On Fermentation

Last night, Abel and I transferred all the lovely beer from our primary fermentation vessel to a secondary fermentation vessel. The primary fermentor was a straight up white plastic bucket with an airlock drilled into the lid; the secondary fermentor is a traditional carboy. Most people who make beer or wine use one of these. Our carboy is plastic instead of glass. The general consensus seems to be that plastic carboys are lighter and safer than glass, and that the difference in material doesn't really affect the taste. In honor of this momentous occasion, I wanted to talk a bit about fermentation (with an emphasis on its application to brewing).

The carboy, full of delicious beer.

As may be no surprise, fermentation is hands down the most important part of brewing. This is the process that allows yeast to transform a syrupy-sweet and somewhat viscous liquid into beer - a thinner, alcoholic, carbonated, and decidedly less sweet beverage.

The organism responsible for all of this is the humble, unicellular yeast. Each yeast cell has one primary goal during its life: make more yeast cells. Doing so requires materials and energy. The materials are taken from the wort - various vitamins, minerals, and soluble fats. The energy instead comes from dissolved sugars. Yeast metabolizes sugar through the process of glycolysis, untapping a small amount of energy for each molecule of sugar processed. Unfortunately, glycolysis is a one-way process. There are many components that start out in one conformation at the beginning and end up quite different by the end. In order to prime these units for additional rounds of glycolysis, the cell undergoes fermentation. This process resets the machinery for glycolysis, but also turns the remnants of consumed sugar molecules into ethanol (alcohol) and carbon dioxide. Ethanol is toxic to the cell, but nature has deemed this an acceptable risk in the name of creating more cells.

Unfortunately for the yeast that Abel and I have added to our wort, they have somewhat limited resources in their five gallons of liquid. Combined with ethanol toxicity, this assures that many of them will die. When this happens they sink to the bottom and begin to break down. For this reason we decided to brew using two-stage fermentation. Many, many yeast cells die within the first couple days of fermentation. By taking the beer away from those dead cells, it is easier to preserve a clean, fresh taste. This is especially helpful if fermentation is taking place in a slightly warmer place. Considering that we are in Houston and the ambient high just fell below 100º F recently, we deemed it an appropriate action.

Pictured: a half inch thick cake of dead yeast at the bottom of the primary fermentor.

There are still plenty of yeast cells kicking about in the secondary fermentation vessel. Those cells will continue turning sugar into beer, creating a lovely beverage from what was once a big steel pot full of sugar water. At this point, we only have another week or so until our beer is ready to bottle! We'll know that the time has come because we are measuring the specific gravity of the beer. Specific gravity references how dense the beer is when compared to water. As sugar molecules are broken down into carbon dioxide and ethanol, the beer becomes less dense. When specific gravity flatlines, we'll know that it's ready to bottle. At that point we add just a wee bit of sugar before bottling and capping. That extra sugar allows the surviving yeast to undergo yet more fermentation. This last fermenting process isn't about creating alcohol, but instead is responsible for the carbonation of the bottled beer.

During some batch in the future, I would like to try transferring only 50% of the beer to a secondary fermentation vessel. That way I would really be able to pick out the differences in taste that come from removing the excess dead yeast.

Fun facts for the day:

  - Dead yeast cake at the bottom of the fermentor does not taste very good. Not at all.

  - A fermentor is a vessel for fermenting. A fermenter is an organism that ferments