Need Some Yeasty Knowledge and Wisdom
The more I brew, the more I realize that I don't quite understand the intricacies of some of the process. Right now I'm very curious about yeast. I understand the basics, I even understand some of the more complex stuff. And theres a lot of literature out there about it but I'm coming up short in understanding just a few things. I'm mostly looking for the wisdom from someone else's experience, since all I know is how I do it in my garage.
Lets say that I decided that I liked a particular strain of liquid yeast for my beer, and I wanted to brew a few big batches of it. Normally I'd make a starter, which multiplies the amount of yeast from the tiny little tube, to a factor of three or four times.
1. But if I need a heck of a lot more (like 3 or 4 more starters worth), how would I go about making that yeast more, or can I even get it to multiply that much?
2. How do bigger brewers (like microbreweries with 400+ gallon batches) do it? Do they buy these strains in bulk? Or do they start with small samples and cultivate it themselves.
3. I know that you can re-use the yeast from a batch of beer several times before it starts to change too much. Does this process only happen with batches of actual beer, or will this happen with your standard starter solution (DME/water at 1.040) as well? Is there a way to multiply it without modifying it?
4. I understand after feeding and multiplying the yeast will essentially go dormant for a while. What is a good amount of time to let it rest before using it? In other words, if I make a big starter that makes a whole bunch of yeast, is that yeast going to need to rest before I can pitch it into a wort? Or is it good to go anytime? (I ask this because I know that after hi-grav fermentations the yeast needs to rest before bottle carbonating will happen. So will a mad rush of making baby yeasts require a rest too, and how long?)
5. How long will healthy yeast survive at room temp? I know to store long term we keep it cold, but what about 70ish degrees? Will it stay dormant or die out? After how long?
Thanks for any answers you can give. I really appreciate it.
That's a lot of questions, so I'll just attempt to comment on a couple.
1. Yeast will continue to multiply under the right conditions, which simply put include amount of sugars and type of sugars available to consume, ABV, temperature, pH, and a few other things. In order to get the yeast count that you need you can do "step starters" in which you make a 1L starter, let it ferment for 36-48 hours, chill and then decant the spent wort. Then you add another volume of fresh wort and let it ferment for 36-48 hours, and so on. Sometimes this is an inefficient technique, i.e. you're not getting the number of multiplies of yeast relative to the amount of wort used. Check out yeastcalc.com for more info and a calculator for step starters.
Another thing you can do is make a full batch small beer, i.e. lower gravity, and then harvest the yeast after it has completed fermented. Then use this yeast on your next batch. Once again, yeastcalc.com will give an estimate of the yeast count after the beer is finished.
4. Yeast do not need to rest. They go dormant as the conditions change. They begin to store food rather than multiply when the conditions in the fermenting beer get to some threshold. So, after a starter has fermented out you can just pitch that yeast directly into your wort. It does not need to rest before pitching. There are optimal times to pitch a yeast starter to avoid fermentation lag times (at high kraussen), but in general if you pitch a healthy yeast starter with the correct number of yeast lag time will not be an issue. And once again, yeast do not need to rest.
I'll let someone else tackle the other questions and suggest you give the "Yeast" book by White and Zainasheff a read.
First, pro brewers buy bulk and they pitch from previous batches in a lot of cases, at least for a few generations of yeast.
Second, I believe yeast will only divide so many times before it can no longer multiply. I'll have to study up on that.
What I would do for a very large or very high gravity beer is to brew a lower gravity beer first, then pitch the big batch, or high gravity beer, right on the yeast. You'll want to brew the starter batch to have a fairly neutral flavor profile and make it not overly hoppy.
Aerate the beer very well. Yeast will reproduce as long as there is O2 in the wort.
I second the recommendation on YEAST by White and Zainasheff. I've been meaning to pick that up myself. I'm sure it covers a lot of territory that most of us haven't even thought to think about.
Yeast can multiply indefinitely. Most likely, all the yeast in the world is descended from a single cell. Younger cells are generally healthier and more vigorous, and produce better beer faster. Quality supersedes quantity.
Larger brewers have in-house yeast labs. Smaller ones buy in bulk, then repitch 5-6 times, or until the yeast character changes in an undesirable way. Some are more regimented than others. Sometimes the change is in a direction that makes the beer better. On a homebrew level, there is no way to prevent your yeast from ultimately drifting from its original profile. However, keeping it in relatively consistent and stress-free environments will slow this process down, as will fastidious sanitation.
Yeast survival is dependent on temp, pH, ABV and other factors. They die off in an exponential decay model, so that you'll have progressively fewer yeast as you go along. I have harvested yeast from a single bottle of 9% beer that was shipped halfway around the world under unknown conditions and stored on a shelf for however long, so it's safe to say that they're pretty tough.
Given adequate nutrients a yeast POPULATION will multiply indefinitely, a single yeast cell has a finite life span before it dies (autolysis).
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