Secondary fermentation

by Ray Daniels

Most homebrewers start out using a single-stage fermentation, often in a covered plastic bucket. Eventually, they hear someone talking about "secondary fermentation" and curiosity kicks in.

Many brewers refer to the bottle fermentation that carbonates their beer as a "secondary fermentation." In addition to homebrewers, many European and even some American craft brewers follow this practice. It is certainly one example of secondary fermentation.

But for homebrewers, the phrase has a different meaning. Here, it more often refers to the final phase of fermentation before bottling. Thus, the activity that goes on inside those plastic buckets can be divided into two phases: primary fermentation and secondary fermentation.

Primary fermentation is marked by the vigorous release of carbon dioxide. The airlock on top of a typical homebrew fermenter will bubble away at a rapid rate during this phase. As this vigorous activity winds down and the bubble count decreases, the primary fermentation is drawing to a close. The entire primary fermentation phase usually only takes three to five days in a normal ale fermentation. (Lagers may take as long as seven to ten days in a homebrew setting.)

The key chemical reaction of fermentation -- the conversion of sugar to alcohol and carbon dioxide -- occurs mostly during the primary phase. Despite this, the yeast still has important work to do in the beer.

During secondary fermentation, the yeast reduces or removes certain undesirable flavor components (diacetyl, acetylaldehyde, and some sulphur compounds), thereby maturing the beer into a more drinkable condition. During this latter phase of fermentation, the majority of the yeast in the beer also settles to the bottom of the fermenter, leaving the beer clear and bright. Thus, at the end of secondary fermentation, beer is generally ready to be carbonated and consumed.

I imagine that you are already familiar with the layer of debris that collects at the bottom of a fermenter. This debris includes flakes of protein called "trub" (pronounced "troob") as well as dead yeast cells and other materials that precipitate from the beer.

The positive flavor effects of secondary fermentation can be improved by separating the young beer from this debris. In commercial breweries this is done by simply draining the debris out of the bottom of the fermenter. To accomplish this at home, we must move the beer and leave the debris behind. As a result, it is common for many homebrewers to employ a second fermenter. At the end of primary fermentation, they will transfer the beer into this second fermenter and allow it to sit for about one week before bottling.

The vessel most commonly used for secondary fermentation is a 5 gallon glass carboy. These containers are easy to use and easy to clean. In addition, they offer the advantage of allowing you to see what is going on inside the fermenter.

After racking beer to a secondary fermenter, you may see a few carbon dioxide bubbles escaping from the air lock, but little other activity will appear. After a few days, you will probably notice that a layer of yeast is developing on the bottom of the fermenter. At the same time, the beer may begin to look darker at the top than at the bottom. This means that the yeast is settling to the bottom. The beer at the top appears darker because there are fewer yeast particles in the beer to reflect light.

When using a secondary fermenter, many brewers choose to wait until the beer has cleared completely before bottling. The clear beer will still contain enough yeast to carbonate, but it will be in a more drinkable condition.