Re using yeast from commercial beer isn't necessarily the fermenting yeast some brewers use a different yeast for bottle conditioning. Article from BYO.
For the benefit of readers who are not up on these methods, I will briefly cover the basics of how a brewer with a basic home microbiology lab can propagate yeast from a bottle of beer. We need an example for this question and I am going to take the easy route and select a bottle of Sierra Nevada Pale Ale.
The first decision that we need to make is do we propagate by simply collecting and growing the yeast solids from the bottom of the bottle, or do we do a little clean up work. I selected a bottle of Sierra Pale Ale because if we really wanted to take the easy route, starting with what is on the bottom of this bottle will most likely work. Why? Because Sierra Nevada Pale Ale is conditioned with the same strain used for fermentation and because their quality control measures are extraordinary. But with all the respect brewers have for Sierra Nevada, simply propping the “dregs” from the bottom of a bottle is pretty crude. The purpose of propagation is to grow cells and whatever ends up in the prop is probably going to grow. So what is a brewer to do? Make some media and streak some plates!
. . . streaking provides a much better shot at growing what we want versus bringing in a bunch of riff raff into the propagation party.
Yeah, this is sounding pretty advanced but it’s really pretty simple. Making your own media at home is just a few steps away from making Jell-O! Mix up an all-purpose media such as Wallerstein Lab Nutrient (WLN), pressure cook in media bottles for 20 minutes at 15 psig (1 barg) pressure, allow to cool to about 86 ˚F (30 ˚C), and pour the media into pre-sterilized Petri dishes. Once the plates have cooled and solidified they can be used to streak out the dregs from the bottom of the bottle. This is one of the things we do at BYO Boot Camp in a conference room. Fun times!
Why do we want to plate the sediment from the bottom? Because by streaking this stuff onto plates, we can select a single colony-forming unit (CFU in microbiology speak) and propagate that one colony. Each CFU is assumed to represent a single yeast cell and proper dilutions and streaking methods help to produce plates with clear separation among colonies. Suffice to say, streaking plates provides a much better shot at growing what we want versus bringing in a bunch of riff raff into the propagation party.
OK, now what? The textbook methods you reference usually start with transferring a single colony from a plate into about 25 mL of sterilized wort. In about 48 hours, the contents of the propagation flask (usually an Erlenmeyer flask that is about twice the volume of the contents) is increased to 10X. This means that 25 mL of propagating yeast slurry is added to 225 mL of sterile wort in a 500 mL Erlenmeyer flask to yield a 250 mL volume. This method continues until there is enough yeast to pitch into wort. Yes, the propagation volume represents a significant (10%) volume and the color and flavor of the propagation wort is important to your finished beer. But that is really a separate question for another deep dive into what-about-this-wort-ism.
In order to isolate a yeast or bacterial strain, streaking plates is the best way to make sure you are not bringing along unwanted guests. Photo courtesy of Omega Yeast
Now that the basic method has been established, let’s go back and explore some possible shortcuts because this process is somewhat of a pain in the neck (this is when I get to thank the yeast sponsors who generously donate to Team Mr. Wizard . . . thank you yeast sponsors, I am still waiting for those cool rugby shirts I suggested on Facebook). Idea #1 is to skip the intermediate steps and add the starter cells, either a single CFU from a plate or a shot of dregs from a beer bottle, to a volume that is ~1⁄10 the fermenter full volume (remember this is not 10% of the wort volume, but 10% of the wort + yeast volume). The real challenge to this method is transferring the starter cells into the sterile wort without bringing other organisms to the party. The idea with pure culture technique, which is what this is about, is growing cells from a single cell line, and the reality of the situation is that it is nearly impossible to transfer yeast into a sterile environment, i.e., the wort in your Erlenmeyer flask, without introducing other organisms.
So why do the textbook methods suggest incrementally increasing the volume? It’s all about competition, hurdles, and speed. While this may sound like track and field, it’s really about putting yeast into an environment where they will outcompete other critters by quickly reducing pH, gobbling up nutrients, and producing alcohol, which are all hurdles that impede the growth of other organisms that snuck past the bouncer at the door (you). As long as the selected yeast population is the super majority, non-selected bacteria and yeast strains will most likely fare poorly in this environment. “Most likely fare poorly” is not an absolute and yeast labs that do this thing day in and day out have stringent quality methods to ensure that the bouncers were effective.
Circling back on your question, if you put a single CFU or shot of dregs into your 3-L (3-qt.) starter, a lot of time passes before significant pH reduction, nutrient depletion, and alcohol formation occurs. This time represents the period between bar closing and sunrise when all sorts of chaos occurs. Yeast labs do not want this sort of craziness happening . . . ever . . . and have found that incremental increases in propagation volume work pretty darn well.
A corollary to this “hurdles theory” that is so prevalent in the practical application of food microbiology, is selection. Our bottle of Sierra Nevada Pale Ale is probably clean and growing the yeast sediment from one of these bottles will probably turn out just fine. But what if you were going to grow the sediment from a great beer brewed in a garage brewery? The beer is great, we just established that, but great beer can contain bacteria that don’t cause product damage. Let’s call these bottles asymptomatic spreaders. You bring the dregs from an asymptomatic spreader into your propagation flask, encourage cell growth, and what you have is a messed-
up starter culture. This can happen with or without incremental changes in volume, and is much more likely to occur when using dregs as the cell source versus a single CFU.
Back to your original question. Can I just pitch the dregs into a 3-L starter at 1.035–1.040 that is well fed, oxygenated, and on a stir plate? This is a really great question and I sincerely hope that my brief explanation has explained why the answer is “yes, but not recommended.”
Response by Ashton Lewis.
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