Dear Mr. Wizard,
I am curious about the best way of going about reusing yeast. I have thought about taking it from my secondary fermenter, but after that I have no idea how to store it or what I need to do. Could you please advise?
Mr. Wizard replies:
Yeast storage and reuse is one of those topics that strikes fear in many homebrewers because of the importance yeast plays in beer quality and the real possibility of ruining a batch of beer with bad yeast. With that said, there are only a few key things to be mindful of with harvesting and storing yeast for use in subsequent brews.
Commercial brewers routinely harvest, store and reuse yeast because beginning every batch of beer with a new culture is not feasible without considerable investments in both time and equipment for large-scale yeast propagation. Furthermore, the quality of yeast available for harvest following fermentation is excellent in most breweries. The optimal time for harvesting yeast is after primary fermentation has completed, when yeast viability is high and the yeast is easy to crop either from the top of an ale fermentation or the bottom of the fermenter. This technique is most easily accomplished with the use of conical fermenters, which have become the norm.
Sanitation is of the utmost importance when harvesting yeast and all tools must be clean and sanitized prior to use. Since yeast slurries are rich in nutrients, especially as yeast ages, dies and autolyzes, bacteria can grow during storage and the slurry can turn into a source of bacterial contamination. With this being said, good techniques can be easily used to successfully harvest and store yeast.
Once harvested, the yeast slurry should be stored cold to minimize metabolic activity and loss of viability. The general practice in commercial breweries is to maintain the yeast slurry between 32 °F and 38 °F (0–4 °C) for a minimal time period before re-pitching. Most large breweries harvest yeast from the fermenter and store it in an agitated, cooled vessel to minimize hot spots in large volumes of yeast. At home, where much smaller volumes are used, a slurry can be easily maintained at a uniform temperature in the refrigerator. Many small brewers leave their yeast in the bottom of their conical fermenters and remove the yeast for re-use immediately before pitching. This method works well as long as the yeast does not sit in the bottom of the tank for an excessive time period following fermentation. Anything beyond two weeks is getting a bit long based on my experience.
A method I have successfully used in 5-gallon (19-L) batches fermented in carboys is to harvest the yeast after primary has completed and the yeast has settled to the bottom of the fermenter. Moving the carboy into a refrigerator greatly helps with yeast flocculation. The beer can be racked off the yeast into a secondary fermenter, keg or bottling bucket after about a week and the yeast can easily be recovered by swirling the sediment in the bottom of the carboy with a little beer left behind after racking. This slurry can then be poured out of the fermenter into a clean and sanitized storage container and placed in the refrigerator. I suggest using a glass container fitted with a sterile cotton plug or a plastic container with a screw top because yeast slurries can build up pressure even when stored cold.
When I was a student at UC Davis we used to go on annual trips to Sierra Nevada that were always a great deal of fun. Not only did we get a great tour of a great brewery, we were also given goodies to take back to Davis. On one such trip, we took two glass bottles used to autoclave and store microbiological media — these served as our yeast containers. When we returned to the lab we placed the yeast-filled bottles in a 39 °F (4 °C) cooler for future use. Later the next day, my friend Bill Cherry and I heard a noise from the cooler and discovered a huge mess caused by an exploding bottle in the cooler. We put on face shields and thick gloves to carefully open the remaining bottle. Suffice to say, these bottles with sealing caps were no longer used as little yeast brinks.
The other thing to consider when harvesting yeast for re-use is its history. I do not suggest harvesting yeast from high alcohol beers, beers that had a sluggish or unusual fermentation or from batches of beer brewed from “high generation” yeast. Every time yeast is used in fermentation its generation number increases. First generation yeast comes from a lab propagation. When the fermentation is complete and yeast is harvested, the next batch or batches contain second generation yeast (it is common to harvest enough yeast from one batch to brew two or three batches).
As the generation number increases, so does the likelihood of using yeast that has mutated and lost some of its desirable brewing qualities (such as flocculation characteristics). The potential for contamination also increases with each generation. Most commercial lager breweries do not use yeast older than 10 generations, while some ale brewers reportedly never go back to a lab culture and are always re-pitching yeast from a fermentation.
The huge difference between commercial brewing and homebrewing is frequency. While commercial brewers brew frequently (packaging breweries typically brew 24 hours a day, five to seven days per week), homebrewers are not so active! This makes rules of thumb about the number of generations between buying yeast of little use because the storage time increases. One summer I had a group that I brewed with and we took turns brewing with our chosen strain to minimize the time between fermentations. This worked well since we all took cleaning and sanitation seriously and passed around the culture for several months without incident.
I hope that I gave you some useful information to address your question. Now for some unsolicited advice.
Bad yeast will wreck a brew, wasting both time and money in addition to creating a shortage in beer! There are several sources of very good homebrewing yeast out there and the price of yeast is relatively inexpensive in the grand scheme of things, especially if one values their free time. There are several reasons to re-use yeast, but if one is in doubt about technique and does not brew relatively often (every couple weeks), I would seriously consider the pitfalls before using this method on a routine basis.
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