminology is pretty straightforward, there is some confusion when it comes to the specific application of terms such as apparent attenuation versus real attenuation. For most homebrewers, apparent attenuation is more than adequate for communicating degree of fermentation.

Information for this article on yeast fermentation terminology was taken from Jamil Zainasheff’s, John Palmer’s How To Brew, Randy Mosher’s Radical Brewing, and Al Korzonas’ Homebrewing Vol.1.

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