It occurred to me soon after posting the previous article on lager brewing that I really didn't mention much about yeast starters. Not that the PROCESS involved in making a yeast starter for a lager is any different than it is for an ale - you still ferment the starter at roughly room temperatures - it's just that lagers require twice the amount of yeast, which can lead to a ridiculously-large starter requirement... easily past 4 L, depending on the size of your beer.
For example, using the yeast calculator at mrmalty.com, if you enter that you're going to brew a 1.060 lager, the calculator tells you to make a 7 L starter (with intermittent shaking selected). Crazy, right? Who has a bottle large enough for a yeast starter of that size? Not to mention how expensive that much dry malt extract would be, if you don't happen to have starter wort frozen and on hand. One may immediately assume that if you made two 3.5 L starters, one right after the other, you'd end up with the same amount of yeast, but that isn't the case. It's kind of complicated, but basically the amount of wort you use changes the yeast inoculation rate. A 4 L starter would result in more yeast, for example, than a 2 L starter, but not proportionally-so (e.g. a 2 L starter, with intermittent shaking, would result in ~250 billion yeast cells; a 4 L starter, ~350 billion cells... not the 500 billion you may first assume).
Sometime last year I came across some steps that someone had thankfully
posted online. Basically, it walks you through using the yeast
calculator to make 2 smaller starters, to gradually build
your yeast up until they're at the required number for the beer that
you're going to brew. This way, you end up using less starter wort overall, and you won't need a 10 L carboy just for a starter. It works well, makes sense math-wise, and saves money. The only REAL catch to this process is that it takes time... you really have to prepare ahead.
Ok, let's do an example. Say we want to brew a 1.052-gravity Oktoberfest. We have a smackpack of Wyeast 2206 Bavarian Lager with a manufacture date of March 15th...
For the first starter...
1) In the yeast calculator, select "Lager" under Fermentation Type.
2) Enter the production date of the yeast - March 15th, 2012.
3) Select "Intermittent Shaking" in the tab that says Simple Starter (unless you have a stir plate, in which case, lucky you!).
4) Slide the Growth Factor bar all the way to the right.
5) Fiddle around with the numbers under O.G. so that "# Liters of starter required" reads 2 L, or as close to 2 L as you can get - 1.032 in this case, requires a 1.95 L starter.
6) Take note of the number under "# Yeast cells needed (in billions)" - 239. This means that with our currently-dated smackpack, making a 1.95 L starter (properly) should result in about 239 billion yeast cells.
7) Make a starter of this size. Shake/swirl it whenever you walk by, letting it ferment out over a couple of days. Then, place it in the fridge for a few days to let the yeast drop out of suspension. Pour out the majority of the liquid off the yeast cake, and leave the yeast at room temp for the day to warm up in preparation for your second starter.
For the second starter...
1) In the calculator, uncheck "Calculate Viability from Date". Enter the number of cells, in billions, that the first starter made - 239 (you're doing this because in theory, a completely fresh smackpack would have 100 billion cells, so viability is therefore 100%... we now have 239 billion cells, so viability is technically 239%).
2) Enter the target O.G. for the beer you're going to brew - 1.052.
3) Now it shows we need to make a 1.56 L starter... this is going to be with the yeast cake left from our FIRST starter.
4) So, make your 1.5 L of starter wort and pitch it on top of the first starter's yeast cake, and follow the same process as before: shake frequently, let it ferment out.
5) If you want to decant off that wort again, put the fermented-starter in the fridge once more to let the yeast drop out. Decant most of the wort, then keep the yeast at a temperature roughly around what you plan on pitching at (e.g. 45 F).
As you can see, if you're going to decant the liquid both times, you need to start this whole process a good 10 days before brew day if you want to be absolutely sure your yeast is ready to go. Most ales can be brewed at much shorter notice, but if you want a proper pitch of yeast for your beer (and you should), you need to prepare well in advance for lagers.
One final, important note; while making smaller starters ultimately saves you money and increases yeast numbers, keep in mind that the more steps you take, the more likely contamination will be. Keep EVERYTHING sanitary during the process; it's just as important when making yeast starters as when brewing.