Serious Eats: Drinks
Homebrewing: Robust Porter Brew-Along Day 3
Fermentation is in full force now!
We're about 36 hours after pitching the yeast into our Robust Porter, and the airlock is bubbling away. The yeast have occupied every milliliter of the wort and they're breaking down sugar at a rapid rate. That thick, ugly foam that is on the top is called Krausen (a word derived from German, pronounced "KROY-zen"). It is caused by CO2 being forced up through the wort and collecting protein and bits of hops and grain along the way. When you're using an ale yeast, like we are with this porter, this is also the place where the highest density of yeast can be found.
This is the phase when the majority of the alcohol is made. The chemistry is pretty simple. Yeast consume a glucose molecule, and produce two ethanol (alcohol) molecules and two carbon dioxide molecules. This process will continue until all of the glucose is gone. The alcohol production is a natural defense system of yeast, since many competing bacteria are unable to survive in this type of environment.
By this point all the oxygen has been completely used up and the reproduction phase has ended. For those who remember their high school chemistry, the lack of oxygen means that this phase is considered an anaerobic process. Any rogue oxygen molecules that are still in suspension will be forced out of solution through the carbon dioxide that is being produced. It's important to not introduce excess oxygen into the beer after fermentation has begun, since it will shorten the time it takes for the beer to go stale. This is why it's best to avoid excessive movement or agitation of the beer after the Krausen is present.
Fermentation will continue for about a week, so not much will change in the next few days, but we'll be back again tomorrow to see how it's progressing. Check back then!
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