With all the writing I’ve been doing about yeast lately, I thought it would probably be a good idea to outline my general yeast propagation and storage procedures. There’s an enormous variation, both philosophically and technically, among homebrewers – from directly pitching a smack pack to acid washing and storage in -80°C freezers. My own techniques are based on two sometimes contradictory goals:
- Emulate professional brewers’ procedures whenever possible, in an endless pursuit of better beer.
- Minimize the expense of brewing, in terms of both time and money.
#1 precludes simply throwing in a smack pack and hoping for the best, and #2 precludes buying new yeast every time I brew. So I find myself a reluctant yeast rancher. There are many ways of storing yeast, and all of them have been tried by other home brewers at one time or another, so I’m not going to go into details here. Suffice it to say that the cheapest and easiest is to store yeast slurries in an ordinary refrigerator. If you’re going to be using the yeast within a few weeks, you can even harvest it directly from the fermenter and re-pitch into the next batch. I don’t really brew often enough for that to be practical, and I also worry about the strain of repeated high-gravity fermentations causing mutations, so I propagate solely in un-hopped 1.030 starter wort. In effect, this means I’m always pitching first-generation yeast, although I’ll probably start over every few dozen batches anyway, just for peace of mind.
Except for when I’m starting a new strain from a smack pack, the cycle begins and ends with a 100 mL glass jar that’s stored in my beer fridge. It’s pretty important to get a reasonable estimate of the number of cells in the starting population; any errors here will be compounded throughout the rest of the process. For the sake of simplicity, I’m going to assume the slurry is a cylinder 3 mm in height and 6 cm in diameter. It’s almost exactly a month old, so the viability should be about 75%. Assuming a density of 4.5 billion/mL, there should therefore be about: π(3 cm)2(0.3 cm)(4.5 billion/mL)(0.75) = 29 billion viable cells. That’s enough that I’m going to step up only twice. For an older slurry (fewer than about 10 billion cells), I would do a three-stage propagation.
Unless poor planning or a heavy brewing schedule have drained my supply, I do all my propagations using the tail runnings from a previous mash that I store in the freezer. This is the ultimate cost-cutting measure; as I continue to use a particular strain, the yeast cost per batch approaches zero. These particular tail runnings are from a doppelbock, so they’re darker than usual. Since I’m going to be decanting the starters, that isn’t a huge issue – and this is being pitched into a stout anyway. For a very light beer I would have to plan ahead, or bite the bullet and buy some DME. They’re also higher-gravity than usual, at about 7°P (1.028). Ordinarily the tail runnings would end up closer to 3-5°P (1.012-1.020) and need to be boiled down by about half.
For the first stage I’m going to use 500 mL. (When building up from an older slurry I would start with 200 mL, then 500.) The chief drawback to using the MrMalty calculator for this kind of thing is that it won’t deal with starter volumes smaller than 1 L – presumably because that’s roughly the minimum size needed to get significant growth out of a smack pack. So for the first stage, I use the Wyeast calculator instead. As you can see, it predicts that my 500 mL starter will produce about 137 million cells/mL, or 68 billion total. So it’s basically back up to the cell count of a smack pack.
Now I can move back to the MrMalty calculator, manually setting the viability to 68%. For this particular beer, I therefore need roughly a 2.5 L starter. I’m using the “continuous aeration” setting, not because I am aerating continuously – only as much as foaming allows – but because that’s the setting that most closely matches my own slurry measurements, adjusted for 1.030 wort. My highly sophisticated aeration setup consists of an “Elite 800″ model aquarium air pump, with two 0.45 μm syringe filters in series, and a disposable plastic air stone. I like the disposable stones because they produce very fine bubbles, and can be thrown away instead of trying to sanitize the microscopic pores. The stainless steel nut is there to keep the air stone from floating to the top of the starter.
Be sure to allow enough time – 3-4 days per stage – to have the yeast ready by brew day. After each stage, I refrigerate the starter overnight, then decant as much liquid as possible, both to remove the alcohol and to maximize the surface:volume ratio. The whole idea here is to keep the yeast as healthy as possible, and avoid any potential long-term complications. Before cooling the final stage, though, I resuspend the yeast and pour off 100 mL of the starter into a jar. This keeps a protective layer of beer over the yeast for storage, and ensures you won’t select a substantial number of mutants that are either more or less flocculent than the population as a whole. The jar is then sealed, labeled with the strain, number of “generations”, and the date, and placed in the fridge for next time. Yeast stored this way can be revived and built back up to a pitchable population after well over a year.
Because I know you’re curious, the yeasts I’m currently keeping on hand are:
- Wyeast 1028 London Ale (Worthington White Shield)
- Wyeast 1056 American Ale (Sierra Nevada)
- Wyeast 1084 Irish Ale (Guinness)
- Wyeast 1272 American Ale II (Anchor Liberty)
- Wyeast 2035 American Lager (August Schell)
- Wyeast 2206 Bavarian Lager (Weihenstephan 206)
- Wyeast 3787 Trappist High Gravity (Westmalle)
- Wyeast 3864 Canadian/Belgian Ale (Unibroue)
- Wyeast 3944 Belgian Witbier (Celis White)
With these nine strains I feel I can brew just about anything (at least anything I brew regularly), without having two that would be substantially similar.