Here is some yeast fermentation terminology that will help you understand the content in homebrewing texts, what’s happening in your carboy or bucket, and enable you to communicate these terms concisely.
Beer Attenuation is typically expressed as a percentage of the total wort that the yeast converts into alcohol and CO2. This is known as “apparent” attenuation. It is calculated by comparing the original gravity (OG) of the beer with the final gravity (FG). The formula is (OG-FG)/(OG), represented as a percentage.
For example, if you have a beer with OG=1.050 and FG=1.010, the apparent attenuation would be (50-10)/50 = 0.80 or 80%. Since alcohol is lighter than water, the apparent attenuation of a beer can exceed 100%.
However, this is not possible, and the “real” attenuation will always be lower. For practical purposes, the apparent attenuation is sufficient for measuring the degree of fermentation.
Yeast strains have different apparent attenuations and they are usually provided as quoted values. However, the actual apparent attenuation will vary depending on factors such as temperature and the amount and types of fermentable sugars in your wort. The quoted values are generally averages obtained under laboratory conditions, but they serve as a useful guide for homebrewers when creating recipes and selecting appropriate yeast strains.
Apparent attenuation for most yeasts ranges from 67% to 77%, but it can be lower or higher, reaching 85% to 90% (although beers with such high attenuation may not be suitable for competitions). The fermentability of your beer, determined by your recipe and mash schedule, etc., generally determines the level of attenuation.
Lag time refers to the period between pitching the yeast into your wort and the onset of fermentation, including the reproductive phase. Signs of fermentation include a color change in the beer to an opaque creamy color, the formation of foam on top of the beer, and bubbles rising from the bottom of the fermenter and through the airlock.
Long lag times may or may not indicate problems. Lager yeasts and ale yeasts pitched at low temperatures typically have longer lag times. However, for ales, lag times exceeding 24 hours may suggest issues with oxygen levels, yeast health, or yeast pitching rates.
Pitching rate refers to the number of yeast cells per volume of wort. Brewing texts mention various pitching rates. I recommend using a calculator to determine the appropriate amount of yeast to pitch for your beers. Pitching the correct quantity of healthy yeast is one less thing to worry about in your brewing process.
Flocculation refers to how yeast cells aggregate and form clumps that settle at the bottom of the fermenter after fermentation. Each yeast strain demonstrates a different degree of flocculation. Some yeast forms compact sediment or lees, while others create a loose mass that easily gets disturbed. Highly flocculant yeast may even settle before fermentation is complete. Collecting the yeast at the bottom of the primary fermenter for reusing favors yeast strains with high flocculation.
This sometimes leads to excessive sweetness in your beer due to leftover fermentable sugars. Additionally, the yeast that flocculate last are the ones responsible for consuming byproducts like diacetyl, which could result in higher levels of this substance in your finished beers.
Understanding yeast fermentation terminology will enhance your understanding of the fermentation process and enable you to communicate effectively with other homebrewers or winemakers. While yeast fermentation terminology is generally straightforward, some confusion may arise when it comes to specific terms such as apparent attenuation versus real attenuation. For most homebrewers, apparent attenuation is more than sufficient for conveying the degree of fermentation.